1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203
204
205
206
207
208
209
210
211
212
213
214
215
216
217
218
219
220
221
222
223
224
225
226
227
228
229
230
231
232
233
234
235
236
237
238
239
240
241
242
243
244
245
246
247
248
249
250
251
252
253
254
255
256
257
258
259
260
261
262
263
264
265
266
267
268
269
270
271
272
273
274
275
276
277
278
279
280
281
282
283
284
285
286
287
288
289
290
291
292
293
294
295
296
297
298
299
300
301
302
303
304
305
306
307
308
309
310
311
312
313
314
315
316
317
318
319
320
321
322
323
324
325
326
327
328
329
330
331
332
333
334
335
336
337
338
339
340
341
342
343
344
345
346
347
348
349
350
351
352
353
354
355
356
357
358
359
360
361
362
363
364
365
366
367
368
369
370
371
372
373
374
375
376
377
378
379
380
381
382
383
384
385
386
387
388
389
390
391
392
393
394
395
396
397
398
399
400
401
402
403
404
405
406
407
408
409
410
411
412
413
414
415
416
417
418
419
420
421
422
423
424
425
426
427
428
429
430
431
432
433
434
435
436
437
438
439
440
441
442
443
444
445
446
447
448
449
450
451
452
453
454
455
456
457
458
459
460
461
462
463
464
465
466
467
468
469
470
471
472
473
474
475
476
477
478
479
480
481
482
483
484
485
486
487
488
489
490
491
492
493
494
495
496
497
498
499
500
501
502
503
504
505
506
507
508
509
510
511
512
513
514
515
516
517
518
519
520
521
522
523
524
525
526
527
528
529
530
531
532
533
534
535
536
537
538
539
540
541
542
543
544
545
546
547
548
549
550
551
552
553
554
555
556
557
558
559
560
561
562
563
564
565
566
567
568
569
570
571
572
573
574
575
576
577
578
579
580
581
582
583
584
585
586
587
588
589
590
591
592
593
594
595
596
597
598
599
600
601
602
603
604
605
606
607
608
609
610
611
612
613
614
615
616
617
618
619
620
621
622
623
624
625
626
627
628
629
630
631
632
633
634
635
636
637
638
639
640
641
642
643
644
645
646
647
648
649
650
651
652
653
654
655
656
657
658
659
660
661
662
663
664
665
666
667
668
669
670
671
672
673
674
675
676
677
678
679
680
681
682
683
684
685
686
687
688
689
690
691
692
693
694
695
696
697
698
699
700
701
702
703
704
705
706
707
708
709
710
711
712
713
714
715
716
717
718
719
720
721
722
723
724
725
726
727
728
729
730
731
732
733
734
735
736
737
738
739
740
741
742
743
744
745
746
747
748
749
750
751
752
753
754
755
756
757
758
759
760
761
762
763
764
765
766
767
768
769
770
771
772
773
774
775
776
777
778
779
780
781
782
783
784
785
786
787
788
789
790
791
792
793
794
795
796
797
798
799
800
801
802
803
804
805
806
807
808
809
810
811
812
813
814
815
816
817
818
819
820
821
822
823
824
825
826
827
828
829
830
831
832
833
834
835
836
837
838
839
840
841
842
843
844
845
846
847
848
849
850
851
852
853
854
855
856
857
858
859
860
861
862
863
864
865
866
867
868
869
870
871
872
873
874
875
876
877
878
879
880
881
882
883
884
885
886
887
888
889
890
891
892
893
894
895
896
897
898
899
900
901
902
903
904
905
906
907
908
909
910
911
912
913
914
915
916
917
918
919
920
921
922
923
924
925
926
927
928
929
930
931
932
933
934
935
936
937
938
939
940
941
942
943
944
945
946
947
948
949
950
951
952
953
954
955
956
957
958
959
960
961
962
963
964
965
966
967
968
969
970
971
972
973
974
975
976
977
978
979
980
981
982
983
984
985
986
987
988
989
990
991
992
993
994
995
996
997
998
999
1000
1001
1002
1003
1004
1005
1006
1007
1008
1009
1010
1011
1012
1013
1014
1015
1016
1017
1018
1019
1020
1021
1022
1023
1024
1025
1026
1027
1028
1029
1030
1031
1032
1033
1034
1035
1036
1037
1038
1039
1040
1041
1042
1043
1044
1045
1046
1047
1048
1049
1050
1051
1052
1053
1054
1055
1056
1057
1058
1059
1060
1061
1062
1063
1064
1065
1066
1067
1068
1069
1070
1071
1072
1073
1074
1075
1076
1077
1078
1079
1080
1081
1082
1083
1084
1085
1086
1087
1088
1089
1090
1091
1092
1093
1094
1095
1096
1097
1098
1099
1100
1101
1102
1103
1104
1105
1106
1107
1108
1109
1110
1111
1112
1113
1114
1115
1116
1117
1118
1119
1120
1121
1122
1123
1124
1125
1126
1127
1128
1129
1130
1131
1132
1133
1134
1135
1136
1137
1138
1139
1140
1141
1142
1143
1144
1145
1146
1147
1148
1149
1150
1151
1152
1153
1154
1155
1156
1157
1158
1159
1160
1161
1162
1163
1164
1165
1166
1167
1168
1169
1170
1171
1172
1173
1174
1175
1176
1177
1178
1179
1180
1181
1182
1183
1184
1185
1186
1187
1188
1189
1190
1191
1192
1193
1194
1195
1196
1197
1198
1199
1200
1201
1202
1203
1204
1205
1206
1207
1208
1209
1210
1211
1212
1213
1214
1215
1216
1217
1218
1219
1220
1221
1222
1223
1224
1225
1226
1227
1228
1229
1230
1231
1232
1233
1234
1235
1236
1237
1238
1239
1240
1241
1242
1243
1244
1245
1246
1247
1248
1249
1250
1251
1252
1253
1254
1255
1256
1257
1258
1259
1260
1261
1262
1263
1264
1265
1266
1267
1268
1269
1270
1271
1272
1273
1274
1275
1276
1277
1278
1279
1280
1281
1282
1283
1284
1285
1286
1287
1288
1289
1290
1291
1292
1293
1294
1295
1296
1297
1298
1299
1300
1301
1302
1303
1304
1305
1306
1307
1308
1309
1310
1311
1312
1313
1314
1315
1316
1317
1318
1319
1320
1321
1322
1323
1324
1325
1326
1327
1328
1329
1330
1331
1332
1333
1334
1335
1336
1337
1338
1339
1340
1341
1342
1343
1344
1345
1346
1347
1348
1349
1350
1351
1352
1353
1354
1355
1356
1357
1358
1359
1360
1361
1362
1363
1364
1365
1366
1367
1368
1369
1370
1371
1372
1373
1374
1375
1376
1377
1378
1379
1380
1381
1382
1383
1384
1385
1386
1387
1388
1389
1390
1391
1392
1393
1394
1395
1396
1397
1398
1399
1400
1401
1402
1403
1404
1405
1406
1407
1408
1409
1410
1411
1412
1413
1414
1415
1416
1417
1418
1419
1420
1421
1422
1423
1424
1425
1426
1427
1428
1429
1430
1431
1432
1433
1434
1435
1436
1437
1438
1439
1440
1441
1442
1443
1444
1445
1446
1447
1448
1449
1450
1451
1452
1453
1454
1455
1456
1457
1458
1459
1460
1461
1462
1463
1464
1465
1466
1467
1468
1469
1470
1471
1472
1473
1474
1475
1476
1477
1478
1479
1480
1481
1482
1483
1484
1485
1486
1487
1488
1489
1490
1491
1492
1493
1494
1495
1496
1497
1498
1499
1500
1501
1502
1503
1504
1505
1506
1507
1508
1509
1510
1511
1512
1513
1514
1515
1516
1517
1518
1519
1520
1521
1522
1523
1524
1525
1526
1527
1528
1529
1530
1531
1532
1533
1534
1535
1536
1537
1538
1539
1540
1541
1542
1543
1544
1545
1546
1547
1548
1549
1550
1551
1552
1553
1554
1555
1556
1557
1558
1559
1560
1561
1562
1563
1564
1565
1566
1567
1568
1569
1570
1571
1572
1573
1574
1575
1576
1577
1578
1579
1580
1581
1582
1583
1584
1585
1586
1587
1588
1589
1590
1591
1592
1593
1594
1595
1596
1597
1598
1599
1600
1601
1602
1603
1604
1605
1606
1607
1608
1609
1610
1611
1612
1613
1614
1615
1616
1617
1618
1619
1620
1621
1622
1623
1624
1625
1626
1627
1628
1629
1630
1631
1632
1633
1634
1635
1636
1637
1638
1639
1640
1641
1642
1643
1644
1645
1646
1647
1648
1649
1650
1651
1652
1653
1654
1655
1656
1657
1658
1659
1660
1661
1662
1663
1664
1665
1666
1667
1668
1669
1670
1671
1672
1673
1674
1675
1676
1677
1678
1679
1680
1681
1682
1683
1684
1685
1686
1687
1688
1689
1690
1691
1692
1693
1694
1695
1696
1697
1698
1699
1700
1701
1702
1703
1704
1705
1706
1707
1708
1709
1710
1711
1712
1713
1714
1715
1716
1717
1718
1719
1720
1721
1722
1723
1724
1725
1726
1727
1728
1729
1730
1731
1732
1733
1734
1735
1736
1737
1738
1739
1740
1741
1742
1743
1744
1745
1746
1747
1748
1749
1750
1751
1752
1753
1754
1755
1756
1757
1758
1759
1760
1761
1762
1763
1764
1765
1766
1767
1768
1769
1770
1771
1772
1773
1774
1775
1776
1777
1778
1779
1780
1781
1782
1783
1784
1785
1786
1787
1788
1789
1790
1791
1792
1793
1794
1795
1796
1797
1798
1799
1800
1801
1802
1803
1804
1805
1806
1807
1808
1809
1810
1811
1812
1813
1814
1815
1816
1817
1818
1819
1820
1821
1822
1823
1824
1825
1826
1827
1828
1829
1830
1831
1832
1833
1834
1835
1836
1837
1838
1839
1840
1841
1842
1843
1844
1845
1846
1847
1848
1849
1850
1851
1852
1853
1854
1855
1856
1857
1858
1859
1860
1861
1862
1863
1864
1865
1866
1867
1868
1869
1870
1871
1872
1873
1874
1875
1876
1877
1878
1879
1880
1881
1882
1883
1884
1885
1886
1887
1888
1889
1890
1891
1892
1893
1894
1895
1896
1897
1898
1899
1900
1901
1902
1903
1904
1905
1906
1907
1908
1909
1910
1911
1912
1913
1914
1915
1916
1917
1918
1919
1920
1921
1922
1923
1924
1925
1926
1927
1928
1929
1930
1931
1932
1933
1934
1935
1936
1937
1938
1939
1940
1941
1942
1943
1944
1945
1946
1947
1948
1949
1950
1951
1952
1953
1954
1955
1956
1957
1958
1959
1960
1961
1962
1963
1964
1965
1966
1967
1968
1969
1970
1971
1972
1973
1974
1975
1976
1977
1978
1979
1980
1981
1982
1983
1984
1985
1986
1987
1988
1989
1990
1991
1992
1993
1994
1995
1996
1997
1998
1999
2000
2001
2002
2003
2004
2005
2006
2007
2008
2009
2010
2011
2012
2013
2014
2015
2016
2017
2018
2019
2020
2021
2022
2023
2024
2025
2026
2027
2028
2029
2030
2031
2032
2033
2034
2035
2036
2037
2038
2039
2040
2041
2042
2043
2044
2045
2046
2047
2048
2049
2050
2051
2052
2053
2054
2055
2056
2057
2058
2059
2060
2061
2062
2063
2064
2065
2066
2067
2068
2069
2070
2071
2072
2073
2074
2075
2076
2077
2078
2079
2080
2081
2082
2083
2084
2085
2086
2087
2088
2089
2090
2091
2092
2093
2094
2095
2096
2097
2098
2099
2100
2101
2102
2103
2104
2105
2106
2107
2108
2109
2110
2111
2112
2113
2114
2115
2116
2117
2118
2119
2120
2121
2122
2123
2124
2125
2126
2127
2128
2129
2130
2131
2132
2133
2134
2135
2136
2137
2138
2139
2140
2141
2142
2143
2144
2145
2146
2147
2148
2149
2150
2151
2152
2153
2154
2155
2156
2157
2158
2159
2160
2161
2162
2163
2164
2165
2166
2167
2168
2169
2170
2171
2172
2173
2174
2175
2176
2177
2178
2179
2180
2181
2182
2183
2184
2185
2186
2187
2188
2189
2190
2191
2192
2193
2194
2195
2196
2197
2198
2199
2200
2201
2202
2203
2204
2205
2206
2207
2208
2209
2210
2211
2212
2213
2214
2215
2216
2217
2218
2219
2220
2221
2222
2223
2224
2225
2226
2227
2228
2229
2230
2231
2232
2233
2234
2235
2236
2237
2238
2239
2240
2241
2242
2243
2244
2245
2246
2247
2248
2249
2250
2251
2252
2253
2254
2255
2256
2257
2258
2259
2260
2261
2262
2263
2264
2265
2266
2267
2268
2269
2270
2271
2272
2273
2274
2275
2276
2277
2278
2279
2280
2281
2282
2283
2284
2285
2286
2287
2288
2289
2290
2291
2292
2293
2294
2295
2296
2297
2298
2299
2300
2301
2302
2303
2304
2305
2306
2307
2308
2309
2310
2311
2312
2313
2314
2315
2316
2317
2318
2319
2320
2321
2322
2323
2324
2325
2326
2327
2328
2329
2330
2331
2332
2333
2334
2335
2336
2337
2338
2339
2340
2341
2342
2343
2344
2345
2346
2347
2348
2349
2350
2351
2352
2353
2354
2355
2356
2357
2358
2359
2360
2361
2362
2363
2364
2365
2366
2367
2368
2369
2370
2371
2372
2373
2374
2375
2376
2377
2378
2379
2380
2381
2382
2383
2384
2385
2386
2387
2388
2389
2390
2391
2392
2393
2394
2395
2396
2397
2398
2399
2400
2401
2402
2403
2404
2405
2406
2407
2408
2409
2410
2411
2412
2413
2414
2415
2416
2417
2418
2419
2420
2421
2422
2423
2424
2425
2426
2427
2428
2429
2430
2431
2432
2433
2434
2435
2436
2437
2438
2439
2440
2441
2442
2443
2444
2445
2446
2447
2448
2449
2450
2451
2452
2453
2454
2455
2456
2457
2458
2459
2460
2461
2462
2463
2464
2465
2466
2467
2468
2469
2470
2471
2472
2473
2474
2475
2476
2477
2478
2479
2480
2481
2482
2483
2484
2485
2486
2487
2488
2489
2490
2491
2492
2493
2494
2495
2496
2497
2498
2499
2500
2501
2502
2503
2504
2505
2506
2507
2508
2509
2510
2511
2512
2513
2514
2515
2516
2517
2518
2519
2520
2521
2522
2523
2524
2525
2526
2527
2528
2529
2530
2531
2532
2533
2534
2535
2536
2537
2538
2539
2540
2541
2542
2543
2544
2545
2546
2547
2548
2549
2550
2551
2552
2553
2554
2555
2556
2557
2558
2559
2560
2561
2562
2563
2564
2565
2566
2567
2568
2569
2570
2571
2572
2573
2574
2575
2576
2577
2578
2579
2580
2581
2582
2583
2584
2585
2586
2587
2588
2589
2590
2591
2592
2593
2594
2595
2596
2597
2598
2599
2600
2601
2602
2603
2604
2605
2606
2607
2608
2609
2610
2611
2612
2613
2614
2615
2616
2617
2618
2619
2620
2621
2622
2623
2624
2625
2626
2627
2628
2629
2630
2631
2632
2633
2634
2635
2636
2637
2638
2639
2640
2641
2642
2643
2644
2645
2646
2647
2648
2649
2650
2651
2652
2653
2654
2655
2656
2657
2658
2659
2660
2661
2662
2663
2664
2665
2666
2667
2668
2669
2670
2671
2672
2673
2674
2675
2676
2677
2678
2679
2680
2681
2682
2683
2684
2685
2686
2687
2688
2689
2690
2691
2692
2693
2694
2695
2696
2697
2698
2699
2700
2701
2702
2703
2704
2705
2706
2707
2708
2709
2710
2711
2712
2713
2714
2715
2716
2717
2718
2719
2720
2721
2722
2723
2724
2725
2726
2727
2728
2729
2730
2731
2732
2733
2734
2735
2736
2737
2738
2739
2740
2741
2742
2743
2744
2745
2746
2747
2748
2749
2750
2751
2752
2753
2754
2755
2756
2757
2758
2759
2760
2761
2762
2763
2764
2765
2766
2767
2768
2769
2770
2771
2772
2773
2774
2775
2776
2777
2778
2779
2780
2781
2782
2783
2784
2785
2786
2787
2788
2789
2790
2791
2792
2793
2794
2795
2796
2797
2798
2799
2800
2801
2802
2803
2804
2805
2806
2807
2808
2809
2810
2811
2812
2813
2814
2815
2816
2817
2818
2819
2820
2821
2822
2823
2824
2825
2826
2827
2828
2829
2830
2831
2832
2833
2834
2835
2836
2837
2838
2839
2840
2841
2842
2843
2844
2845
2846
2847
2848
2849
2850
2851
2852
2853
2854
2855
2856
2857
2858
2859
2860
2861
2862
2863
2864
2865
2866
2867
2868
2869
2870
2871
2872
2873
2874
2875
2876
2877
2878
2879
2880
2881
2882
2883
2884
2885
2886
2887
2888
2889
2890
2891
2892
2893
2894
2895
2896
2897
2898
2899
2900
2901
2902
2903
2904
2905
2906
2907
2908
2909
2910
2911
2912
2913
2914
2915
2916
2917
2918
2919
2920
2921
2922
2923
2924
2925
2926
2927
2928
2929
2930
2931
2932
2933
2934
2935
2936
2937
2938
2939
2940
2941
2942
2943
2944
2945
2946
2947
2948
2949
2950
2951
2952
2953
2954
2955
2956
2957
2958
2959
2960
2961
2962
2963
2964
2965
2966
2967
2968
2969
2970
2971
2972
2973
2974
2975
2976
2977
2978
2979
2980
2981
2982
2983
2984
2985
2986
2987
2988
2989
2990
2991
2992
2993
2994
2995
2996
2997
2998
2999
3000
3001
3002
3003
3004
3005
3006
3007
3008
3009
3010
3011
3012
3013
3014
3015
3016
3017
3018
3019
3020
3021
3022
3023
3024
3025
3026
3027
3028
3029
3030
3031
3032
3033
3034
3035
3036
3037
3038
3039
3040
3041
3042
3043
3044
3045
3046
3047
3048
3049
3050
3051
3052
3053
3054
3055
3056
3057
3058
3059
3060
3061
3062
3063
3064
3065
3066
3067
3068
3069
3070
3071
3072
3073
3074
3075
3076
3077
3078
3079
3080
3081
3082
3083
3084
3085
3086
3087
3088
3089
3090
3091
3092
3093
3094
3095
3096
3097
3098
3099
3100
3101
3102
3103
3104
3105
3106
3107
3108
3109
3110
3111
3112
3113
3114
3115
3116
3117
3118
3119
3120
3121
3122
3123
3124
3125
3126
3127
3128
3129
3130
3131
3132
3133
3134
3135
3136
3137
3138
3139
3140
3141
3142
3143
3144
3145
3146
3147
3148
3149
3150
3151
3152
3153
3154
3155
3156
3157
3158
3159
3160
3161
3162
3163
3164
3165
3166
3167
3168
3169
3170
3171
3172
3173
3174
3175
3176
3177
3178
3179
3180
3181
3182
3183
3184
//! A mechanism to specify policy.
//!
//! A major goal of the Sequoia OpenPGP crate is to be policy free.
//! However, many mid-level operations build on low-level primitives.
//! For instance, finding a certificate's primary User ID means
//! examining each of its User IDs and their current self-signature.
//! Some algorithms are considered broken (e.g., MD5) and some are
//! considered weak (e.g. SHA-1).  When dealing with data from an
//! untrusted source, for instance, callers will often prefer to
//! ignore signatures that rely on these algorithms even though [RFC
//! 4880] says that "\[i\]mplementations MUST implement SHA-1."  When
//! trying to decrypt old archives, however, users probably don't want
//! to ignore keys using MD5, even though [RFC 4880] deprecates MD5.
//!
//! Rather than not provide this mid-level functionality, the `Policy`
//! trait allows callers to specify their preferred policy.  This can be
//! highly customized by providing a custom implementation of the
//! `Policy` trait, or it can be slightly refined by tweaking the
//! `StandardPolicy`'s parameters.
//!
//! When implementing the `Policy` trait, it is *essential* that the
//! functions are [pure].  That is, if the same `Policy` is used
//! to determine whether a given `Signature` is valid, it must always
//! return the same value.
//!
//! [RFC 4880]: https://tools.ietf.org/html/rfc4880#section-9.4
//! [pure]: https://en.wikipedia.org/wiki/Pure_function
use std::fmt;
use std::time::{SystemTime, Duration};
use std::u32;

use anyhow::Context;

use crate::{
    cert::prelude::*,
    Error,
    Packet,
    packet::{
        key,
        Signature,
        signature::subpacket::{
            SubpacketTag,
            SubpacketValue,
        },
        Tag,
    },
    Result,
    types,
    types::{
        AEADAlgorithm,
        HashAlgorithm,
        SignatureType,
        SymmetricAlgorithm,
        Timestamp,
    },
};

#[macro_use] mod cutofflist;
use cutofflist::{
    CutoffList,
    REJECT,
    ACCEPT,
    VersionedCutoffList,
};

/// A policy for cryptographic operations.
pub trait Policy : fmt::Debug + Send + Sync {
    /// Returns an error if the signature violates the policy.
    ///
    /// This function performs the last check before the library
    /// decides that a signature is valid.  That is, after the library
    /// has determined that the signature is well-formed, alive, not
    /// revoked, etc., it calls this function to allow you to
    /// implement any additional policy.  For instance, you may reject
    /// signatures that make use of cryptographically insecure
    /// algorithms like SHA-1.
    ///
    /// Note: Whereas it is generally better to reject suspicious
    /// signatures, one should be more liberal when considering
    /// revocations: if you reject a revocation certificate, it may
    /// inadvertently make something else valid!
    fn signature(&self, _sig: &Signature, _sec: HashAlgoSecurity) -> Result<()> {
        Err(Error::PolicyViolation(
            "By default all signatures are rejected.".into(), None).into())
    }

    /// Returns an error if the key violates the policy.
    ///
    /// This function performs one of the last checks before a
    /// `KeyAmalgamation` or a related data structures is turned into
    /// a `ValidKeyAmalgamation`, or similar.
    ///
    /// Internally, the library always does this before using a key.
    /// The sole exception is when creating a key using `CertBuilder`.
    /// In that case, the primary key is not validated before it is
    /// used to create any binding signatures.
    ///
    /// Thus, you can prevent keys that make use of insecure
    /// algorithms, don't have a sufficiently high security margin
    /// (e.g., 1024-bit RSA keys), are on a bad list, etc. from being
    /// used here.
    ///
    /// If you implement this function, make sure to consider the Key
    /// Derivation Function and Key Encapsulation parameters of ECDH
    /// keys, see [`PublicKey::ECDH`].
    ///
    /// [`PublicKey::ECDH`]: crate::crypto::mpi::PublicKey::ECDH
    fn key(&self, _ka: &ValidErasedKeyAmalgamation<key::PublicParts>)
        -> Result<()>
    {
        Err(Error::PolicyViolation(
            "By default all keys are rejected.".into(), None).into())
    }

    /// Returns an error if the symmetric encryption algorithm
    /// violates the policy.
    ///
    /// This function performs the last check before an encryption
    /// container is decrypted by the streaming decryptor.
    ///
    /// With this function, you can prevent the use of insecure
    /// symmetric encryption algorithms.
    fn symmetric_algorithm(&self, _algo: SymmetricAlgorithm) -> Result<()> {
        Err(Error::PolicyViolation(
            "By default all symmetric algorithms are rejected.".into(), None).into())
    }

    /// Returns an error if the AEAD mode violates the policy.
    ///
    /// This function performs the last check before an encryption
    /// container is decrypted by the streaming decryptor.
    ///
    /// With this function, you can prevent the use of insecure AEAD
    /// constructions.
    ///
    /// This feature is [experimental](super#experimental-features).
    fn aead_algorithm(&self, _algo: AEADAlgorithm) -> Result<()> {
        Err(Error::PolicyViolation(
            "By default all AEAD algorithms are rejected.".into(), None).into())
    }

    /// Returns an error if the packet violates the policy.
    ///
    /// This function performs the last check before a packet is
    /// considered by the streaming verifier and decryptor.
    ///
    /// With this function, you can prevent the use of insecure
    /// encryption containers, notably the *Symmetrically Encrypted
    /// Data Packet*.
    fn packet(&self, _packet: &Packet) -> Result<()> {
        Err(Error::PolicyViolation(
            "By default all packets are rejected.".into(), None).into())
    }
}

/// Whether the signed data requires a hash algorithm with collision
/// resistance.
///
/// Since the context of a signature is not passed to
/// `Policy::signature`, it is not possible to determine from that
/// function whether the signature requires a hash algorithm with
/// collision resistance.  This enum indicates this.
///
/// In short, many self signatures only require second pre-image
/// resistance.  This can be used to extend the life of hash
/// algorithms whose collision resistance has been partially
/// compromised.  Be careful.  Read the background and the warning
/// before accepting the use of weak hash algorithms!
///
/// # Warning
///
/// Although distinguishing whether signed data requires collision
/// resistance can be used to permit the continued use of a hash
/// algorithm in certain situations, once attacks against a hash
/// algorithm are known, it is imperative to retire the use of the
/// hash algorithm as soon as it is feasible.  Cryptoanalytic attacks
/// improve quickly, as demonstrated by the attacks on SHA-1.
///
/// # Background
///
/// Cryptographic hash functions normally have three security
/// properties:
///
///   - Pre-image resistance,
///   - Second pre-image resistance, and
///   - Collision resistance.
///
/// A hash algorithm has pre-image resistance if given a hash `h`, it
/// is impractical for an attacker to find a message `m` such that `h
/// = hash(m)`.  In other words, a hash algorithm has pre-image
/// resistance if it is hard to invert.  A hash algorithm has second
/// pre-image resistance if it is impractical for an attacker to find
/// a second message with the same hash as the first.  That is, given
/// `m1`, it is hard for an attacker to find an `m2` such that
/// `hash(m1) = hash(m2)`.  And, a hash algorithm has collision
/// resistance if it is impractical for an attacker to find two
/// messages with the same hash.  That is, it is hard for an attacker
/// to find an `m1` and an `m2` such that `hash(m1) = hash(m2)`.
///
/// In the context of verifying an OpenPGP signature, we don't need a
/// hash algorithm with pre-image resistance.  Pre-image resistance is
/// only required when the message is a secret, e.g., a password.  We
/// always need a hash algorithm with second pre-image resistance,
/// because an attacker must not be able to repurpose an arbitrary
/// signature, i.e., create a collision with respect to a *known*
/// hash.  And, we need collision resistance when a signature is over
/// data that could have been influenced by an attacker: if an
/// attacker creates a pair of colliding messages and convinces the
/// user to sign one of them, then the attacker can copy the signature
/// to the other message.
///
/// Collision resistance implies second pre-image resistance, but not
/// vice versa.  If an attacker can find a second message with the
/// same hash as some known message, they can also create a collision
/// by choosing an arbitrary message and using their pre-image attack
/// to find a colliding message.  Thus, a context that requires
/// collision resistance also requires second pre-image resistance.
///
/// Because collision resistance is with respect to two arbitrary
/// messages, collision resistance is always susceptible to a
/// [birthday paradox].  This means that the security margin of a hash
/// algorithm's collision resistance is half of the security margin of
/// its second pre-image resistance.  And, in practice, the collision
/// resistance of industry standard hash algorithms has been
/// practically attacked multiple times.  In the context of SHA-1,
/// Wang et al. described how to find collisions in SHA-1 in their
/// 2005 paper [Finding Collisions in the Full SHA-1].  In 2017,
/// Stevens et al. published [The First Collision for Full SHA-1],
/// which demonstrates the first practical attack on SHA-1's collision
/// resistance, an identical-prefix collision attack.  This attack
/// only gives the attacker limited control over the content of the
/// collided messages, which limits its applicability.  However, in
/// 2020, Leurent and Peyrin published [SHA-1 is a Shambles], which
/// demonstrates a practical chosen-prefix collision attack.  This
/// attack gives the attacker complete control over the prefixes of
/// the collided messages.
///
///   [birthday paradox]: https://en.wikipedia.org/wiki/Birthday_attack#Digital_signature_susceptibility
///   [Finding Collisions in the Full SHA-1]: https://link.springer.com/chapter/10.1007/11535218_2
///   [The first collision for full SHA-1]: https://shattered.io/
///   [SHA-1 is a Shambles]: https://sha-mbles.github.io/
///
/// A chosen-prefix collision attack works as follows: an attacker
/// chooses two arbitrary message prefixes, and then searches for
/// so-called near collision blocks.  These near collision blocks
/// cause the internal state of the hashes to converge and eventually
/// result in a collision, i.e., an identical hash value.  The attack
/// described in the [SHA-1 is a Shambles] paper requires 8 to 10 near
/// collision blocks (512 to 640 bytes) to fully synchronize the
/// internal state.
///
/// SHA-1 is a [Merkle-Damgård hash function].  This means that the
/// hash function processes blocks one after the other, and the
/// internal state of the hash function at any given point only
/// depends on earlier blocks in the stream.  A consequence of this is
/// that it is possible to append a common suffix to the collided
/// messages without any additional computational effort.  That is, if
/// `hash(m1) = hash(m2)`, then it necessarily holds that `hash(m1 ||
/// suffix) = hash(m2 || suffix)`.  This is called a [length extension
/// attack].
///
///   [Merkle-Damgård hash function]: https://en.wikipedia.org/wiki/Merkle%E2%80%93Damg%C3%A5rd_construction
///   [length extension attack]: https://en.wikipedia.org/wiki/Length_extension_attack
///
/// Thus, the [SHA-1 is a Shambles] attack solves the following:
///
/// ```text
/// hash(m1 || collision blocks 1 || suffix) = hash(m2 || collision blocks 2 || suffix)
/// ```
///
/// Where `m1`, `m2`, and `suffix` are controlled by the attacker, and
/// only the collision blocks are controlled by the algorithm.
///
/// If an attacker can convince an OpenPGP user to sign a message of
/// their choosing (some `m1 || collision blocks 1 || suffix`), then
/// the attacker also has a valid signature from the victim for a
/// colliding message (some `m2 || collision blocks 2 || suffix`).
///
/// The OpenPGP format imposes some additional constraints on the
/// attacker.  Although the attacker may control the message, the
/// signature is also over a [signature packet], and a trailer.
/// Specifically, [the following is signed] when signing a document:
///
/// ```text
/// hash(document || sig packet || 0x04 || sig packet len)
/// ```
///
/// and the [following is signed] when signing a binding signature:
///
/// ```text
/// hash(public key || subkey || sig packet || 0x04 || sig packet len)
/// ```
///
///  [signature packet]: https://tools.ietf.org/html/rfc4880#section-5.2.3
///  [the following is signed]: https://tools.ietf.org/html/rfc4880#section-5.2.4
///
/// Since the signature packet is chosen by the victim's OpenPGP
/// implementation, the attacker may be able to predict it, but they
/// cannot store the collision blocks there.  Thus, the signature
/// packet is necessarily part of the common suffix, and the collision
/// blocks must occur earlier in the stream.
///
/// This restriction on the signature packet means that an attacker
/// cannot convince the victim to sign a document, and then transfer
/// that signature to a colliding binding signature.  These signatures
/// necessarily have different [signature packet]s: the value of the
/// [signature type] field is different.  And, as just described, for
/// this attack, the signature packets must be identical, because they
/// are part of the common suffix.  Finally, the trailer, which
/// contains the signature packet's length, prevents hiding a
/// signature in a signature.
///
///   [signature type]: https://tools.ietf.org/html/rfc4880#section-5.2.1
///
/// Given this, if we know for a given signature type that an attacker
/// cannot control any of the data that is signed, then that type of
/// signature does not need collision resistance; it is still
/// vulnerable to an attack on the hash's second pre-image resistance
/// (a collision with a specific message), but not one on its
/// collision resistance (a collision with any message).  This is the
/// case for binding signatures, and direct key signatures.  But, it
/// is not normally the case for documents (the attacker may be able
/// to control the content of the document), certifications (the
/// attacker may be able to control the the key packet, the User ID
/// packet, or the User Attribute packet), or certificate revocations
/// (the attacker may be able to control the key packet).
///
/// Certification signatures and revocations signatures can be further
/// divided into self signatures and third-party signatures.  If an
/// attacker can convince a victim into signing a third-party
/// signature, as was done in the [SHA-1 is a Shambles], they may be
/// able to transfer the signature to a colliding self signature.  If
/// we can show that an attacker can't collide a self signature, and a
/// third-party signature, then we may be able to show that self
/// signatures don't require collision resistance.  The same
/// consideration holds for revocations and third-party revocations.
///
/// We first consider revocations, which are more straightforward.
/// The attack is the following: an attacker creates a fake
/// certificate (A), and sets the victim as a designated revoker.
/// They then ask the victim to revoke their certificate (V).  The
/// attacker than transfers the signature to a colliding self
/// revocation, which causes the victim's certificate (V) to be
/// revoked.
///
/// A revocation is over a public key packet and a signature packet.
/// In this scenario, the attacker controls the fake certificate (A)
/// and thus the public key packet that the victim actually signs.
/// But the victim's public key packet is determined by their
/// certificate (V).  Thus, the attacker would have to insert the near
/// collision blocks in the signature packet, which, as we argued
/// before, is not possible.  Thus, it is safe to only use a hash with
/// pre-image resistance to protect a self-revocation.
///
/// We now turn to self signatures.  The attack is similar to the
/// [SHA-1 is a Shambles] attack.  An attacker creates a certificate
/// (A) and convinces the victim to sign it.  The attacker can then
/// transfer the third-party certification to a colliding self
/// signature for the victim's certificate (V).  If successful, this
/// attack allows the attacker to add a User ID or a User Attribute to
/// the victim's certificate (V).  This can confuse people who use the
/// victim's certificate.  For instance, if the attacker adds the
/// identity `alice@example.org` to the victim's certificate, and Bob
/// receives a message signed using the victim's certificate (V), he
/// may think that Alice signed the message instead of the victim.
/// Bob won't be tricked if he uses strong authentication, but many
/// OpenPGP users use weak authentication (e.g., TOFU) or don't
/// authenticate keys at all.
///
/// A certification is over a public key packet, a User ID or User
/// Attribute packet, and a signature packet.  The attacker controls
/// the fake certificate (A) and therefore the public key packet, and
/// the User ID or User Attribute packet that the victim signs.
/// However, to trick the victim, the User ID packet or User Attribute
/// packet needs to correspond to an identity that the attacker
/// appears to control.  Thus, if the near collision blocks are stored
/// in the User ID or User Attribute packet of A, they have to be
/// hidden to avoid making the victim suspicious.  This is
/// straightforward for User Attributes, which are currently images,
/// and have many places to hide this type of data.  However, User IDs
/// are are normally [UTF-8 encoded RFC 2822 mailbox]es, which makes
/// hiding half a kilobyte of binary data impractical.  The attacker
/// does not control the victim's public key (in V).  But, they do
/// control the malicious User ID or User Attribute that they want to
/// attack to the victim's certificate (V).  But again, the near
/// collision blocks have to be hidden in order to trick Bob, the
/// second victim.  Thus, the attack has two possibilities: they can
/// hide the near collision blocks in the fake public key (in A), and
/// the User ID or User Attribute (added to V); or, they can hide them
/// in the fake User IDs or User Attributes (in A and the one added to
/// V).
///
/// As evidenced by the [SHA-1 is a Shambles] attack, it is possible
/// to hide near collision blocks in User Attribute packets.  Thus,
/// this attack can be used to transfer a third-party certification
/// over a User Attribute to a self signature over a User Attribute.
/// As such, self signatures over User Attributes need collision
/// resistance.
///
/// The final case to consider is hiding the near collision blocks in
/// the User ID that the attacker wants to add to the victim's
/// certificate.  Again, it is possible to store the near collision
/// blocks there.  However, there are two mitigating factors.  First,
/// there is no place to hide the blocks.  As such, the user must be
/// convinced to ignore them.  Second, a User ID is structure: it
/// normally contains a [UTF-8 encoded RFC 2822 mailbox].  Thus, if we
/// only consider valid UTF-8 strings, and limit the maximum size, we
/// can dramatically increase the workfactor, which can extend the life
/// of a hash algorithm whose collision resistance has been weakened.
///
///   [UTF-8 encoded RFC 2822 mailbox]: https://tools.ietf.org/html/rfc4880#section-5.11
#[derive(Debug, Clone, Copy, Eq, PartialEq)]
pub enum HashAlgoSecurity {
    /// The signed data only requires second pre-image resistance.
    ///
    /// If a signature is over data that an attacker cannot influence,
    /// then the hash function does not need to provide collision
    /// resistance.  This is **only** the case for:
    ///
    ///   - Subkey binding signatures
    ///   - Primary key binding signatures
    ///   - Self revocations
    ///
    /// Due to the structure of User IDs (they are normally short,
    /// UTF-8 encoded RFC 2822 mailboxes), self signatures over short,
    /// reasonable User IDs (**not** User Attributes) also don't
    /// require strong collision resistance.  Thus, we also only
    /// require a signature with second pre-image resistance for:
    ///
    ///   - Self signatures over reasonable User IDs
    SecondPreImageResistance,
    /// The signed data requires collision resistance.
    ///
    /// If a signature is over data that an attacker can influence,
    /// then the hash function must provide collision resistance.
    /// This is the case for documents, third-party certifications,
    /// and third-party revocations.
    ///
    /// Note: collision resistance implies second pre-image
    /// resistance.  Thus, when evaluating whether a hash algorithm
    /// has collision resistance, we also check whether it has second
    /// pre-image resistance.
    CollisionResistance,
}

impl Default for HashAlgoSecurity {
    /// The default is the most conservative policy.
    fn default() -> Self {
        HashAlgoSecurity::CollisionResistance
    }
}

/// The standard policy.
///
/// The standard policy stores when each algorithm in a family of
/// algorithms is no longer considered safe.  Attempts to use an
/// algorithm after its cutoff time should fail.
///
/// A `StandardPolicy` can be configured using Rust.  Sometimes it is
/// useful to configure it via a configuration file.  This can be done
/// using the [`sequoia-policy-config`] crate.
///
///   [`sequoia-policy-config`]: https://docs.rs/sequoia-policy-config/latest/sequoia_policy_config/
///
/// It is recommended to support using a configuration file when the
/// program should respect the system's crypto policy.  This is
/// required on Fedora, for instance.  See the [Fedora Crypto
/// Policies] project for more information.
///
///   [Fedora]: https://gitlab.com/redhat-crypto/fedora-crypto-policies
///
/// When validating a signature, we normally want to know whether the
/// algorithms used are safe *now*.  That is, we don't use the
/// signature's alleged creation time when considering whether an
/// algorithm is safe, because if an algorithm is discovered to be
/// compromised at time X, then an attacker could forge a message
/// after time X with a signature creation time that is prior to X,
/// which would be incorrectly accepted.
///
/// Occasionally, we know that a signature has not been tampered with
/// since some time in the past.  We might know this if the signature
/// was stored on some tamper-proof medium.  In those cases, it is
/// reasonable to use the time that the signature was saved, since an
/// attacker could not have taken advantage of any weaknesses found
/// after that time.
///
/// # Examples
///
/// A `StandardPolicy` object can be used to build specialized policies.
/// For example the following policy filters out Persona certifications mimicking
/// what GnuPG does when calculating the Web of Trust.
///
/// ```rust
/// use sequoia_openpgp as openpgp;
/// use std::io::{Cursor, Read};
/// use openpgp::Result;
/// use openpgp::packet::{Packet, Signature, key::PublicParts};
/// use openpgp::cert::prelude::*;
/// use openpgp::parse::Parse;
/// use openpgp::armor::{Reader, ReaderMode, Kind};
/// use openpgp::policy::{HashAlgoSecurity, Policy, StandardPolicy};
/// use openpgp::types::{
///    SymmetricAlgorithm,
///    AEADAlgorithm,
///    SignatureType
/// };
///
/// #[derive(Debug)]
/// struct RejectPersonaCertificationsPolicy<'a>(StandardPolicy<'a>);
///
/// impl Policy for RejectPersonaCertificationsPolicy<'_> {
///     fn key(&self, ka: &ValidErasedKeyAmalgamation<PublicParts>)
///            -> Result<()>
///     {
///         self.0.key(ka)
///     }
///
///     fn signature(&self, sig: &Signature, sec: HashAlgoSecurity) -> Result<()> {
///         if sig.typ() == SignatureType::PersonaCertification {
///             Err(anyhow::anyhow!("Persona certifications are ignored."))
///         } else {
///             self.0.signature(sig, sec)
///         }
///     }
///
///     fn symmetric_algorithm(&self, algo: SymmetricAlgorithm) -> Result<()> {
///         self.0.symmetric_algorithm(algo)
///     }
///
///     fn aead_algorithm(&self, algo: AEADAlgorithm) -> Result<()> {
///         self.0.aead_algorithm(algo)
///     }
///
///     fn packet(&self, packet: &Packet) -> Result<()> {
///         self.0.packet(packet)
///     }
/// }
///
/// impl RejectPersonaCertificationsPolicy<'_> {
///     fn new() -> Self {
///         Self(StandardPolicy::new())
///     }
/// }
///
/// # fn main() -> Result<()> {
/// // this key has one persona certification
/// let data = r#"
/// -----BEGIN PGP PUBLIC KEY BLOCK-----
///
/// mDMEX7JGrxYJKwYBBAHaRw8BAQdASKGcnowaZBDc2Z3rZZlWb6jEjne9sK76afbJ
/// trd5Uw+0BlRlc3QgMoiQBBMWCAA4FiEEyZ6oBYFia3z+ooCBqR9BqiGp8AQFAl+y
/// Rq8CGwMFCwkIBwIGFQoJCAsCBBYCAwECHgECF4AACgkQqR9BqiGp8ASfxwEAvEb0
/// bFr7ZgFZSDOITNptm+FEynib8mmLACsvHAmCjvIA+gOaSNyxMW6N59q7/j0sDjp1
/// aYNgpNFLbYBZpkXXVL0GiHUEERYIAB0WIQTE4QfdkkisIbWVOcHmlsuS3dbWEwUC
/// X7JG4gAKCRDmlsuS3dbWExEwAQCpqfiVMhjDwVFMsMpwd5r0N/8rAx8/nmgpCsK3
/// M9TUrAD7BhTYVPRbkJqTZYd9DlLtBcbF3yNPTHlB+F2sFjI+cgo=
/// =ZfYu
/// -----END PGP PUBLIC KEY BLOCK-----
/// "#;
///
/// let mut cursor = Cursor::new(&data);
/// let mut reader = Reader::from_reader(&mut cursor, ReaderMode::Tolerant(Some(Kind::PublicKey)));
///
/// let mut buf = Vec::new();
/// reader.read_to_end(&mut buf)?;
/// let cert = Cert::from_bytes(&buf)?;
///
/// let ref sp = StandardPolicy::new();
/// let u = cert.with_policy(sp, None)?.userids().nth(0).unwrap();
///
/// // Under the standard policy the persona certification is visible.
/// assert_eq!(u.certifications().count(), 1);
///
/// // Under our custom policy the persona certification is not available.
/// let ref p = RejectPersonaCertificationsPolicy::new();
/// assert_eq!(u.with_policy(p, None)?.certifications().count(), 0);
/// #
/// # Ok(())
/// # }
/// ```
#[derive(Clone, Debug)]
pub struct StandardPolicy<'a> {
    // The time.  If None, the current time is used.
    time: Option<Timestamp>,

    // Hash algorithms.
    collision_resistant_hash_algos:
        CollisionResistantHashCutoffList,
    second_pre_image_resistant_hash_algos:
        SecondPreImageResistantHashCutoffList,
    hash_revocation_tolerance: types::Duration,

    // Critical subpacket tags.
    critical_subpackets: SubpacketTagCutoffList,

    // Critical notation good-list.
    good_critical_notations: &'a [&'a str],

    // Packet types.
    packet_tags: PacketTagCutoffList,

    // Symmetric algorithms.
    symmetric_algos: SymmetricAlgorithmCutoffList,

    // AEAD algorithms.
    aead_algos: AEADAlgorithmCutoffList,

    // Asymmetric algorithms.
    asymmetric_algos: AsymmetricAlgorithmCutoffList,
}

assert_send_and_sync!(StandardPolicy<'_>);

impl<'a> Default for StandardPolicy<'a> {
    fn default() -> Self {
        Self::new()
    }
}

impl<'a> From<&'a StandardPolicy<'a>> for Option<&'a dyn Policy> {
    fn from(p: &'a StandardPolicy<'a>) -> Self {
        Some(p as &dyn Policy)
    }
}

// Signatures that require a hash with collision Resistance and second
// Pre-image Resistance.  See the documentation for HashAlgoSecurity
// for more details.
a_cutoff_list!(CollisionResistantHashCutoffList, HashAlgorithm, 12,
               [
                   REJECT,                   // 0. Not assigned.
                   Some(Timestamp::Y1997M2), // 1. MD5
                   Some(Timestamp::Y2013M2), // 2. SHA-1
                   Some(Timestamp::Y2013M2), // 3. RIPE-MD/160
                   REJECT,                   // 4. Reserved.
                   REJECT,                   // 5. Reserved.
                   REJECT,                   // 6. Reserved.
                   REJECT,                   // 7. Reserved.
                   ACCEPT,                   // 8. SHA256
                   ACCEPT,                   // 9. SHA384
                   ACCEPT,                   // 10. SHA512
                   ACCEPT,                   // 11. SHA224
               ]);
// Signatures that *only* require a hash with Second Pre-image
// Resistance.  See the documentation for HashAlgoSecurity for more
// details.
a_cutoff_list!(SecondPreImageResistantHashCutoffList, HashAlgorithm, 12,
               [
                   REJECT,                   // 0. Not assigned.
                   Some(Timestamp::Y2004M2), // 1. MD5
                   Some(Timestamp::Y2023M2), // 2. SHA-1
                   Some(Timestamp::Y2013M2), // 3. RIPE-MD/160
                   REJECT,                   // 4. Reserved.
                   REJECT,                   // 5. Reserved.
                   REJECT,                   // 6. Reserved.
                   REJECT,                   // 7. Reserved.
                   ACCEPT,                   // 8. SHA256
                   ACCEPT,                   // 9. SHA384
                   ACCEPT,                   // 10. SHA512
                   ACCEPT,                   // 11. SHA224
               ]);

a_cutoff_list!(SubpacketTagCutoffList, SubpacketTag, 38,
               [
                   REJECT,                 // 0. Reserved.
                   REJECT,                 // 1. Reserved.
                   ACCEPT,                 // 2. SignatureCreationTime.
                   ACCEPT,                 // 3. SignatureExpirationTime.
                   ACCEPT,                 // 4. ExportableCertification.
                   ACCEPT,                 // 5. TrustSignature.
                   ACCEPT,                 // 6. RegularExpression.
                   // Note: Even though we don't explicitly honor the
                   // Revocable flag, we don't support signature
                   // revocations, hence it is safe to ACCEPT it.
                   ACCEPT,                 // 7. Revocable.
                   REJECT,                 // 8. Reserved.
                   ACCEPT,                 // 9. KeyExpirationTime.
                   REJECT,                 // 10. PlaceholderForBackwardCompatibility.
                   ACCEPT,                 // 11. PreferredSymmetricAlgorithms.
                   ACCEPT,                 // 12. RevocationKey.
                   REJECT,                 // 13. Reserved.
                   REJECT,                 // 14. Reserved.
                   REJECT,                 // 15. Reserved.
                   ACCEPT,                 // 16. Issuer.
                   REJECT,                 // 17. Reserved.
                   REJECT,                 // 18. Reserved.
                   REJECT,                 // 19. Reserved.
                   ACCEPT,                 // 20. NotationData.
                   ACCEPT,                 // 21. PreferredHashAlgorithms.
                   ACCEPT,                 // 22. PreferredCompressionAlgorithms.
                   ACCEPT,                 // 23. KeyServerPreferences.
                   ACCEPT,                 // 24. PreferredKeyServer.
                   ACCEPT,                 // 25. PrimaryUserID.
                   ACCEPT,                 // 26. PolicyURI.
                   ACCEPT,                 // 27. KeyFlags.
                   ACCEPT,                 // 28. SignersUserID.
                   ACCEPT,                 // 29. ReasonForRevocation.
                   ACCEPT,                 // 30. Features.
                   REJECT,                 // 31. SignatureTarget.
                   ACCEPT,                 // 32. EmbeddedSignature.
                   ACCEPT,                 // 33. IssuerFingerprint.
                   ACCEPT,                 // 34. PreferredAEADAlgorithms.
                   ACCEPT,                 // 35. IntendedRecipient.
                   REJECT,                 // 36. Reserved.
                   ACCEPT,                 // 37. AttestedCertifications.
               ]);

a_cutoff_list!(AsymmetricAlgorithmCutoffList, AsymmetricAlgorithm, 19,
               [
                   Some(Timestamp::Y2014M2), // 0. RSA1024.
                   ACCEPT,                   // 1. RSA2048.
                   ACCEPT,                   // 2. RSA3072.
                   ACCEPT,                   // 3. RSA4096.
                   Some(Timestamp::Y2014M2), // 4. ElGamal1024.
                   ACCEPT,                   // 5. ElGamal2048.
                   ACCEPT,                   // 6. ElGamal3072.
                   ACCEPT,                   // 7. ElGamal4096.
                   Some(Timestamp::Y2014M2), // 8. DSA1024.
                   ACCEPT,                   // 9. DSA2048.
                   ACCEPT,                   // 10. DSA3072.
                   ACCEPT,                   // 11. DSA4096.
                   ACCEPT,                   // 12. NistP256.
                   ACCEPT,                   // 13. NistP384.
                   ACCEPT,                   // 14. NistP521.
                   ACCEPT,                   // 15. BrainpoolP256.
                   ACCEPT,                   // 16. BrainpoolP512.
                   ACCEPT,                   // 17. Cv25519.
                   ACCEPT,                   // 16. BrainpoolP384.
               ]);

a_cutoff_list!(SymmetricAlgorithmCutoffList, SymmetricAlgorithm, 14,
               [
                   REJECT,                   // 0. Unencrypted.
                   ACCEPT,                   // 1. IDEA.
                   Some(Timestamp::Y2017M2), // 2. TripleDES.
                   ACCEPT,                   // 3. CAST5.
                   ACCEPT,                   // 4. Blowfish.
                   REJECT,                   // 5. Reserved.
                   REJECT,                   // 6. Reserved.
                   ACCEPT,                   // 7. AES128.
                   ACCEPT,                   // 8. AES192.
                   ACCEPT,                   // 9. AES256.
                   ACCEPT,                   // 10. Twofish.
                   ACCEPT,                   // 11. Camellia128.
                   ACCEPT,                   // 12. Camellia192.
                   ACCEPT,                   // 13. Camellia256.
               ]);

a_cutoff_list!(AEADAlgorithmCutoffList, AEADAlgorithm, 4,
               [
                   REJECT,                 // 0. Reserved.
                   ACCEPT,                 // 1. EAX.
                   ACCEPT,                 // 2. OCB.
                   ACCEPT,                 // 3. GCM.
               ]);

a_versioned_cutoff_list!(PacketTagCutoffList, Tag, 21,
    [
        REJECT,                   // 0. Reserved.
        ACCEPT,                   // 1. PKESK.
        ACCEPT,                   // 2. Signature.
        ACCEPT,                   // 3. SKESK.
        ACCEPT,                   // 4. OnePassSig.
        ACCEPT,                   // 5. SecretKey.
        ACCEPT,                   // 6. PublicKey.
        ACCEPT,                   // 7. SecretSubkey.
        ACCEPT,                   // 8. CompressedData.
        Some(Timestamp::Y2004M2), // 9. SED.
        ACCEPT,                   // 10. Marker.
        ACCEPT,                   // 11. Literal.
        ACCEPT,                   // 12. Trust.
        ACCEPT,                   // 13. UserID.
        ACCEPT,                   // 14. PublicSubkey.
        REJECT,                   // 15. Not assigned.
        REJECT,                   // 16. Not assigned.
        ACCEPT,                   // 17. UserAttribute.
        ACCEPT,                   // 18. SEIP.
        ACCEPT,                   // 19. MDC.
        REJECT,                   // 20. "v5" AED.
    ],
    // The versioned list overrides the unversioned list.  So we only
    // need to tweak the above.
    //
    // Note: this list must be sorted and the tag and version must be unique!
    2,
    [
        (Tag::Signature, 3, Some(Timestamp::Y2021M2)),
        (Tag::Signature, 5, REJECT), // "v5" Signatures.
    ]);

// We need to convert a `SystemTime` to a `Timestamp` in
// `StandardPolicy::reject_hash_at`.  Unfortunately, a `SystemTime`
// can represent a larger range of time than a `Timestamp` can.  Since
// the times passed to this function are cutoff points, and we only
// compare them to OpenPGP timestamps, any `SystemTime` that is prior
// to the Unix Epoch is equivalent to the Unix Epoch: it will reject
// all timestamps.  Similarly, any `SystemTime` that is later than the
// latest time representable by a `Timestamp` is equivalent to
// accepting all time stamps, which is equivalent to passing None.
fn system_time_cutoff_to_timestamp(t: SystemTime) -> Option<Timestamp> {
    let t = t
        .duration_since(SystemTime::UNIX_EPOCH)
        // An error can only occur if the SystemTime is less than the
        // reference time (SystemTime::UNIX_EPOCH).  Map that to
        // SystemTime::UNIX_EPOCH, as above.
        .unwrap_or_else(|_| Duration::new(0, 0));
    let t = t.as_secs();
    if t > u32::MAX as u64 {
        // Map to None, as above.
        None
    } else {
        Some((t as u32).into())
    }
}

impl<'a> StandardPolicy<'a> {
    /// Instantiates a new `StandardPolicy` with the default parameters.
    pub const fn new() -> Self {
        const EMPTY_LIST: &[&str] = &[];
        Self {
            time: None,
            collision_resistant_hash_algos:
                CollisionResistantHashCutoffList::Default(),
            second_pre_image_resistant_hash_algos:
                SecondPreImageResistantHashCutoffList::Default(),
            // There are 365.2425 days in a year.  Use a reasonable
            // approximation.
            hash_revocation_tolerance:
                types::Duration::seconds((7 * 365 + 2) * 24 * 60 * 60),
            critical_subpackets: SubpacketTagCutoffList::Default(),
            good_critical_notations: EMPTY_LIST,
            asymmetric_algos: AsymmetricAlgorithmCutoffList::Default(),
            symmetric_algos: SymmetricAlgorithmCutoffList::Default(),
            aead_algos: AEADAlgorithmCutoffList::Default(),
            packet_tags: PacketTagCutoffList::Default(),
        }
    }

    /// Instantiates a new `StandardPolicy` with parameters
    /// appropriate for `time`.
    ///
    /// `time` is a meta-parameter that selects a security profile
    /// that is appropriate for the given point in time.  When
    /// evaluating an object, the reference time should be set to the
    /// time that the object was stored to non-tamperable storage.
    /// Since most applications don't record when they received an
    /// object, they should conservatively use the current time.
    ///
    /// Note that the reference time is a security parameter and is
    /// different from the time that the object was allegedly created.
    /// Consider evaluating a signature whose `Signature Creation
    /// Time` subpacket indicates that it was created in 2007.  Since
    /// the subpacket is under the control of the sender, setting the
    /// reference time according to the subpacket means that the
    /// sender chooses the security profile.  If the sender were an
    /// attacker, she could have forged this to take advantage of
    /// security weaknesses found since 2007.  This is why the
    /// reference time must be set---at the earliest---to the time
    /// that the message was stored to non-tamperable storage.  When
    /// that is not available, the current time should be used.
    pub fn at<T>(time: T) -> Self
        where T: Into<SystemTime>,
    {
        let time = time.into();
        let mut p = Self::new();
        p.time = Some(system_time_cutoff_to_timestamp(time)
                          // Map "ACCEPT" to the end of time (None
                          // here means the current time).
                          .unwrap_or(Timestamp::MAX));
        p
    }

    /// Returns the policy's reference time.
    ///
    /// The current time is None.
    ///
    /// See [`StandardPolicy::at`] for details.
    ///
    /// [`StandardPolicy::at`]: StandardPolicy::at()
    pub fn time(&self) -> Option<SystemTime> {
        self.time.map(Into::into)
    }

    /// Always considers `h` to be secure.
    ///
    /// A cryptographic hash algorithm normally has three security
    /// properties:
    ///
    ///   - Pre-image resistance,
    ///   - Second pre-image resistance, and
    ///   - Collision resistance.
    ///
    /// A hash algorithm should only be unconditionally accepted if it
    /// has all three of these properties.  See the documentation for
    /// [`HashAlgoSecurity`] for more details.
    pub fn accept_hash(&mut self, h: HashAlgorithm) {
        self.accept_hash_property(h, HashAlgoSecurity::CollisionResistance);
        self.accept_hash_property(h, HashAlgoSecurity::SecondPreImageResistance);
    }

    /// Considers hash algorithm `h` to be secure for the specified
    /// security property `sec`.
    ///
    /// For instance, an application may choose to allow an algorithm
    /// like SHA-1 in contexts like User ID binding signatures where
    /// only [second preimage
    /// resistance][`HashAlgoSecurity::SecondPreImageResistance`] is
    /// required but not in contexts like signatures over data where
    /// [collision
    /// resistance][`HashAlgoSecurity::CollisionResistance`] is also
    /// required. Whereas SHA-1's collision resistance is
    /// [definitively broken](https://shattered.io/), depending on the
    /// application's threat model, it may be acceptable to continue
    /// to accept SHA-1 in these specific contexts.
    pub fn accept_hash_property(&mut self, h: HashAlgorithm, sec: HashAlgoSecurity)
    {
        self.reject_hash_property_at(h, sec, None);
    }

    /// Considers `h` to be insecure in all security contexts.
    ///
    /// A cryptographic hash algorithm normally has three security
    /// properties:
    ///
    ///   - Pre-image resistance,
    ///   - Second pre-image resistance, and
    ///   - Collision resistance.
    ///
    /// This method causes the hash algorithm to be considered unsafe
    /// in all security contexts.
    ///
    /// See the documentation for [`HashAlgoSecurity`] for more
    /// details.
    ///
    ///
    /// To express a more nuanced policy, use
    /// [`StandardPolicy::reject_hash_at`] or
    /// [`StandardPolicy::reject_hash_property_at`].
    ///
    ///   [`StandardPolicy::reject_hash_at`]: StandardPolicy::reject_hash_at()
    ///   [`StandardPolicy::reject_hash_property_at`]: StandardPolicy::reject_hash_property_at()
    pub fn reject_hash(&mut self, h: HashAlgorithm) {
        self.collision_resistant_hash_algos.set(h, REJECT);
        self.second_pre_image_resistant_hash_algos.set(h, REJECT);
    }

    /// Considers all hash algorithms to be insecure.
    ///
    /// Causes all hash algorithms to be considered insecure in all
    /// security contexts.
    ///
    /// This is useful when using a good list to determine what
    /// algorithms are allowed.
    pub fn reject_all_hashes(&mut self) {
        self.collision_resistant_hash_algos.reject_all();
        self.second_pre_image_resistant_hash_algos.reject_all();
    }

    /// Considers `h` to be insecure in all security contexts starting
    /// at time `t`.
    ///
    /// A cryptographic hash algorithm normally has three security
    /// properties:
    ///
    ///   - Pre-image resistance,
    ///   - Second pre-image resistance, and
    ///   - Collision resistance.
    ///
    /// This method causes the hash algorithm to be considered unsafe
    /// in all security contexts starting at time `t`.
    ///
    /// See the documentation for [`HashAlgoSecurity`] for more
    /// details.
    ///
    ///
    /// To express a more nuanced policy, use
    /// [`StandardPolicy::reject_hash_property_at`].
    ///
    ///   [`StandardPolicy::reject_hash_property_at`]: StandardPolicy::reject_hash_property_at()
    pub fn reject_hash_at<T>(&mut self, h: HashAlgorithm, t: T)
        where T: Into<Option<SystemTime>>,
    {
        let t = t.into().and_then(system_time_cutoff_to_timestamp);
        self.collision_resistant_hash_algos.set(h, t);
        self.second_pre_image_resistant_hash_algos.set(h, t);
    }

    /// Considers `h` to be insecure starting at `t` for the specified
    /// security property.
    ///
    /// A hash algorithm is considered secure if it has all of the
    /// following security properties:
    ///
    ///   - Pre-image resistance,
    ///   - Second pre-image resistance, and
    ///   - Collision resistance.
    ///
    /// Some contexts only require a subset of these security
    /// properties.  Specifically, if an attacker is unable to
    /// influence the data that a user signs, then the hash algorithm
    /// only needs second pre-image resistance; it doesn't need
    /// collision resistance.  See the documentation for
    /// [`HashAlgoSecurity`] for more details.
    ///
    ///
    /// This method makes it possible to specify different policies
    /// depending on the security requirements.
    ///
    /// A cutoff of `None` means that there is no cutoff and the
    /// algorithm has no known vulnerabilities for the specified
    /// security policy.
    ///
    /// As a rule of thumb, collision resistance is easier to attack
    /// than second pre-image resistance.  And in practice there are
    /// practical attacks against several widely-used hash algorithms'
    /// collision resistance, but only theoretical attacks against
    /// their second pre-image resistance.  Nevertheless, once one
    /// property of a hash has been compromised, we want to deprecate
    /// its use as soon as it is feasible.  Unfortunately, because
    /// OpenPGP certificates are long-lived, this can take years.
    ///
    /// Given this, we start rejecting [MD5] in cases where collision
    /// resistance is required in 1997 and completely reject it
    /// starting in 2004:
    ///
    /// >  In 1996, Dobbertin announced a collision of the
    /// >  compression function of MD5 (Dobbertin, 1996). While this
    /// >  was not an attack on the full MD5 hash function, it was
    /// >  close enough for cryptographers to recommend switching to
    /// >  a replacement, such as SHA-1 or RIPEMD-160.
    /// >
    /// >  MD5CRK ended shortly after 17 August 2004, when collisions
    /// >  for the full MD5 were announced by Xiaoyun Wang, Dengguo
    /// >  Feng, Xuejia Lai, and Hongbo Yu. Their analytical attack
    /// >  was reported to take only one hour on an IBM p690 cluster.
    /// >
    /// > (Accessed Feb. 2020.)
    ///
    ///   [MD5]: https://en.wikipedia.org/wiki/MD5
    ///
    /// And we start rejecting [SHA-1] in cases where collision
    /// resistance is required in 2013, and completely reject it in
    /// 2023:
    ///
    /// > Since 2005 SHA-1 has not been considered secure against
    /// > well-funded opponents, as of 2010 many organizations have
    /// > recommended its replacement. NIST formally deprecated use
    /// > of SHA-1 in 2011 and disallowed its use for digital
    /// > signatures in 2013. As of 2020, attacks against SHA-1 are
    /// > as practical as against MD5; as such, it is recommended to
    /// > remove SHA-1 from products as soon as possible and use
    /// > instead SHA-256 or SHA-3. Replacing SHA-1 is urgent where
    /// > it's used for signatures.
    /// >
    /// > (Accessed Feb. 2020.)
    ///
    ///   [SHA-1]: https://en.wikipedia.org/wiki/SHA-1
    ///
    /// There are two main reasons why we have decided to accept SHA-1
    /// for so long.  First, as of the end of 2020, there are still a
    /// large number of [certificates that rely on SHA-1].  Second,
    /// Sequoia uses a variant of SHA-1 called [SHA1CD], which is able
    /// to detect and *mitigate* the known attacks on SHA-1's
    /// collision resistance.
    ///
    ///   [certificates that rely on SHA-1]: https://gitlab.com/sequoia-pgp/sequoia/-/issues/595
    ///   [SHA1CD]: https://github.com/cr-marcstevens/sha1collisiondetection
    ///
    /// Since RIPE-MD is structured similarly to SHA-1, we
    /// conservatively consider it to be broken as well.  But, because
    /// it is not widely used in the OpenPGP ecosystem, we don't make
    /// provisions for it.
    ///
    /// Note: if a context indicates that it requires collision
    /// resistance, then it requires both collision resistance and
    /// second pre-image resistance, and both policies must indicate
    /// that the hash algorithm can be safely used at the specified
    /// time.
    pub fn reject_hash_property_at<T>(&mut self, h: HashAlgorithm,
                                      sec: HashAlgoSecurity, t: T)
        where T: Into<Option<SystemTime>>,
    {
        let t = t.into().and_then(system_time_cutoff_to_timestamp);
        match sec {
            HashAlgoSecurity::CollisionResistance =>
                self.collision_resistant_hash_algos.set(h, t),
            HashAlgoSecurity::SecondPreImageResistance =>
                self.second_pre_image_resistant_hash_algos.set(h, t),
        }
    }

    /// Returns the cutoff time for the specified hash algorithm and
    /// security policy.
    pub fn hash_cutoff(&self, h: HashAlgorithm, sec: HashAlgoSecurity)
        -> Option<SystemTime>
    {
        match sec {
            HashAlgoSecurity::CollisionResistance =>
                self.collision_resistant_hash_algos.cutoff(h),
            HashAlgoSecurity::SecondPreImageResistance =>
                self.second_pre_image_resistant_hash_algos.cutoff(h),
        }.map(|t| t.into())
    }

    /// Sets the amount of time to continue to accept revocation
    /// certificates after a hash algorithm should be rejected.
    ///
    /// Using [`StandardPolicy::reject_hash_at`], it is possible to
    /// indicate when a hash algorithm's security has been
    /// compromised, and, as such, should no longer be accepted.
    ///
    ///   [`StandardPolicy::reject_hash_at`]: StandardPolicy::reject_hash_at()
    ///
    /// Applying this policy to revocation certificates can have some
    /// unfortunate side effects.  In particular, if a certificate has
    /// been revoked using a revocation certificate that relies on a
    /// broken hash algorithm, but the most recent self signature uses
    /// a strong acceptable hash algorithm, then rejecting the
    /// revocation certificate would mean considering the certificate
    /// to not be revoked!  This would be a catastrophe if the secret
    /// key material were compromised.
    ///
    /// Unfortunately, this happens in practice.  A common example
    /// appears to be a certificate that has been updated many times,
    /// and is then revoked using a revocation certificate that was
    /// generated when the certificate was generated.
    ///
    /// Since the consequences of allowing an invalid revocation
    /// certificate are significantly less severe (a denial of
    /// service) than ignoring a valid revocation certificate
    /// (compromised confidentiality, integrity, and authentication),
    /// this option makes it possible to accept revocations using weak
    /// hash algorithms longer than other types of signatures.
    ///
    /// By default, the standard policy accepts revocation
    /// certificates seven years after the hash they are using was
    /// initially compromised.
    pub fn hash_revocation_tolerance<D>(&mut self, d: D)
        where D: Into<types::Duration>
    {
        self.hash_revocation_tolerance = d.into();
    }

    /// Sets the amount of time to continue to accept revocation
    /// certificates after a hash algorithm should be rejected.
    ///
    /// See [`StandardPolicy::hash_revocation_tolerance`] for details.
    ///
    ///   [`StandardPolicy::hash_revocation_tolerance`]: StandardPolicy::hash_revocation_tolerance()
    pub fn get_hash_revocation_tolerance(&self) -> types::Duration {
        self.hash_revocation_tolerance
    }

    /// Always considers `s` to be secure.
    pub fn accept_critical_subpacket(&mut self, s: SubpacketTag) {
        self.critical_subpackets.set(s, ACCEPT);
    }

    /// Always considers `s` to be insecure.
    pub fn reject_critical_subpacket(&mut self, s: SubpacketTag) {
        self.critical_subpackets.set(s, REJECT);
    }

    /// Considers all critical subpackets to be insecure.
    ///
    /// This is useful when using a good list to determine what
    /// critical subpackets are allowed.
    pub fn reject_all_critical_subpackets(&mut self) {
        self.critical_subpackets.reject_all();
    }

    /// Considers `s` to be insecure starting at `cutoff`.
    ///
    /// A cutoff of `None` means that there is no cutoff and the
    /// subpacket has no known vulnerabilities.
    ///
    /// By default, we accept all critical subpackets that Sequoia
    /// understands and honors.
    pub fn reject_critical_subpacket_at<C>(&mut self, s: SubpacketTag,
                                       cutoff: C)
        where C: Into<Option<SystemTime>>,
    {
        self.critical_subpackets.set(
            s,
            cutoff.into().and_then(system_time_cutoff_to_timestamp));
    }

    /// Returns the cutoff times for the specified subpacket tag.
    pub fn critical_subpacket_cutoff(&self, s: SubpacketTag)
                                 -> Option<SystemTime> {
        self.critical_subpackets.cutoff(s).map(|t| t.into())
    }

    /// Sets the list of accepted critical notations.
    ///
    /// By default, we reject all critical notations.
    pub fn good_critical_notations(&mut self, good_list: &'a [&'a str]) {
        self.good_critical_notations = good_list;
    }

    /// Always considers `s` to be secure.
    pub fn accept_asymmetric_algo(&mut self, a: AsymmetricAlgorithm) {
        self.asymmetric_algos.set(a, ACCEPT);
    }

    /// Always considers `s` to be insecure.
    pub fn reject_asymmetric_algo(&mut self, a: AsymmetricAlgorithm) {
        self.asymmetric_algos.set(a, REJECT);
    }

    /// Considers all asymmetric algorithms to be insecure.
    ///
    /// This is useful when using a good list to determine what
    /// algorithms are allowed.
    pub fn reject_all_asymmetric_algos(&mut self) {
        self.asymmetric_algos.reject_all();
    }

    /// Considers `a` to be insecure starting at `cutoff`.
    ///
    /// A cutoff of `None` means that there is no cutoff and the
    /// algorithm has no known vulnerabilities.
    ///
    /// By default, we reject the use of asymmetric key sizes lower
    /// than 2048 bits starting in 2014 following [NIST Special
    /// Publication 800-131A].
    ///
    ///   [NIST Special Publication 800-131A]: https://nvlpubs.nist.gov/nistpubs/SpecialPublications/NIST.SP.800-131Ar2.pdf
    pub fn reject_asymmetric_algo_at<C>(&mut self, a: AsymmetricAlgorithm,
                                       cutoff: C)
        where C: Into<Option<SystemTime>>,
    {
        self.asymmetric_algos.set(
            a,
            cutoff.into().and_then(system_time_cutoff_to_timestamp));
    }

    /// Returns the cutoff times for the specified hash algorithm.
    pub fn asymmetric_algo_cutoff(&self, a: AsymmetricAlgorithm)
                                 -> Option<SystemTime> {
        self.asymmetric_algos.cutoff(a).map(|t| t.into())
    }

    /// Always considers `s` to be secure.
    pub fn accept_symmetric_algo(&mut self, s: SymmetricAlgorithm) {
        self.symmetric_algos.set(s, ACCEPT);
    }

    /// Always considers `s` to be insecure.
    pub fn reject_symmetric_algo(&mut self, s: SymmetricAlgorithm) {
        self.symmetric_algos.set(s, REJECT);
    }

    /// Considers all symmetric algorithms to be insecure.
    ///
    /// This is useful when using a good list to determine what
    /// algorithms are allowed.
    pub fn reject_all_symmetric_algos(&mut self) {
        self.symmetric_algos.reject_all();
    }

    /// Considers `s` to be insecure starting at `cutoff`.
    ///
    /// A cutoff of `None` means that there is no cutoff and the
    /// algorithm has no known vulnerabilities.
    ///
    /// By default, we reject the use of TripleDES (3DES) starting in
    /// the year 2017.  While 3DES is still a ["MUST implement"]
    /// algorithm in RFC4880, released in 2007, there are plenty of
    /// other symmetric algorithms defined in RFC4880, and it says
    /// AES-128 SHOULD be implemented.  Support for other algorithms
    /// in OpenPGP implementations is [excellent].  We chose 2017 as
    /// the cutoff year because [NIST deprecated 3DES] that year.
    ///
    ///   ["MUST implement"]: https://tools.ietf.org/html/rfc4880#section-9.2
    ///   [excellent]: https://tests.sequoia-pgp.org/#Symmetric_Encryption_Algorithm_support
    ///   [NIST deprecated 3DES]: https://csrc.nist.gov/News/2017/Update-to-Current-Use-and-Deprecation-of-TDEA
    pub fn reject_symmetric_algo_at<C>(&mut self, s: SymmetricAlgorithm,
                                       cutoff: C)
        where C: Into<Option<SystemTime>>,
    {
        self.symmetric_algos.set(
            s,
            cutoff.into().and_then(system_time_cutoff_to_timestamp));
    }

    /// Returns the cutoff times for the specified hash algorithm.
    pub fn symmetric_algo_cutoff(&self, s: SymmetricAlgorithm)
                                 -> Option<SystemTime> {
        self.symmetric_algos.cutoff(s).map(|t| t.into())
    }

    /// Always considers `s` to be secure.
    ///
    /// This feature is [experimental](super#experimental-features).
    pub fn accept_aead_algo(&mut self, a: AEADAlgorithm) {
        self.aead_algos.set(a, ACCEPT);
    }

    /// Always considers `s` to be insecure.
    ///
    /// This feature is [experimental](super#experimental-features).
    pub fn reject_aead_algo(&mut self, a: AEADAlgorithm) {
        self.aead_algos.set(a, REJECT);
    }

    /// Considers all AEAD algorithms to be insecure.
    ///
    /// This is useful when using a good list to determine what
    /// algorithms are allowed.
    pub fn reject_all_aead_algos(&mut self) {
        self.aead_algos.reject_all();
    }

    /// Considers `a` to be insecure starting at `cutoff`.
    ///
    /// A cutoff of `None` means that there is no cutoff and the
    /// algorithm has no known vulnerabilities.
    ///
    /// By default, we accept all AEAD modes.
    ///
    /// This feature is [experimental](super#experimental-features).
    pub fn reject_aead_algo_at<C>(&mut self, a: AEADAlgorithm,
                                       cutoff: C)
        where C: Into<Option<SystemTime>>,
    {
        self.aead_algos.set(
            a,
            cutoff.into().and_then(system_time_cutoff_to_timestamp));
    }

    /// Returns the cutoff times for the specified hash algorithm.
    ///
    /// This feature is [experimental](super#experimental-features).
    pub fn aead_algo_cutoff(&self, a: AEADAlgorithm)
                                 -> Option<SystemTime> {
        self.aead_algos.cutoff(a).map(|t| t.into())
    }

    /// Always accept the specified version of the packet.
    ///
    /// If a packet does not have a version field, then its version is
    /// `0`.
    pub fn accept_packet_tag_version(&mut self, tag: Tag, version: u8) {
        self.packet_tags.set_versioned(tag, version, ACCEPT);
    }

    /// Always accept packets with the given tag independent of their
    /// version.
    ///
    /// If you previously set a cutoff for a specific version of a
    /// packet, this overrides that.
    pub fn accept_packet_tag(&mut self, tag: Tag) {
        self.packet_tags.set_unversioned(tag, ACCEPT);
    }

    /// Always reject the specified version of the packet.
    ///
    /// If a packet does not have a version field, then its version is
    /// `0`.
    pub fn reject_packet_tag_version(&mut self, tag: Tag, version: u8) {
        self.packet_tags.set_versioned(tag, version, REJECT);
    }

    /// Always reject packets with the given tag.
    pub fn reject_packet_tag(&mut self, tag: Tag) {
        self.packet_tags.set_unversioned(tag, REJECT);
    }

    /// Considers all packets to be insecure.
    ///
    /// This is useful when using a good list to determine what
    /// packets are allowed.
    pub fn reject_all_packet_tags(&mut self) {
        self.packet_tags.reject_all();
    }

    /// Start rejecting the specified version of packets with the
    /// given tag at `t`.
    ///
    /// A cutoff of `None` means that there is no cutoff and the
    /// packet has no known vulnerabilities.
    ///
    /// By default, we consider the *Symmetrically Encrypted Data
    /// Packet* (SED) insecure in messages created in the year 2004 or
    /// later.  The rationale here is that *Symmetrically Encrypted
    /// Integrity Protected Data Packet* (SEIP) can be downgraded to
    /// SED packets, enabling attacks exploiting the malleability of
    /// the CFB stream (see [EFAIL]).
    ///
    ///   [EFAIL]: https://en.wikipedia.org/wiki/EFAIL
    ///
    /// We chose 2004 as a cutoff-date because [Debian 3.0] (Woody),
    /// released on 2002-07-19, was the first release of Debian to
    /// ship a version of GnuPG that emitted SEIP packets by default.
    /// The first version that emitted SEIP packets was [GnuPG 1.0.3],
    /// released on 2000-09-18.  Mid 2002 plus a 18 months grace
    /// period of people still using older versions is 2004.
    ///
    ///   [Debian 3.0]: https://www.debian.org/News/2002/20020719
    ///   [GnuPG 1.0.3]: https://lists.gnupg.org/pipermail/gnupg-announce/2000q3/000075.html
    pub fn reject_packet_tag_version_at<C>(&mut self, tag: Tag, version: u8,
                                           cutoff: C)
        where C: Into<Option<SystemTime>>,
    {
        self.packet_tags.set_versioned(
            tag, version,
            cutoff.into().and_then(system_time_cutoff_to_timestamp));
    }

    /// Start rejecting packets with the given tag at `t`.
    ///
    /// See the documentation for
    /// [`StandardPolicy::reject_packet_tag_version_at`].
    pub fn reject_packet_tag_at<C>(&mut self, tag: Tag, cutoff: C)
        where C: Into<Option<SystemTime>>,
    {
        self.packet_tags.set_unversioned(
            tag,
            cutoff.into().and_then(system_time_cutoff_to_timestamp));
    }

    /// Returns the cutoff for the specified version of the specified
    /// packet tag.
    ///
    /// This first considers the versioned cutoff list.  If there is
    /// no entry in the versioned list, it fallsback to the
    /// unversioned cutoff list.  If there is also no entry there,
    /// then it falls back to the default.
    pub fn packet_tag_version_cutoff(&self, tag: Tag, version: u8)
        -> Option<SystemTime>
    {
        self.packet_tags.cutoff(tag, version).map(|t| t.into())
    }

    /// Returns the cutoff time for the specified packet tag.
    ///
    /// This function returns the maximum cutoff for all versions of
    /// the packet.  That is, if one version has a cutoff of `t1`, and
    /// another version has a cutoff of `t2`, this returns `max(t1,
    /// t2)`.  These semantics answer the question: "Up to which point
    /// can we use this packet?"
    #[deprecated(note = "Since 1.11.  Use `packet_tag_version_cutoff`.")]
    pub fn packet_tag_cutoff(&self, tag: Tag) -> Option<SystemTime> {
        // Versioned policy.
        self.packet_tags.versioned_cutoffs
            .iter()
            .filter_map(|(t, _v, cutoff)| {
                if t == &tag {
                    Some(cutoff)
                } else {
                    None
                }
            })
            // Unversioned policy or default, if nont.
            .chain(
                std::iter::once(
                    self.packet_tags.unversioned_cutoffs.get(
                        u8::from(tag) as usize)
                        .unwrap_or(&cutofflist::DEFAULT_POLICY)))
            // Prefer None.
            .max_by(|a, b| a.is_none().cmp(&b.is_none()).then(a.cmp(b)))
            .expect("have one")
            .map(Into::into)
    }
}

impl<'a> Policy for StandardPolicy<'a> {
    fn signature(&self, sig: &Signature, sec: HashAlgoSecurity) -> Result<()> {
        let time = self.time.unwrap_or_else(Timestamp::now);

        let rev = matches!(sig.typ(), SignatureType::KeyRevocation
                | SignatureType::SubkeyRevocation
                | SignatureType::CertificationRevocation);

        // Note: collision resistance requires 2nd pre-image resistance.
        if sec == HashAlgoSecurity::CollisionResistance {
            if rev {
                self
                    .collision_resistant_hash_algos
                    .check(sig.hash_algo(), time,
                           Some(self.hash_revocation_tolerance))
                    .with_context(|| format!(
                        "Policy rejected revocation signature ({}) requiring \
                         collision resistance", sig.typ()))?
            } else {
                self
                    .collision_resistant_hash_algos
                    .check(sig.hash_algo(), time, None)
                    .with_context(|| format!(
                        "Policy rejected non-revocation signature ({}) requiring \
                         collision resistance", sig.typ()))?
            }
        }

        if rev {
            self
                .second_pre_image_resistant_hash_algos
                .check(sig.hash_algo(), time,
                       Some(self.hash_revocation_tolerance))
                .with_context(|| format!(
                    "Policy rejected revocation signature ({}) requiring \
                     second pre-image resistance", sig.typ()))?
        } else {
            self
                .second_pre_image_resistant_hash_algos
                .check(sig.hash_algo(), time, None)
                .with_context(|| format!(
                    "Policy rejected non-revocation signature ({}) requiring \
                     second pre-image resistance", sig.typ()))?
        }

        for csp in sig.hashed_area().iter().filter(|sp| sp.critical()) {
            self.critical_subpackets.check(csp.tag(), time, None)
                .context("Policy rejected critical signature subpacket")?;
            if let SubpacketValue::NotationData(n) = csp.value() {
                if ! self.good_critical_notations.contains(&n.name()) {
                    return Err(anyhow::Error::from(
                        Error::PolicyViolation(
                            format!("Critical notation {:?}",
                                    n.name()), None))
                               .context("Policy rejected critical notation"));
                }
            }
        }

        Ok(())
    }

    fn key(&self, ka: &ValidErasedKeyAmalgamation<key::PublicParts>)
        -> Result<()>
    {
        use self::AsymmetricAlgorithm::{*, Unknown};
        use crate::types::PublicKeyAlgorithm::*;
        use crate::crypto::mpi::PublicKey;

        #[allow(deprecated)]
        let a = match (ka.pk_algo(), ka.mpis().bits()) {
            // RSA.
            (RSAEncryptSign, Some(b))
                | (RSAEncrypt, Some(b))
                | (RSASign, Some(b)) if b < 2048 => RSA1024,
            (RSAEncryptSign, Some(b))
                | (RSAEncrypt, Some(b))
                | (RSASign, Some(b)) if b < 3072 => RSA2048,
            (RSAEncryptSign, Some(b))
                | (RSAEncrypt, Some(b))
                | (RSASign, Some(b)) if b < 4096 => RSA3072,
            (RSAEncryptSign, Some(_))
                | (RSAEncrypt, Some(_))
                | (RSASign, Some(_)) => RSA4096,
            (RSAEncryptSign, None)
                | (RSAEncrypt, None)
                | (RSASign, None) => unreachable!(),

            // ElGamal.
            (ElGamalEncryptSign, Some(b))
                | (ElGamalEncrypt, Some(b)) if b < 2048 => ElGamal1024,
            (ElGamalEncryptSign, Some(b))
                | (ElGamalEncrypt, Some(b)) if b < 3072 => ElGamal2048,
            (ElGamalEncryptSign, Some(b))
                | (ElGamalEncrypt, Some(b)) if b < 4096 => ElGamal3072,
            (ElGamalEncryptSign, Some(_))
                | (ElGamalEncrypt, Some(_)) => ElGamal4096,
            (ElGamalEncryptSign, None)
                | (ElGamalEncrypt, None) => unreachable!(),

            // DSA.
            (DSA, Some(b)) if b < 2048 => DSA1024,
            (DSA, Some(b)) if b < 3072 => DSA2048,
            (DSA, Some(b)) if b < 4096 => DSA3072,
            (DSA, Some(_)) => DSA4096,
            (DSA, None) => unreachable!(),

            // ECC.
            (ECDH, _) | (ECDSA, _) | (EdDSA, _) => {
                let curve = match ka.mpis() {
                    PublicKey::EdDSA { curve, .. } => curve,
                    PublicKey::ECDSA { curve, .. } => curve,
                    PublicKey::ECDH { curve, .. } => curve,
                    _ => unreachable!(),
                };
                use crate::types::Curve;
                match curve {
                    Curve::NistP256 => NistP256,
                    Curve::NistP384 => NistP384,
                    Curve::NistP521 => NistP521,
                    Curve::BrainpoolP256 => BrainpoolP256,
                    Curve::Unknown(_) if curve.is_brainpoolp384()
                        => BrainpoolP384,
                    Curve::BrainpoolP512 => BrainpoolP512,
                    Curve::Ed25519 => Cv25519,
                    Curve::Cv25519 => Cv25519,
                    Curve::Unknown(_) => Unknown,
                }
            },

            _ => Unknown,
        };

        let time = self.time.unwrap_or_else(Timestamp::now);
        self.asymmetric_algos.check(a, time, None)
            .context("Policy rejected asymmetric algorithm")?;

        // Check ECDH KDF and KEK parameters.
        if let PublicKey::ECDH { hash, sym, .. } = ka.mpis() {
            self.symmetric_algorithm(*sym)
                .context("Policy rejected ECDH \
                          key encapsulation algorithm")?;

            // RFC6637 says:
            //
            // > Refer to Section 13 for the details regarding the
            // > choice of the KEK algorithm, which SHOULD be one of
            // > three AES algorithms.
            //
            // Furthermore, GnuPG rejects anything other than AES.
            // I checked the SKS dump, and there are no keys out
            // there that use a different KEK algorithm.
            match sym {
                SymmetricAlgorithm::AES128
                    | SymmetricAlgorithm::AES192
                    | SymmetricAlgorithm::AES256
                    => (), // Good.
                _ =>
                    return Err(anyhow::Error::from(
                        Error::PolicyViolation(sym.to_string(), None))
                               .context("Policy rejected ECDH \
                                         key encapsulation algorithm")),
            }

            // For use in a KDF the hash algorithm does not
            // necessarily be collision resistant, but this is the
            // weakest property that we otherwise care for, so
            // (somewhat arbitrarily) use this.
            self
                .collision_resistant_hash_algos
                .check(*hash, time, None)
                .context("Policy rejected ECDH \
                          key derivation hash function")?;
        }

        Ok(())
    }

    fn packet(&self, packet: &Packet) -> Result<()> {
        let time = self.time.unwrap_or_else(Timestamp::now);
        self.packet_tags
            .check(
                packet.tag(),
                packet.version().unwrap_or(0),
                time, None)
            .context("Policy rejected packet type")
    }

    fn symmetric_algorithm(&self, algo: SymmetricAlgorithm) -> Result<()> {
        let time = self.time.unwrap_or_else(Timestamp::now);
        self.symmetric_algos.check(algo, time, None)
            .context("Policy rejected symmetric encryption algorithm")
    }

    fn aead_algorithm(&self, algo: AEADAlgorithm) -> Result<()> {
        let time = self.time.unwrap_or_else(Timestamp::now);
        self.aead_algos.check(algo, time, None)
            .context("Policy rejected authenticated encryption algorithm")
    }
}

/// Asymmetric encryption algorithms.
///
/// This type is for refining the [`StandardPolicy`] with respect to
/// asymmetric algorithms.  In contrast to [`PublicKeyAlgorithm`], it
/// does not concern itself with the use (encryption or signing), and
/// it does include key sizes (if applicable) and elliptic curves.
///
///   [`PublicKeyAlgorithm`]: crate::types::PublicKeyAlgorithm
///
/// Key sizes put into are buckets, rounding down to the nearest
/// bucket.  For example, a 3253-bit RSA key is categorized as
/// `RSA3072`.
///
/// Note: This enum cannot be exhaustively matched to allow future
/// extensions.
#[non_exhaustive]
#[derive(Clone, Debug, PartialEq, Eq, Copy)]
pub enum AsymmetricAlgorithm {
    /// RSA with key sizes up to 2048-1 bit.
    RSA1024,
    /// RSA with key sizes up to 3072-1 bit.
    RSA2048,
    /// RSA with key sizes up to 4096-1 bit.
    RSA3072,
    /// RSA with key sizes larger or equal to 4096 bit.
    RSA4096,
    /// ElGamal with key sizes up to 2048-1 bit.
    ElGamal1024,
    /// ElGamal with key sizes up to 3072-1 bit.
    ElGamal2048,
    /// ElGamal with key sizes up to 4096-1 bit.
    ElGamal3072,
    /// ElGamal with key sizes larger or equal to 4096 bit.
    ElGamal4096,
    /// DSA with key sizes up to 2048-1 bit.
    DSA1024,
    /// DSA with key sizes up to 3072-1 bit.
    DSA2048,
    /// DSA with key sizes up to 4096-1 bit.
    DSA3072,
    /// DSA with key sizes larger or equal to 4096 bit.
    DSA4096,
    /// NIST curve P-256.
    NistP256,
    /// NIST curve P-384.
    NistP384,
    /// NIST curve P-521.
    NistP521,
    /// brainpoolP256r1.
    BrainpoolP256,
    /// brainpoolP384r1.
    BrainpoolP384,
    /// brainpoolP512r1.
    BrainpoolP512,
    /// D.J. Bernstein's Curve25519.
    Cv25519,
    /// Unknown algorithm.
    Unknown,
}
assert_send_and_sync!(AsymmetricAlgorithm);

const ASYMMETRIC_ALGORITHM_VARIANTS: [AsymmetricAlgorithm; 19] = [
    AsymmetricAlgorithm::RSA1024,
    AsymmetricAlgorithm::RSA2048,
    AsymmetricAlgorithm::RSA3072,
    AsymmetricAlgorithm::RSA4096,
    AsymmetricAlgorithm::ElGamal1024,
    AsymmetricAlgorithm::ElGamal2048,
    AsymmetricAlgorithm::ElGamal3072,
    AsymmetricAlgorithm::ElGamal4096,
    AsymmetricAlgorithm::DSA1024,
    AsymmetricAlgorithm::DSA2048,
    AsymmetricAlgorithm::DSA3072,
    AsymmetricAlgorithm::DSA4096,
    AsymmetricAlgorithm::NistP256,
    AsymmetricAlgorithm::NistP384,
    AsymmetricAlgorithm::NistP521,
    AsymmetricAlgorithm::BrainpoolP256,
    AsymmetricAlgorithm::BrainpoolP384,
    AsymmetricAlgorithm::BrainpoolP512,
    AsymmetricAlgorithm::Cv25519,
];

impl AsymmetricAlgorithm {
    /// Returns an iterator over all valid variants.
    ///
    /// Returns an iterator over all known variants.  This does not
    /// include the [`AsymmetricAlgorithm::Unknown`] variant.
    pub fn variants() -> impl Iterator<Item=AsymmetricAlgorithm> {
        ASYMMETRIC_ALGORITHM_VARIANTS.iter().cloned()
    }
}

impl std::fmt::Display for AsymmetricAlgorithm {
    fn fmt(&self, f: &mut std::fmt::Formatter) -> std::fmt::Result {
        write!(f, "{:?}", self)
    }
}

impl From<AsymmetricAlgorithm> for u8 {
    fn from(a: AsymmetricAlgorithm) -> Self {
        use self::AsymmetricAlgorithm::*;
        match a {
            RSA1024 => 0,
            RSA2048 => 1,
            RSA3072 => 2,
            RSA4096 => 3,
            ElGamal1024 => 4,
            ElGamal2048 => 5,
            ElGamal3072 => 6,
            ElGamal4096 => 7,
            DSA1024 => 8,
            DSA2048 => 9,
            DSA3072 => 10,
            DSA4096 => 11,
            NistP256 => 12,
            NistP384 => 13,
            NistP521 => 14,
            BrainpoolP256 => 15,
            BrainpoolP384 => 18,
            BrainpoolP512 => 16,
            Cv25519 => 17,
            Unknown => 255,
        }
    }
}

/// The Null Policy.
///
/// Danger, here be dragons.
///
/// This policy imposes no additional policy, i.e., accepts
/// everything.  This includes the MD5 hash algorithm, and SED
/// packets.
///
/// The Null policy has a limited set of valid use cases, e.g., packet statistics.
/// For other purposes, it is more advisable to use the [`StandardPolicy`] and
/// adjust it by selectively allowing items considered insecure by default, e.g.,
/// via [`StandardPolicy::accept_hash`] function. If this is still too inflexible
/// consider creating a specialized policy based on the [`StandardPolicy`] as
/// [the example for `StandardPolicy`] illustrates.
///
///   [`StandardPolicy::accept_hash`]: StandardPolicy::accept_hash()
///   [the example for `StandardPolicy`]: StandardPolicy#examples
#[derive(Debug)]
pub struct NullPolicy {
}

assert_send_and_sync!(NullPolicy);

impl NullPolicy {
    /// Instantiates a new `NullPolicy`.
    pub const fn new() -> Self {
        NullPolicy {}
    }
}

impl Policy for NullPolicy {
    fn signature(&self, _sig: &Signature, _sec: HashAlgoSecurity) -> Result<()> {
        Ok(())
    }

    fn key(&self, _ka: &ValidErasedKeyAmalgamation<key::PublicParts>)
        -> Result<()>
    {
        Ok(())
    }

    fn symmetric_algorithm(&self, _algo: SymmetricAlgorithm) -> Result<()> {
        Ok(())
    }

    fn aead_algorithm(&self, _algo: AEADAlgorithm) -> Result<()> {
        Ok(())
    }

    fn packet(&self, _packet: &Packet) -> Result<()> {
        Ok(())
    }

}

#[cfg(test)]
mod test {
    use std::io::Read;
    use std::time::Duration;

    use super::*;
    use crate::Error;
    use crate::Fingerprint;
    use crate::crypto::SessionKey;
    use crate::packet::key::Key4;
    use crate::packet::signature;
    use crate::packet::{PKESK, SKESK};
    use crate::parse::Parse;
    use crate::parse::stream::DecryptionHelper;
    use crate::parse::stream::DecryptorBuilder;
    use crate::parse::stream::DetachedVerifierBuilder;
    use crate::parse::stream::MessageLayer;
    use crate::parse::stream::MessageStructure;
    use crate::parse::stream::VerificationHelper;
    use crate::parse::stream::VerifierBuilder;
    use crate::policy::StandardPolicy as P;
    use crate::types::Curve;
    use crate::types::KeyFlags;
    use crate::types::SymmetricAlgorithm;

    // Test that the constructor is const.
    const _A_STANDARD_POLICY: StandardPolicy = StandardPolicy::new();

    #[test]
    fn binding_signature() {
        let p = &P::new();

        // A primary and two subkeys.
        let (cert, _) = CertBuilder::new()
            .add_signing_subkey()
            .add_transport_encryption_subkey()
            .generate().unwrap();

        assert_eq!(cert.keys().with_policy(p, None).count(), 3);

        // Reject all direct key signatures.
        #[derive(Debug)]
        struct NoDirectKeySigs;
        impl Policy for NoDirectKeySigs {
            fn signature(&self, sig: &Signature, _sec: HashAlgoSecurity)
                -> Result<()>
            {
                use crate::types::SignatureType::*;

                match sig.typ() {
                    DirectKey => Err(anyhow::anyhow!("direct key!")),
                    _ => Ok(()),
                }
            }

            fn key(&self, _ka: &ValidErasedKeyAmalgamation<key::PublicParts>)
                -> Result<()>
            {
                Ok(())
            }

            fn symmetric_algorithm(&self, _algo: SymmetricAlgorithm) -> Result<()> {
                Ok(())
            }

            fn aead_algorithm(&self, _algo: AEADAlgorithm) -> Result<()> {
                Ok(())
            }

            fn packet(&self, _packet: &Packet) -> Result<()> {
                Ok(())
            }
        }

        let p = &NoDirectKeySigs {};
        assert_eq!(cert.keys().with_policy(p, None).count(), 0);

        // Reject all subkey signatures.
        #[derive(Debug)]
        struct NoSubkeySigs;
        impl Policy for NoSubkeySigs {
            fn signature(&self, sig: &Signature, _sec: HashAlgoSecurity)
                -> Result<()>
            {
                use crate::types::SignatureType::*;

                match sig.typ() {
                    SubkeyBinding => Err(anyhow::anyhow!("subkey signature!")),
                    _ => Ok(()),
                }
            }

            fn key(&self, _ka: &ValidErasedKeyAmalgamation<key::PublicParts>)
                -> Result<()>
            {
                Ok(())
            }

            fn symmetric_algorithm(&self, _algo: SymmetricAlgorithm) -> Result<()> {
                Ok(())
            }

            fn aead_algorithm(&self, _algo: AEADAlgorithm) -> Result<()> {
                Ok(())
            }

            fn packet(&self, _packet: &Packet) -> Result<()> {
                Ok(())
            }
        }

        let p = &NoSubkeySigs {};
        assert_eq!(cert.keys().with_policy(p, None).count(), 1);
    }

    #[test]
    fn revocation() -> Result<()> {
        use crate::cert::prelude::*;
        use crate::types::SignatureType;
        use crate::types::ReasonForRevocation;

        let p = &P::new();

        // A primary and two subkeys.
        let (cert, _) = CertBuilder::new()
            .add_userid("Alice")
            .add_signing_subkey()
            .add_transport_encryption_subkey()
            .generate()?;

        // Make sure we have all keys and all user ids.
        assert_eq!(cert.keys().with_policy(p, None).count(), 3);
        assert_eq!(cert.userids().with_policy(p, None).count(), 1);

        // Reject all user id signatures.
        #[derive(Debug)]
        struct NoPositiveCertifications;
        impl Policy for NoPositiveCertifications {
            fn signature(&self, sig: &Signature, _sec: HashAlgoSecurity)
                -> Result<()>
            {
                use crate::types::SignatureType::*;
                match sig.typ() {
                    PositiveCertification =>
                        Err(anyhow::anyhow!("positive certification!")),
                    _ => Ok(()),
                }
            }

            fn key(&self, _ka: &ValidErasedKeyAmalgamation<key::PublicParts>)
                -> Result<()>
            {
                Ok(())
            }

            fn symmetric_algorithm(&self, _algo: SymmetricAlgorithm) -> Result<()> {
                Ok(())
            }

            fn aead_algorithm(&self, _algo: AEADAlgorithm) -> Result<()> {
                Ok(())
            }

            fn packet(&self, _packet: &Packet) -> Result<()> {
                Ok(())
            }
        }
        let p = &NoPositiveCertifications {};
        assert_eq!(cert.userids().with_policy(p, None).count(), 0);


        // Revoke it.
        let mut keypair = cert.primary_key().key().clone()
            .parts_into_secret()?.into_keypair()?;
        let ca = cert.userids().next().unwrap();

        // Generate the revocation for the first and only UserID.
        let revocation =
            UserIDRevocationBuilder::new()
            .set_reason_for_revocation(
                ReasonForRevocation::KeyRetired,
                b"Left example.org.")?
            .build(&mut keypair, &cert, ca.userid(), None)?;
        assert_eq!(revocation.typ(), SignatureType::CertificationRevocation);

        // Now merge the revocation signature into the Cert.
        let cert = cert.insert_packets(revocation.clone())?;

        // Check that it is revoked.
        assert_eq!(cert.userids().with_policy(p, None).revoked(false).count(), 0);

        // Reject all user id signatures.
        #[derive(Debug)]
        struct NoCertificationRevocation;
        impl Policy for NoCertificationRevocation {
            fn signature(&self, sig: &Signature, _sec: HashAlgoSecurity)
                -> Result<()>
            {
                use crate::types::SignatureType::*;
                match sig.typ() {
                    CertificationRevocation =>
                        Err(anyhow::anyhow!("certification certification!")),
                    _ => Ok(()),
                }
            }

            fn key(&self, _ka: &ValidErasedKeyAmalgamation<key::PublicParts>)
                -> Result<()>
            {
                Ok(())
            }

            fn symmetric_algorithm(&self, _algo: SymmetricAlgorithm) -> Result<()> {
                Ok(())
            }

            fn aead_algorithm(&self, _algo: AEADAlgorithm) -> Result<()> {
                Ok(())
            }

            fn packet(&self, _packet: &Packet) -> Result<()> {
                Ok(())
            }
        }
        let p = &NoCertificationRevocation {};

        // Check that the user id is no longer revoked.
        assert_eq!(cert.userids().with_policy(p, None).revoked(false).count(), 1);


        // Generate the revocation for the first subkey.
        let subkey = cert.keys().subkeys().next().unwrap();
        let revocation =
            SubkeyRevocationBuilder::new()
                .set_reason_for_revocation(
                    ReasonForRevocation::KeyRetired,
                    b"Smells funny.").unwrap()
                .build(&mut keypair, &cert, subkey.key(), None)?;
        assert_eq!(revocation.typ(), SignatureType::SubkeyRevocation);

        // Now merge the revocation signature into the Cert.
        assert_eq!(cert.keys().with_policy(p, None).revoked(false).count(), 3);
        let cert = cert.insert_packets(revocation.clone())?;
        assert_eq!(cert.keys().with_policy(p, None).revoked(false).count(), 2);

        // Reject all subkey revocations.
        #[derive(Debug)]
        struct NoSubkeyRevocation;
        impl Policy for NoSubkeyRevocation {
            fn signature(&self, sig: &Signature, _sec: HashAlgoSecurity)
                -> Result<()>
            {
                use crate::types::SignatureType::*;
                match sig.typ() {
                    SubkeyRevocation =>
                        Err(anyhow::anyhow!("subkey revocation!")),
                    _ => Ok(()),
                }
            }

            fn key(&self, _ka: &ValidErasedKeyAmalgamation<key::PublicParts>)
                -> Result<()>
            {
                Ok(())
            }

            fn symmetric_algorithm(&self, _algo: SymmetricAlgorithm) -> Result<()> {
                Ok(())
            }

            fn aead_algorithm(&self, _algo: AEADAlgorithm) -> Result<()> {
                Ok(())
            }

            fn packet(&self, _packet: &Packet) -> Result<()> {
                Ok(())
            }
        }
        let p = &NoSubkeyRevocation {};

        // Check that the key is no longer revoked.
        assert_eq!(cert.keys().with_policy(p, None).revoked(false).count(), 3);

        Ok(())
    }


    #[test]
    fn binary_signature() -> Result<()> {
        #[derive(PartialEq, Debug)]
        struct VHelper {
            good: usize,
            errors: usize,
            keys: Vec<Cert>,
        }

        impl VHelper {
            fn new(keys: Vec<Cert>) -> Self {
                VHelper {
                    good: 0,
                    errors: 0,
                    keys,
                }
            }
        }

        impl VerificationHelper for VHelper {
            fn get_certs(&mut self, _ids: &[crate::KeyHandle])
                -> Result<Vec<Cert>>
            {
                Ok(self.keys.clone())
            }

            fn check(&mut self, structure: MessageStructure) -> Result<()>
            {
                for layer in structure {
                    match layer {
                        MessageLayer::SignatureGroup { ref results } =>
                            for result in results {
                                eprintln!("result: {:?}", result);
                                match result {
                                    Ok(_) => self.good += 1,
                                    Err(_) => self.errors += 1,
                                }
                            }
                        MessageLayer::Compression { .. } => (),
                        _ => unreachable!(),
                    }
                }

                Ok(())
            }
        }

        impl DecryptionHelper for VHelper {
            fn decrypt<D>(&mut self, _: &[PKESK], _: &[SKESK],
                          _: Option<SymmetricAlgorithm>,_: D)
                          -> Result<Option<Fingerprint>>
                where D: FnMut(SymmetricAlgorithm, &SessionKey) -> bool
            {
                unreachable!();
            }
        }

        // Reject all data (binary) signatures.
        #[derive(Debug)]
        struct NoBinarySigantures;
        impl Policy for NoBinarySigantures {
            fn signature(&self, sig: &Signature, _sec: HashAlgoSecurity)
                -> Result<()>
            {
                use crate::types::SignatureType::*;
                eprintln!("{:?}", sig.typ());
                match sig.typ() {
                    Binary =>
                        Err(anyhow::anyhow!("binary!")),
                    _ => Ok(()),
                }
            }

            fn key(&self, _ka: &ValidErasedKeyAmalgamation<key::PublicParts>)
                -> Result<()>
            {
                Ok(())
            }

            fn symmetric_algorithm(&self, _algo: SymmetricAlgorithm) -> Result<()> {
                Ok(())
            }

            fn aead_algorithm(&self, _algo: AEADAlgorithm) -> Result<()> {
                Ok(())
            }

            fn packet(&self, _packet: &Packet) -> Result<()> {
                Ok(())
            }
        }
        let no_binary_signatures = &NoBinarySigantures {};

        // Reject all subkey signatures.
        #[derive(Debug)]
        struct NoSubkeySigs;
        impl Policy for NoSubkeySigs {
            fn signature(&self, sig: &Signature, _sec: HashAlgoSecurity)
                -> Result<()>
            {
                use crate::types::SignatureType::*;

                match sig.typ() {
                    SubkeyBinding => Err(anyhow::anyhow!("subkey signature!")),
                    _ => Ok(()),
                }
            }

            fn key(&self, _ka: &ValidErasedKeyAmalgamation<key::PublicParts>)
                -> Result<()>
            {
                Ok(())
            }

            fn symmetric_algorithm(&self, _algo: SymmetricAlgorithm) -> Result<()> {
                Ok(())
            }

            fn aead_algorithm(&self, _algo: AEADAlgorithm) -> Result<()> {
                Ok(())
            }

            fn packet(&self, _packet: &Packet) -> Result<()> {
                Ok(())
            }
        }
        let no_subkey_signatures = &NoSubkeySigs {};

        let standard = &P::new();

        let keys = [
            "neal.pgp",
        ].iter()
            .map(|f| Cert::from_bytes(crate::tests::key(f)).unwrap())
            .collect::<Vec<_>>();
        let data = "messages/signed-1.gpg";

        let reference = crate::tests::manifesto();



        // Test Verifier.

        // Standard policy => ok.
        let h = VHelper::new(keys.clone());
        let mut v = VerifierBuilder::from_bytes(crate::tests::file(data))?
            .with_policy(standard, crate::frozen_time(), h)?;
        assert!(v.message_processed());
        assert_eq!(v.helper_ref().good, 1);
        assert_eq!(v.helper_ref().errors, 0);

        let mut content = Vec::new();
        v.read_to_end(&mut content).unwrap();
        assert_eq!(reference.len(), content.len());
        assert_eq!(reference, &content[..]);


        // Kill the subkey.
        let h = VHelper::new(keys.clone());
        let mut v = VerifierBuilder::from_bytes(crate::tests::file(data))?
            .with_policy(no_subkey_signatures, crate::frozen_time(), h)?;
        assert!(v.message_processed());
        assert_eq!(v.helper_ref().good, 0);
        assert_eq!(v.helper_ref().errors, 1);

        let mut content = Vec::new();
        v.read_to_end(&mut content).unwrap();
        assert_eq!(reference.len(), content.len());
        assert_eq!(reference, &content[..]);


        // Kill the data signature.
        let h = VHelper::new(keys.clone());
        let mut v = VerifierBuilder::from_bytes(crate::tests::file(data))?
            .with_policy(no_binary_signatures, crate::frozen_time(), h)?;
        assert!(v.message_processed());
        assert_eq!(v.helper_ref().good, 0);
        assert_eq!(v.helper_ref().errors, 1);

        let mut content = Vec::new();
        v.read_to_end(&mut content).unwrap();
        assert_eq!(reference.len(), content.len());
        assert_eq!(reference, &content[..]);



        // Test Decryptor.

        // Standard policy.
        let h = VHelper::new(keys.clone());
        let mut v = DecryptorBuilder::from_bytes(crate::tests::file(data))?
            .with_policy(standard, crate::frozen_time(), h)?;
        assert!(v.message_processed());
        assert_eq!(v.helper_ref().good, 1);
        assert_eq!(v.helper_ref().errors, 0);

        let mut content = Vec::new();
        v.read_to_end(&mut content).unwrap();
        assert_eq!(reference.len(), content.len());
        assert_eq!(reference, &content[..]);


        // Kill the subkey.
        let h = VHelper::new(keys.clone());
        let mut v = DecryptorBuilder::from_bytes(crate::tests::file(data))?
            .with_policy(no_subkey_signatures, crate::frozen_time(), h)?;
        assert!(v.message_processed());
        assert_eq!(v.helper_ref().good, 0);
        assert_eq!(v.helper_ref().errors, 1);

        let mut content = Vec::new();
        v.read_to_end(&mut content).unwrap();
        assert_eq!(reference.len(), content.len());
        assert_eq!(reference, &content[..]);


        // Kill the data signature.
        let h = VHelper::new(keys.clone());
        let mut v = DecryptorBuilder::from_bytes(crate::tests::file(data))?
            .with_policy(no_binary_signatures, crate::frozen_time(), h)?;
        assert!(v.message_processed());
        assert_eq!(v.helper_ref().good, 0);
        assert_eq!(v.helper_ref().errors, 1);

        let mut content = Vec::new();
        v.read_to_end(&mut content).unwrap();
        assert_eq!(reference.len(), content.len());
        assert_eq!(reference, &content[..]);
        Ok(())
    }

    #[test]
    fn hash_algo() -> Result<()> {
        use crate::types::RevocationStatus;
        use crate::types::ReasonForRevocation;

        const SECS_IN_YEAR : u64 = 365 * 24 * 60 * 60;

        // A `const fn` is only guaranteed to be evaluated at compile
        // time if the result is assigned to a `const` variable.  Make
        // sure that works.
        const DEFAULT : StandardPolicy = StandardPolicy::new();

        let (cert, _) = CertBuilder::new()
            .add_userid("Alice")
            .generate()?;

        let algo = cert.primary_key()
            .binding_signature(&DEFAULT, None).unwrap().hash_algo();

        eprintln!("{:?}", algo);

        // Create a revoked version.
        let mut keypair = cert.primary_key().key().clone()
            .parts_into_secret()?.into_keypair()?;
        let rev = cert.revoke(
            &mut keypair,
            ReasonForRevocation::KeyCompromised,
            b"It was the maid :/")?;
        let cert_revoked = cert.clone().insert_packets(rev)?;

        match cert_revoked.revocation_status(&DEFAULT, None) {
            RevocationStatus::Revoked(sigs) => {
                assert_eq!(sigs.len(), 1);
                assert_eq!(sigs[0].hash_algo(), algo);
            }
            _ => panic!("not revoked"),
        }


        // Reject the hash algorithm unconditionally.
        let mut reject : StandardPolicy = StandardPolicy::new();
        reject.reject_hash(algo);
        assert!(cert.primary_key()
                    .binding_signature(&reject, None).is_err());
        assert_match!(RevocationStatus::NotAsFarAsWeKnow
                      = cert_revoked.revocation_status(&reject, None));

        // Reject the hash algorithm next year.
        let mut reject : StandardPolicy = StandardPolicy::new();
        reject.reject_hash_at(
            algo,
            crate::now().checked_add(Duration::from_secs(SECS_IN_YEAR)));
        reject.hash_revocation_tolerance(0);
        cert.primary_key().binding_signature(&reject, None)?;
        assert_match!(RevocationStatus::Revoked(_)
                      = cert_revoked.revocation_status(&reject, None));

        // Reject the hash algorithm last year.
        let mut reject : StandardPolicy = StandardPolicy::new();
        reject.reject_hash_at(
            algo,
            crate::now().checked_sub(Duration::from_secs(SECS_IN_YEAR)));
        reject.hash_revocation_tolerance(0);
        assert!(cert.primary_key()
                    .binding_signature(&reject, None).is_err());
        assert_match!(RevocationStatus::NotAsFarAsWeKnow
                      = cert_revoked.revocation_status(&reject, None));

        // Reject the hash algorithm for normal signatures last year,
        // and revocations next year.
        let mut reject : StandardPolicy = StandardPolicy::new();
        reject.reject_hash_at(
            algo,
            crate::now().checked_sub(Duration::from_secs(SECS_IN_YEAR)));
        reject.hash_revocation_tolerance(2 * SECS_IN_YEAR as u32);
        assert!(cert.primary_key()
                    .binding_signature(&reject, None).is_err());
        assert_match!(RevocationStatus::Revoked(_)
                      = cert_revoked.revocation_status(&reject, None));

        // Accept algo, but reject the algos with id - 1 and id + 1.
        let mut reject : StandardPolicy = StandardPolicy::new();
        let algo_u8 : u8 = algo.into();
        assert!(algo_u8 != 0u8);
        reject.reject_hash_at(
            (algo_u8 - 1).into(),
            crate::now().checked_sub(Duration::from_secs(SECS_IN_YEAR)));
        reject.reject_hash_at(
            (algo_u8 + 1).into(),
            crate::now().checked_sub(Duration::from_secs(SECS_IN_YEAR)));
        reject.hash_revocation_tolerance(0);
        cert.primary_key().binding_signature(&reject, None)?;
        assert_match!(RevocationStatus::Revoked(_)
                      = cert_revoked.revocation_status(&reject, None));

        // Reject the hash algorithm since before the Unix epoch.
        // Since the earliest representable time using a Timestamp is
        // the Unix epoch, this is equivalent to rejecting everything.
        let mut reject : StandardPolicy = StandardPolicy::new();
        reject.reject_hash_at(
            algo,
            crate::now().checked_sub(Duration::from_secs(SECS_IN_YEAR)));
        reject.hash_revocation_tolerance(0);
        assert!(cert.primary_key()
                    .binding_signature(&reject, None).is_err());
        assert_match!(RevocationStatus::NotAsFarAsWeKnow
                      = cert_revoked.revocation_status(&reject, None));

        // Reject the hash algorithm after the end of time that is
        // representable by a Timestamp (2106).  This should accept
        // everything.
        let mut reject : StandardPolicy = StandardPolicy::new();
        reject.reject_hash_at(
            algo,
            SystemTime::UNIX_EPOCH.checked_add(Duration::from_secs(500 * SECS_IN_YEAR)));
        reject.hash_revocation_tolerance(0);
        cert.primary_key().binding_signature(&reject, None)?;
        assert_match!(RevocationStatus::Revoked(_)
                      = cert_revoked.revocation_status(&reject, None));

        Ok(())
    }

    #[test]
    fn key_verify_self_signature() -> Result<()> {
        let p = &P::new();

        #[derive(Debug)]
        struct NoRsa;
        impl Policy for NoRsa {
            fn key(&self, ka: &ValidErasedKeyAmalgamation<key::PublicParts>)
                   -> Result<()>
            {
                use crate::types::PublicKeyAlgorithm::*;

                eprintln!("algo: {}", ka.key().pk_algo());
                if ka.key().pk_algo() == RSAEncryptSign {
                    Err(anyhow::anyhow!("RSA!"))
                } else {
                    Ok(())
                }
            }

            fn signature(&self, _sig: &Signature, _sec: HashAlgoSecurity) -> Result<()> {
                Ok(())
            }

            fn symmetric_algorithm(&self, _algo: SymmetricAlgorithm) -> Result<()> {
                Ok(())
            }

            fn aead_algorithm(&self, _algo: AEADAlgorithm) -> Result<()> {
                Ok(())
            }

            fn packet(&self, _packet: &Packet) -> Result<()> {
                Ok(())
            }
        }
        let norsa = &NoRsa {};

        // Generate a certificate with an RSA primary and two RSA
        // subkeys.
        let (cert,_) = CertBuilder::new()
            .set_cipher_suite(CipherSuite::RSA2k)
            .add_signing_subkey()
            .add_signing_subkey()
            .generate()?;
        assert_eq!(cert.keys().with_policy(p, None).count(), 3);
        assert_eq!(cert.keys().with_policy(norsa, None).count(), 0);
        assert!(cert.primary_key().with_policy(p, None).is_ok());
        assert!(cert.primary_key().with_policy(norsa, None).is_err());

        // Generate a certificate with an ECC primary, an ECC subkey,
        // and an RSA subkey.
        let (cert,_) = CertBuilder::new()
            .set_cipher_suite(CipherSuite::Cv25519)
            .add_signing_subkey()
            .generate()?;

        let pk = cert.primary_key().key().parts_as_secret()?;
        let subkey: key::SecretSubkey
            = Key4::generate_rsa(2048)?.into();
        let binding = signature::SignatureBuilder::new(SignatureType::SubkeyBinding)
            .set_key_flags(KeyFlags::empty().set_transport_encryption())?
            .sign_subkey_binding(&mut pk.clone().into_keypair()?,
                                 pk.parts_as_public(), &subkey)?;

        let cert = cert.insert_packets(
            vec![ Packet::from(subkey), binding.into() ])?;

        assert_eq!(cert.keys().with_policy(p, None).count(), 3);
        assert_eq!(cert.keys().with_policy(norsa, None).count(), 2);
        assert!(cert.primary_key().with_policy(p, None).is_ok());
        assert!(cert.primary_key().with_policy(norsa, None).is_ok());

        // Generate a certificate with an RSA primary, an RSA subkey,
        // and an ECC subkey.
        let (cert,_) = CertBuilder::new()
            .set_cipher_suite(CipherSuite::RSA2k)
            .add_signing_subkey()
            .generate()?;

        let pk = cert.primary_key().key().parts_as_secret()?;
        let subkey: key::SecretSubkey
            = key::Key4::generate_ecc(true, Curve::Ed25519)?.into();
        let binding = signature::SignatureBuilder::new(SignatureType::SubkeyBinding)
            .set_key_flags(KeyFlags::empty().set_transport_encryption())?
            .sign_subkey_binding(&mut pk.clone().into_keypair()?,
                                 pk.parts_as_public(), &subkey)?;

        let cert = cert.insert_packets(
            vec![ Packet::from(subkey), binding.into() ])?;

        assert_eq!(cert.keys().with_policy(p, None).count(), 3);
        assert_eq!(cert.keys().with_policy(norsa, None).count(), 0);
        assert!(cert.primary_key().with_policy(p, None).is_ok());
        assert!(cert.primary_key().with_policy(norsa, None).is_err());

        // Generate a certificate with an ECC primary and two ECC
        // subkeys.
        let (cert,_) = CertBuilder::new()
            .set_cipher_suite(CipherSuite::Cv25519)
            .add_signing_subkey()
            .add_signing_subkey()
            .generate()?;
        assert_eq!(cert.keys().with_policy(p, None).count(), 3);
        assert_eq!(cert.keys().with_policy(norsa, None).count(), 3);
        assert!(cert.primary_key().with_policy(p, None).is_ok());
        assert!(cert.primary_key().with_policy(norsa, None).is_ok());

        Ok(())
    }

    #[test]
    fn key_verify_binary_signature() -> Result<()> {
        use crate::packet::signature;
        use crate::serialize::Serialize;
        use crate::Packet;
        use crate::types::KeyFlags;

        let p = &P::new();

        #[derive(Debug)]
        struct NoRsa;
        impl Policy for NoRsa {
            fn key(&self, ka: &ValidErasedKeyAmalgamation<key::PublicParts>)
                   -> Result<()>
            {
                use crate::types::PublicKeyAlgorithm::*;

                eprintln!("algo: {} is {}",
                          ka.fingerprint(), ka.key().pk_algo());
                if ka.key().pk_algo() == RSAEncryptSign {
                    Err(anyhow::anyhow!("RSA!"))
                } else {
                    Ok(())
                }
            }

            fn signature(&self, _sig: &Signature, _sec: HashAlgoSecurity) -> Result<()> {
                Ok(())
            }

            fn symmetric_algorithm(&self, _algo: SymmetricAlgorithm) -> Result<()> {
                Ok(())
            }

            fn aead_algorithm(&self, _algo: AEADAlgorithm) -> Result<()> {
                Ok(())
            }

            fn packet(&self, _packet: &Packet) -> Result<()> {
                Ok(())
            }
        }
        let norsa = &NoRsa {};

        #[derive(PartialEq, Debug)]
        struct VHelper {
            good: usize,
            errors: usize,
            keys: Vec<Cert>,
        }

        impl VHelper {
            fn new(keys: Vec<Cert>) -> Self {
                VHelper {
                    good: 0,
                    errors: 0,
                    keys,
                }
            }
        }

        impl VerificationHelper for VHelper {
            fn get_certs(&mut self, _ids: &[crate::KeyHandle])
                -> Result<Vec<Cert>>
            {
                Ok(self.keys.clone())
            }

            fn check(&mut self, structure: MessageStructure) -> Result<()>
            {
                for layer in structure {
                    match layer {
                        MessageLayer::SignatureGroup { ref results } =>
                            for result in results {
                                match result {
                                    Ok(_) => self.good += 1,
                                    Err(_) => self.errors += 1,
                                }
                            }
                        MessageLayer::Compression { .. } => (),
                        _ => unreachable!(),
                    }
                }

                Ok(())
            }
        }

        impl DecryptionHelper for VHelper {
            fn decrypt<D>(&mut self, _: &[PKESK], _: &[SKESK],
                          _: Option<SymmetricAlgorithm>,_: D)
                          -> Result<Option<Fingerprint>>
                where D: FnMut(SymmetricAlgorithm, &SessionKey) -> bool
            {
                unreachable!();
            }
        }

        // Sign msg using cert's first subkey, return the signature.
        fn sign_and_verify(p: &dyn Policy, cert: &Cert, good: bool) {
            eprintln!("Expect verification to be {}",
                      if good { "good" } else { "bad" });
            for (i, k) in cert.keys().enumerate() {
                eprintln!("  {}. {}", i, k.fingerprint());
            }

            let msg = b"Hello, World";

            // We always use the first subkey.
            let key = cert.keys().nth(1).unwrap().key();
            let mut keypair = key.clone()
                .parts_into_secret().unwrap()
                .into_keypair().unwrap();

            // Create a signature.
            let sig =
                signature::SignatureBuilder::new(SignatureType::Binary)
                .sign_message(&mut keypair, msg).unwrap();

            // Make sure the signature is ok.
            sig.verify_message(key, msg).unwrap();

            // Turn it into a detached signature.
            let sig = {
                let mut v = Vec::new();
                let sig : Packet = sig.into();
                sig.serialize(&mut v).unwrap();
                v
            };

            let h = VHelper::new(vec![ cert.clone() ]);
            let mut v = DetachedVerifierBuilder::from_bytes(&sig).unwrap()
                .with_policy(p, None, h).unwrap();
            v.verify_bytes(msg).unwrap();
            assert_eq!(v.helper_ref().good, if good { 1 } else { 0 });
            assert_eq!(v.helper_ref().errors, if good { 0 } else { 1 });
        }


        // A certificate with an ECC primary and an ECC signing
        // subkey.
        eprintln!("Trying ECC primary, ECC sub:");
        let (cert,_) = CertBuilder::new()
            .set_cipher_suite(CipherSuite::Cv25519)
            .add_subkey(KeyFlags::empty().set_signing(), None,
                        None)
            .generate()?;

        assert_eq!(cert.keys().with_policy(p, None).count(), 2);
        assert_eq!(cert.keys().with_policy(norsa, None).count(), 2);
        assert!(cert.primary_key().with_policy(p, None).is_ok());
        assert!(cert.primary_key().with_policy(norsa, None).is_ok());

        sign_and_verify(p, &cert, true);
        sign_and_verify(norsa, &cert, true);

        // A certificate with an RSA primary and an RCC signing
        // subkey.
        eprintln!("Trying RSA primary, ECC sub:");
        let (cert,_) = CertBuilder::new()
            .set_cipher_suite(CipherSuite::RSA2k)
            .add_subkey(KeyFlags::empty().set_signing(), None,
                        CipherSuite::Cv25519)
            .generate()?;

        assert_eq!(cert.keys().with_policy(p, None).count(), 2);
        assert_eq!(cert.keys().with_policy(norsa, None).count(), 0);
        assert!(cert.primary_key().with_policy(p, None).is_ok());
        assert!(cert.primary_key().with_policy(norsa, None).is_err());

        sign_and_verify(p, &cert, true);
        sign_and_verify(norsa, &cert, false);

        // A certificate with an ECC primary and an RSA signing
        // subkey.
        eprintln!("Trying ECC primary, RSA sub:");
        let (cert,_) = CertBuilder::new()
            .set_cipher_suite(CipherSuite::Cv25519)
            .add_subkey(KeyFlags::empty().set_signing(), None,
                        CipherSuite::RSA2k)
            .generate()?;

        assert_eq!(cert.keys().with_policy(p, None).count(), 2);
        assert_eq!(cert.keys().with_policy(norsa, None).count(), 1);
        assert!(cert.primary_key().with_policy(p, None).is_ok());
        assert!(cert.primary_key().with_policy(norsa, None).is_ok());

        sign_and_verify(p, &cert, true);
        sign_and_verify(norsa, &cert, false);

        Ok(())
    }

    #[test]
    fn reject_seip_packet() -> Result<()> {
        #[derive(PartialEq, Debug)]
        struct Helper {}
        impl VerificationHelper for Helper {
            fn get_certs(&mut self, _: &[crate::KeyHandle])
                -> Result<Vec<Cert>> {
                unreachable!()
            }

            fn check(&mut self, _: MessageStructure) -> Result<()> {
                unreachable!()
            }
        }

        impl DecryptionHelper for Helper {
            fn decrypt<D>(&mut self, _: &[PKESK], _: &[SKESK],
                          _: Option<SymmetricAlgorithm>, _: D)
                          -> Result<Option<Fingerprint>>
                where D: FnMut(SymmetricAlgorithm, &SessionKey) -> bool {
                Ok(None)
            }
        }

        let p = &P::new();
        let r = DecryptorBuilder::from_bytes(crate::tests::message(
                "encrypted-to-testy.gpg"))?
            .with_policy(p, crate::frozen_time(), Helper {});
        match r {
            Ok(_) => panic!(),
            Err(e) => assert_match!(Error::MissingSessionKey(_)
                                    = e.downcast().unwrap()),
        }

        // Reject the SEIP packet.
        let p = &mut P::new();
        p.reject_packet_tag(Tag::SEIP);
        let r = DecryptorBuilder::from_bytes(crate::tests::message(
                "encrypted-to-testy.gpg"))?
            .with_policy(p, crate::frozen_time(), Helper {});
        match r {
            Ok(_) => panic!(),
            Err(e) => assert_match!(Error::PolicyViolation(_, _)
                                    = e.downcast().unwrap()),
        }
        Ok(())
    }

    #[test]
    fn reject_cipher() -> Result<()> {
        struct Helper {}
        impl VerificationHelper for Helper {
            fn get_certs(&mut self, _: &[crate::KeyHandle])
                -> Result<Vec<Cert>> {
                Ok(Default::default())
            }

            fn check(&mut self, _: MessageStructure) -> Result<()> {
                Ok(())
            }
        }

        impl DecryptionHelper for Helper {
            fn decrypt<D>(&mut self, pkesks: &[PKESK], _: &[SKESK],
                          algo: Option<SymmetricAlgorithm>, mut decrypt: D)
                          -> Result<Option<Fingerprint>>
                where D: FnMut(SymmetricAlgorithm, &SessionKey) -> bool
            {
                let p = &P::new();
                let mut pair = Cert::from_bytes(
                    crate::tests::key("testy-private.pgp"))?
                    .keys().with_policy(p, None)
                    .for_transport_encryption().secret().next().unwrap()
                    .key().clone().into_keypair()?;
                pkesks[0].decrypt(&mut pair, algo)
                    .map(|(algo, session_key)| decrypt(algo, &session_key));
                Ok(None)
            }
        }

        let p = &P::new();
        DecryptorBuilder::from_bytes(crate::tests::message(
                "encrypted-to-testy-no-compression.gpg"))?
            .with_policy(p, crate::frozen_time(), Helper {})?;

        // Reject the AES256.
        let p = &mut P::new();
        p.reject_symmetric_algo(SymmetricAlgorithm::AES256);
        let r = DecryptorBuilder::from_bytes(crate::tests::message(
                "encrypted-to-testy-no-compression.gpg"))?
            .with_policy(p, crate::frozen_time(), Helper {});
        match r {
            Ok(_) => panic!(),
            Err(e) => assert_match!(Error::PolicyViolation(_, _)
                                    = e.downcast().unwrap()),
        }
        Ok(())
    }

    #[test]
    fn reject_asymmetric_algos() -> Result<()> {
        let cert = Cert::from_bytes(crate::tests::key("neal.pgp"))?;
        let p = &mut P::new();
        let t = crate::frozen_time();

        assert_eq!(cert.with_policy(p, t).unwrap().keys().count(), 4);
        p.reject_asymmetric_algo(AsymmetricAlgorithm::RSA1024);
        assert_eq!(cert.with_policy(p, t).unwrap().keys().count(), 4);
        p.reject_asymmetric_algo(AsymmetricAlgorithm::RSA2048);
        assert_eq!(cert.with_policy(p, t).unwrap().keys().count(), 1);
        Ok(())
    }

    #[test]
    fn reject_all_hashes() -> Result<()> {
        let mut p = StandardPolicy::new();

        let set_variants = [
            HashAlgorithm::MD5,
            HashAlgorithm::Unknown(234),
        ];
        let check_variants = [
            HashAlgorithm::SHA512,
            HashAlgorithm::Unknown(239),
        ];

        // Accept a few hashes explicitly.
        for v in set_variants.iter().cloned() {
            p.accept_hash(v);
            assert_eq!(
                p.hash_cutoff(
                    v,
                    HashAlgoSecurity::SecondPreImageResistance),
                ACCEPT.map(Into::into));
            assert_eq!(
                p.hash_cutoff(
                    v,
                    HashAlgoSecurity::CollisionResistance),
                ACCEPT.map(Into::into));
        }

        // Reject all hashes.
        p.reject_all_hashes();

        for v in set_variants.iter().chain(check_variants.iter()).cloned() {
            assert_eq!(
                p.hash_cutoff(
                    v,
                    HashAlgoSecurity::SecondPreImageResistance),
                REJECT.map(Into::into));
            assert_eq!(
                p.hash_cutoff(
                    v,
                    HashAlgoSecurity::CollisionResistance),
                REJECT.map(Into::into));
        }

        Ok(())
    }

    macro_rules! reject_all_check {
        ($reject_all:ident, $accept_one:ident, $cutoff:ident,
         $set_variants:expr, $check_variants:expr) => {
            #[test]
            fn $reject_all() -> Result<()> {
                let mut p = StandardPolicy::new();

                // Accept a few hashes explicitly.
                for v in $set_variants.iter().cloned() {
                    p.$accept_one(v);
                    assert_eq!(p.$cutoff(v), ACCEPT.map(Into::into));
                }

                // Reject all hashes.
                p.$reject_all();

                for v in $set_variants.iter()
                    .chain($check_variants.iter()).cloned()
                {
                    assert_eq!(
                        p.$cutoff(v),
                        REJECT.map(Into::into));
                }
                Ok(())
            }
        }
    }

    reject_all_check!(reject_all_critical_subpackets,
                      accept_critical_subpacket,
                      critical_subpacket_cutoff,
                      &[ SubpacketTag::TrustSignature,
                         SubpacketTag::Unknown(252) ],
                      &[ SubpacketTag::Unknown(253),
                         SubpacketTag::SignatureCreationTime ]);

    reject_all_check!(reject_all_asymmetric_algos,
                      accept_asymmetric_algo,
                      asymmetric_algo_cutoff,
                      &[ AsymmetricAlgorithm::RSA3072,
                         AsymmetricAlgorithm::Cv25519 ],
                      &[ AsymmetricAlgorithm::Unknown,
                         AsymmetricAlgorithm::NistP256 ]);

    reject_all_check!(reject_all_symmetric_algos,
                      accept_symmetric_algo,
                      symmetric_algo_cutoff,
                      &[ SymmetricAlgorithm::Unencrypted,
                         SymmetricAlgorithm::Unknown(252) ],
                      &[ SymmetricAlgorithm::AES256,
                         SymmetricAlgorithm::Unknown(230) ]);

    reject_all_check!(reject_all_aead_algos,
                      accept_aead_algo,
                      aead_algo_cutoff,
                      &[ AEADAlgorithm::OCB ],
                      &[ AEADAlgorithm::EAX ]);

    #[test]
    fn reject_all_packets() -> Result<()> {
        let mut p = StandardPolicy::new();

        let set_variants = [
            (Tag::SEIP, 4),
            (Tag::Unknown(252), 17),
        ];
        let check_variants = [
            (Tag::Signature, 4),
            (Tag::Unknown(230), 9),
        ];

        // Accept a few packets explicitly.
        for (t, v) in set_variants.iter().cloned() {
            p.accept_packet_tag_version(t, v);
            assert_eq!(
                p.packet_tag_version_cutoff(t, v),
                ACCEPT.map(Into::into));
        }

        // Reject all hashes.
        p.reject_all_packet_tags();

        for (t, v) in set_variants.iter().chain(check_variants.iter()).cloned() {
            assert_eq!(
                p.packet_tag_version_cutoff(t, v),
                REJECT.map(Into::into));
        }

        Ok(())
    }

    #[test]
    fn packet_versions() -> Result<()> {
        // Accept the version of a packet.  Optionally make sure a
        // different version is not accepted.
        fn accept_and_check(p: &mut StandardPolicy,
                            tag: Tag,
                            accept_versions: &[u8],
                            good_versions: &[u8],
                            bad_versions: &[u8]) {
            for v in accept_versions {
                p.accept_packet_tag_version(tag, *v);
                assert_eq!(
                    p.packet_tag_version_cutoff(tag, *v),
                    ACCEPT.map(Into::into));
            }

            for v in good_versions.iter() {
                assert_eq!(
                    p.packet_tag_version_cutoff(tag, *v),
                    ACCEPT.map(Into::into));
            }
            for v in bad_versions.iter() {
                assert_eq!(
                    p.packet_tag_version_cutoff(tag, *v),
                    REJECT.map(Into::into));
            }
        }

        use rand::seq::SliceRandom;
        let mut rng = rand::thread_rng();

        let mut all_versions = (0..=u8::MAX).collect::<Vec<_>>();
        all_versions.shuffle(&mut rng);
        let all_versions = &all_versions[..];
        let mut not_v5 = all_versions.iter()
            .filter(|&&v| v != 5)
            .cloned()
            .collect::<Vec<_>>();
        not_v5.shuffle(&mut rng);
        let not_v5 = &not_v5[..];

        let p = &mut StandardPolicy::new();
        p.reject_all_packet_tags();

        // First only use the versioned interfaces.
        accept_and_check(p, Tag::Signature, &[3], &[], &[4, 5]);
        accept_and_check(p, Tag::Signature, &[4], &[3], &[5]);

        // Only use an unversioned policy.
        accept_and_check(p, Tag::SEIP,
                         &[], // set to accept
                         &[], // good
                         all_versions, // bad
        );
        p.accept_packet_tag(Tag::SEIP);
        accept_and_check(p, Tag::SEIP,
                         &[], // set to accept
                         all_versions, // good
                         &[], // bad
        );

        // Set an unversioned policy and then a versioned policy.
        accept_and_check(p, Tag::PKESK,
                         &[], // set to accept
                         &[], // good
                         all_versions, // bad
        );
        p.accept_packet_tag(Tag::PKESK);
        accept_and_check(p, Tag::PKESK,
                         &[], // set to accept
                         &(0..u8::MAX).collect::<Vec<_>>()[..], // good
                         &[], // bad
        );
        p.reject_packet_tag_version(Tag::PKESK, 5);
        accept_and_check(p, Tag::PKESK,
                         &[], // set to accept
                         not_v5, // good
                         &[5], // bad
        );

        // Set a versioned policy and then an unversioned policy.
        // Make sure that the versioned policy is cleared by the
        // unversioned policy.
        accept_and_check(p, Tag::SKESK,
                         &[], // set to accept
                         &[], // good
                         all_versions, // bad
        );
        p.accept_packet_tag_version(Tag::SKESK, 5);
        accept_and_check(p, Tag::SKESK,
                         &[], // set to accept
                         &[5], // good
                         not_v5, // bad
        );
        p.reject_packet_tag(Tag::SKESK);
        // All versions should be bad now...
        accept_and_check(p, Tag::SKESK,
                         &[], // set to accept
                         &[], // good
                         all_versions, // bad
        );

        Ok(())
    }

    #[test]
    #[allow(deprecated)]
    fn packet_tag_cutoff() {
        // The semantics of packet_tag_cutoff are: max of all
        // versioned cutoffs and the unversioned cutoff.

        let p = &mut StandardPolicy::new();
        p.reject_all_packet_tags();

        assert_eq!(p.packet_tag_cutoff(Tag::Signature),
                   REJECT.map(Into::into));

        p.reject_packet_tag_version_at(Tag::Signature, 5,
                                       Timestamp::Y2007M2);
        assert_eq!(p.packet_tag_cutoff(Tag::Signature),
                   Some(Timestamp::Y2007M2.into()));

        p.reject_packet_tag_version_at(Tag::Signature, 3,
                                       Timestamp::Y2005M2);
        assert_eq!(p.packet_tag_cutoff(Tag::Signature),
                   Some(Timestamp::Y2007M2.into()));

        p.reject_packet_tag_version_at(Tag::Signature, 6,
                                       ACCEPT.map(Into::into));
        assert_eq!(p.packet_tag_cutoff(Tag::Signature),
                   ACCEPT.map(Into::into));

        p.reject_packet_tag_version_at(Tag::Signature, 6,
                                       Timestamp::Y2005M2);
        assert_eq!(p.packet_tag_cutoff(Tag::Signature),
                   Some(Timestamp::Y2007M2.into()));
    }
}