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
// Copyright 2018 Developers of the Rand project.
//
// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
// https://www.apache.org/licenses/LICENSE-2.0> or the MIT license
// <LICENSE-MIT or https://opensource.org/licenses/MIT>, at your
// option. This file may not be copied, modified, or distributed
// except according to those terms.

//! The ChaCha random number generator.

#[cfg(not(feature = "std"))] use core;
#[cfg(feature = "std")] use std as core;

use self::core::fmt;
use crate::guts::ChaCha;
use rand_core::block::{BlockRng, BlockRngCore};
use rand_core::{CryptoRng, Error, RngCore, SeedableRng};

#[cfg(feature = "serde1")] use serde::{Serialize, Deserialize, Serializer, Deserializer};

// NB. this must remain consistent with some currently hard-coded numbers in this module
const BUF_BLOCKS: u8 = 4;
// number of 32-bit words per ChaCha block (fixed by algorithm definition)
const BLOCK_WORDS: u8 = 16;

#[repr(transparent)]
pub struct Array64<T>([T; 64]);
impl<T> Default for Array64<T>
where T: Default
{
    #[rustfmt::skip]
    fn default() -> Self {
        Self([
            T::default(), T::default(), T::default(), T::default(), T::default(), T::default(), T::default(), T::default(),
            T::default(), T::default(), T::default(), T::default(), T::default(), T::default(), T::default(), T::default(),
            T::default(), T::default(), T::default(), T::default(), T::default(), T::default(), T::default(), T::default(),
            T::default(), T::default(), T::default(), T::default(), T::default(), T::default(), T::default(), T::default(),
            T::default(), T::default(), T::default(), T::default(), T::default(), T::default(), T::default(), T::default(),
            T::default(), T::default(), T::default(), T::default(), T::default(), T::default(), T::default(), T::default(),
            T::default(), T::default(), T::default(), T::default(), T::default(), T::default(), T::default(), T::default(),
            T::default(), T::default(), T::default(), T::default(), T::default(), T::default(), T::default(), T::default(),
        ])
    }
}
impl<T> AsRef<[T]> for Array64<T> {
    fn as_ref(&self) -> &[T] {
        &self.0
    }
}
impl<T> AsMut<[T]> for Array64<T> {
    fn as_mut(&mut self) -> &mut [T] {
        &mut self.0
    }
}
impl<T> Clone for Array64<T>
where T: Copy + Default
{
    fn clone(&self) -> Self {
        let mut new = Self::default();
        new.0.copy_from_slice(&self.0);
        new
    }
}
impl<T> fmt::Debug for Array64<T> {
    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
        write!(f, "Array64 {{}}")
    }
}

macro_rules! chacha_impl {
    ($ChaChaXCore:ident, $ChaChaXRng:ident, $rounds:expr, $doc:expr, $abst:ident) => {
        #[doc=$doc]
        #[derive(Clone, PartialEq, Eq)]
        pub struct $ChaChaXCore {
            state: ChaCha,
        }

        // Custom Debug implementation that does not expose the internal state
        impl fmt::Debug for $ChaChaXCore {
            fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
                write!(f, "ChaChaXCore {{}}")
            }
        }

        impl BlockRngCore for $ChaChaXCore {
            type Item = u32;
            type Results = Array64<u32>;
            #[inline]
            fn generate(&mut self, r: &mut Self::Results) {
                // Fill slice of words by writing to equivalent slice of bytes, then fixing endianness.
                self.state.refill4($rounds, unsafe {
                    &mut *(&mut *r as *mut Array64<u32> as *mut [u8; 256])
                });
                for x in r.as_mut() {
                    *x = x.to_le();
                }
            }
        }

        impl SeedableRng for $ChaChaXCore {
            type Seed = [u8; 32];
            #[inline]
            fn from_seed(seed: Self::Seed) -> Self {
                $ChaChaXCore { state: ChaCha::new(&seed, &[0u8; 8]) }
            }
        }

        impl CryptoRng for $ChaChaXCore {}

        /// A cryptographically secure random number generator that uses the ChaCha algorithm.
        ///
        /// ChaCha is a stream cipher designed by Daniel J. Bernstein[^1], that we use as an RNG. It is
        /// an improved variant of the Salsa20 cipher family, which was selected as one of the "stream
        /// ciphers suitable for widespread adoption" by eSTREAM[^2].
        ///
        /// ChaCha uses add-rotate-xor (ARX) operations as its basis. These are safe against timing
        /// attacks, although that is mostly a concern for ciphers and not for RNGs. We provide a SIMD
        /// implementation to support high throughput on a variety of common hardware platforms.
        ///
        /// With the ChaCha algorithm it is possible to choose the number of rounds the core algorithm
        /// should run. The number of rounds is a tradeoff between performance and security, where 8
        /// rounds is the minimum potentially secure configuration, and 20 rounds is widely used as a
        /// conservative choice.
        ///
        /// We use a 64-bit counter and 64-bit stream identifier as in Bernstein's implementation[^1]
        /// except that we use a stream identifier in place of a nonce. A 64-bit counter over 64-byte
        /// (16 word) blocks allows 1 ZiB of output before cycling, and the stream identifier allows
        /// 2<sup>64</sup> unique streams of output per seed. Both counter and stream are initialized
        /// to zero but may be set via the `set_word_pos` and `set_stream` methods.
        ///
        /// The word layout is:
        ///
        /// ```text
        /// constant  constant  constant  constant
        /// seed      seed      seed      seed
        /// seed      seed      seed      seed
        /// counter   counter   stream_id stream_id
        /// ```
        ///
        /// This implementation uses an output buffer of sixteen `u32` words, and uses
        /// [`BlockRng`] to implement the [`RngCore`] methods.
        ///
        /// [^1]: D. J. Bernstein, [*ChaCha, a variant of Salsa20*](
        ///       https://cr.yp.to/chacha.html)
        ///
        /// [^2]: [eSTREAM: the ECRYPT Stream Cipher Project](
        ///       http://www.ecrypt.eu.org/stream/)
        #[derive(Clone, Debug)]
        pub struct $ChaChaXRng {
            rng: BlockRng<$ChaChaXCore>,
        }

        impl SeedableRng for $ChaChaXRng {
            type Seed = [u8; 32];
            #[inline]
            fn from_seed(seed: Self::Seed) -> Self {
                let core = $ChaChaXCore::from_seed(seed);
                Self {
                    rng: BlockRng::new(core),
                }
            }
        }

        impl RngCore for $ChaChaXRng {
            #[inline]
            fn next_u32(&mut self) -> u32 {
                self.rng.next_u32()
            }
            #[inline]
            fn next_u64(&mut self) -> u64 {
                self.rng.next_u64()
            }
            #[inline]
            fn fill_bytes(&mut self, bytes: &mut [u8]) {
                self.rng.fill_bytes(bytes)
            }
            #[inline]
            fn try_fill_bytes(&mut self, bytes: &mut [u8]) -> Result<(), Error> {
                self.rng.try_fill_bytes(bytes)
            }
        }

        impl $ChaChaXRng {
            // The buffer is a 4-block window, i.e. it is always at a block-aligned position in the
            // stream but if the stream has been seeked it may not be self-aligned.

            /// Get the offset from the start of the stream, in 32-bit words.
            ///
            /// Since the generated blocks are 16 words (2<sup>4</sup>) long and the
            /// counter is 64-bits, the offset is a 68-bit number. Sub-word offsets are
            /// not supported, hence the result can simply be multiplied by 4 to get a
            /// byte-offset.
            #[inline]
            pub fn get_word_pos(&self) -> u128 {
                let buf_start_block = {
                    let buf_end_block = self.rng.core.state.get_block_pos();
                    u64::wrapping_sub(buf_end_block, BUF_BLOCKS.into())
                };
                let (buf_offset_blocks, block_offset_words) = {
                    let buf_offset_words = self.rng.index() as u64;
                    let blocks_part = buf_offset_words / u64::from(BLOCK_WORDS);
                    let words_part = buf_offset_words % u64::from(BLOCK_WORDS);
                    (blocks_part, words_part)
                };
                let pos_block = u64::wrapping_add(buf_start_block, buf_offset_blocks);
                let pos_block_words = u128::from(pos_block) * u128::from(BLOCK_WORDS);
                pos_block_words + u128::from(block_offset_words)
            }

            /// Set the offset from the start of the stream, in 32-bit words.
            ///
            /// As with `get_word_pos`, we use a 68-bit number. Since the generator
            /// simply cycles at the end of its period (1 ZiB), we ignore the upper
            /// 60 bits.
            #[inline]
            pub fn set_word_pos(&mut self, word_offset: u128) {
                let block = (word_offset / u128::from(BLOCK_WORDS)) as u64;
                self.rng
                    .core
                    .state
                    .set_block_pos(block);
                self.rng.generate_and_set((word_offset % u128::from(BLOCK_WORDS)) as usize);
            }

            /// Set the stream number.
            ///
            /// This is initialized to zero; 2<sup>64</sup> unique streams of output
            /// are available per seed/key.
            ///
            /// Note that in order to reproduce ChaCha output with a specific 64-bit
            /// nonce, one can convert that nonce to a `u64` in little-endian fashion
            /// and pass to this function. In theory a 96-bit nonce can be used by
            /// passing the last 64-bits to this function and using the first 32-bits as
            /// the most significant half of the 64-bit counter (which may be set
            /// indirectly via `set_word_pos`), but this is not directly supported.
            #[inline]
            pub fn set_stream(&mut self, stream: u64) {
                self.rng
                    .core
                    .state
                    .set_nonce(stream);
                if self.rng.index() != 64 {
                    let wp = self.get_word_pos();
                    self.set_word_pos(wp);
                }
            }

            /// Get the stream number.
            #[inline]
            pub fn get_stream(&self) -> u64 {
                self.rng
                    .core
                    .state
                    .get_nonce()
            }

            /// Get the seed.
            #[inline]
            pub fn get_seed(&self) -> [u8; 32] {
                self.rng
                    .core
                    .state
                    .get_seed()
            }
        }

        impl CryptoRng for $ChaChaXRng {}

        impl From<$ChaChaXCore> for $ChaChaXRng {
            fn from(core: $ChaChaXCore) -> Self {
                $ChaChaXRng {
                    rng: BlockRng::new(core),
                }
            }
        }

        impl PartialEq<$ChaChaXRng> for $ChaChaXRng {
            fn eq(&self, rhs: &$ChaChaXRng) -> bool {
                let a: $abst::$ChaChaXRng = self.into();
                let b: $abst::$ChaChaXRng = rhs.into();
                a == b
            }
        }
        impl Eq for $ChaChaXRng {}

        #[cfg(feature = "serde1")]
        impl Serialize for $ChaChaXRng {
            fn serialize<S>(&self, s: S) -> Result<S::Ok, S::Error>
            where S: Serializer {
                $abst::$ChaChaXRng::from(self).serialize(s)
            }
        }
        #[cfg(feature = "serde1")]
        impl<'de> Deserialize<'de> for $ChaChaXRng {
            fn deserialize<D>(d: D) -> Result<Self, D::Error> where D: Deserializer<'de> {
                $abst::$ChaChaXRng::deserialize(d).map(|x| Self::from(&x))
            }
        }

        mod $abst {
            #[cfg(feature = "serde1")] use serde::{Serialize, Deserialize};

            // The abstract state of a ChaCha stream, independent of implementation choices. The
            // comparison and serialization of this object is considered a semver-covered part of
            // the API.
            #[derive(Debug, PartialEq, Eq)]
            #[cfg_attr(
                feature = "serde1",
                derive(Serialize, Deserialize),
            )]
            pub(crate) struct $ChaChaXRng {
                seed: [u8; 32],
                stream: u64,
                word_pos: u128,
            }

            impl From<&super::$ChaChaXRng> for $ChaChaXRng {
                // Forget all information about the input except what is necessary to determine the
                // outputs of any sequence of pub API calls.
                fn from(r: &super::$ChaChaXRng) -> Self {
                    Self {
                        seed: r.get_seed(),
                        stream: r.get_stream(),
                        word_pos: r.get_word_pos(),
                    }
                }
            }

            impl From<&$ChaChaXRng> for super::$ChaChaXRng {
                // Construct one of the possible concrete RNGs realizing an abstract state.
                fn from(a: &$ChaChaXRng) -> Self {
                    use rand_core::SeedableRng;
                    let mut r = Self::from_seed(a.seed);
                    r.set_stream(a.stream);
                    r.set_word_pos(a.word_pos);
                    r
                }
            }
        }
    }
}

chacha_impl!(ChaCha20Core, ChaCha20Rng, 10, "ChaCha with 20 rounds", abstract20);
chacha_impl!(ChaCha12Core, ChaCha12Rng, 6, "ChaCha with 12 rounds", abstract12);
chacha_impl!(ChaCha8Core, ChaCha8Rng, 4, "ChaCha with 8 rounds", abstract8);

#[cfg(test)]
mod test {
    use rand_core::{RngCore, SeedableRng};

    #[cfg(feature = "serde1")] use super::{ChaCha20Rng, ChaCha12Rng, ChaCha8Rng};

    type ChaChaRng = super::ChaCha20Rng;

    #[cfg(feature = "serde1")]
    #[test]
    fn test_chacha_serde_roundtrip() {
        let seed = [
            1, 0, 52, 0, 0, 0, 0, 0, 1, 0, 10, 0, 22, 32, 0, 0, 2, 0, 55, 49, 0, 11, 0, 0, 3, 0, 0, 0, 0,
            0, 2, 92,
        ];
        let mut rng1 = ChaCha20Rng::from_seed(seed);
        let mut rng2 = ChaCha12Rng::from_seed(seed);
        let mut rng3 = ChaCha8Rng::from_seed(seed);

        let encoded1 = serde_json::to_string(&rng1).unwrap();
        let encoded2 = serde_json::to_string(&rng2).unwrap();
        let encoded3 = serde_json::to_string(&rng3).unwrap();

        let mut decoded1: ChaCha20Rng = serde_json::from_str(&encoded1).unwrap();
        let mut decoded2: ChaCha12Rng = serde_json::from_str(&encoded2).unwrap();
        let mut decoded3: ChaCha8Rng = serde_json::from_str(&encoded3).unwrap();

        assert_eq!(rng1, decoded1);
        assert_eq!(rng2, decoded2);
        assert_eq!(rng3, decoded3);

        assert_eq!(rng1.next_u32(), decoded1.next_u32());
        assert_eq!(rng2.next_u32(), decoded2.next_u32());
        assert_eq!(rng3.next_u32(), decoded3.next_u32());
    }

    // This test validates that:
    // 1. a hard-coded serialization demonstrating the format at time of initial release can still
    //    be deserialized to a ChaChaRng
    // 2. re-serializing the resultant object produces exactly the original string
    //
    // Condition 2 is stronger than necessary: an equivalent serialization (e.g. with field order
    // permuted, or whitespace differences) would also be admissible, but would fail this test.
    // However testing for equivalence of serialized data is difficult, and there shouldn't be any
    // reason we need to violate the stronger-than-needed condition, e.g. by changing the field
    // definition order.
    #[cfg(feature = "serde1")]
    #[test]
    fn test_chacha_serde_format_stability() {
        let j = r#"{"seed":[4,8,15,16,23,42,4,8,15,16,23,42,4,8,15,16,23,42,4,8,15,16,23,42,4,8,15,16,23,42,4,8],"stream":27182818284,"word_pos":314159265359}"#;
        let r: ChaChaRng = serde_json::from_str(&j).unwrap();
        let j1 = serde_json::to_string(&r).unwrap();
        assert_eq!(j, j1);
    }

    #[test]
    fn test_chacha_construction() {
        let seed = [
            0, 0, 0, 0, 0, 0, 0, 0, 1, 0, 0, 0, 0, 0, 0, 0, 2, 0, 0, 0, 0, 0, 0, 0, 3, 0, 0, 0, 0,
            0, 0, 0,
        ];
        let mut rng1 = ChaChaRng::from_seed(seed);
        assert_eq!(rng1.next_u32(), 137206642);

        let mut rng2 = ChaChaRng::from_rng(rng1).unwrap();
        assert_eq!(rng2.next_u32(), 1325750369);
    }

    #[test]
    fn test_chacha_true_values_a() {
        // Test vectors 1 and 2 from
        // https://tools.ietf.org/html/draft-nir-cfrg-chacha20-poly1305-04
        let seed = [0u8; 32];
        let mut rng = ChaChaRng::from_seed(seed);

        let mut results = [0u32; 16];
        for i in results.iter_mut() {
            *i = rng.next_u32();
        }
        let expected = [
            0xade0b876, 0x903df1a0, 0xe56a5d40, 0x28bd8653, 0xb819d2bd, 0x1aed8da0, 0xccef36a8,
            0xc70d778b, 0x7c5941da, 0x8d485751, 0x3fe02477, 0x374ad8b8, 0xf4b8436a, 0x1ca11815,
            0x69b687c3, 0x8665eeb2,
        ];
        assert_eq!(results, expected);

        for i in results.iter_mut() {
            *i = rng.next_u32();
        }
        let expected = [
            0xbee7079f, 0x7a385155, 0x7c97ba98, 0x0d082d73, 0xa0290fcb, 0x6965e348, 0x3e53c612,
            0xed7aee32, 0x7621b729, 0x434ee69c, 0xb03371d5, 0xd539d874, 0x281fed31, 0x45fb0a51,
            0x1f0ae1ac, 0x6f4d794b,
        ];
        assert_eq!(results, expected);
    }

    #[test]
    fn test_chacha_true_values_b() {
        // Test vector 3 from
        // https://tools.ietf.org/html/draft-nir-cfrg-chacha20-poly1305-04
        let seed = [
            0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0,
            0, 0, 1,
        ];
        let mut rng = ChaChaRng::from_seed(seed);

        // Skip block 0
        for _ in 0..16 {
            rng.next_u32();
        }

        let mut results = [0u32; 16];
        for i in results.iter_mut() {
            *i = rng.next_u32();
        }
        let expected = [
            0x2452eb3a, 0x9249f8ec, 0x8d829d9b, 0xddd4ceb1, 0xe8252083, 0x60818b01, 0xf38422b8,
            0x5aaa49c9, 0xbb00ca8e, 0xda3ba7b4, 0xc4b592d1, 0xfdf2732f, 0x4436274e, 0x2561b3c8,
            0xebdd4aa6, 0xa0136c00,
        ];
        assert_eq!(results, expected);
    }

    #[test]
    fn test_chacha_true_values_c() {
        // Test vector 4 from
        // https://tools.ietf.org/html/draft-nir-cfrg-chacha20-poly1305-04
        let seed = [
            0, 0xff, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0,
            0, 0, 0, 0,
        ];
        let expected = [
            0xfb4dd572, 0x4bc42ef1, 0xdf922636, 0x327f1394, 0xa78dea8f, 0x5e269039, 0xa1bebbc1,
            0xcaf09aae, 0xa25ab213, 0x48a6b46c, 0x1b9d9bcb, 0x092c5be6, 0x546ca624, 0x1bec45d5,
            0x87f47473, 0x96f0992e,
        ];
        let expected_end = 3 * 16;
        let mut results = [0u32; 16];

        // Test block 2 by skipping block 0 and 1
        let mut rng1 = ChaChaRng::from_seed(seed);
        for _ in 0..32 {
            rng1.next_u32();
        }
        for i in results.iter_mut() {
            *i = rng1.next_u32();
        }
        assert_eq!(results, expected);
        assert_eq!(rng1.get_word_pos(), expected_end);

        // Test block 2 by using `set_word_pos`
        let mut rng2 = ChaChaRng::from_seed(seed);
        rng2.set_word_pos(2 * 16);
        for i in results.iter_mut() {
            *i = rng2.next_u32();
        }
        assert_eq!(results, expected);
        assert_eq!(rng2.get_word_pos(), expected_end);

        // Test skipping behaviour with other types
        let mut buf = [0u8; 32];
        rng2.fill_bytes(&mut buf[..]);
        assert_eq!(rng2.get_word_pos(), expected_end + 8);
        rng2.fill_bytes(&mut buf[0..25]);
        assert_eq!(rng2.get_word_pos(), expected_end + 15);
        rng2.next_u64();
        assert_eq!(rng2.get_word_pos(), expected_end + 17);
        rng2.next_u32();
        rng2.next_u64();
        assert_eq!(rng2.get_word_pos(), expected_end + 20);
        rng2.fill_bytes(&mut buf[0..1]);
        assert_eq!(rng2.get_word_pos(), expected_end + 21);
    }

    #[test]
    fn test_chacha_multiple_blocks() {
        let seed = [
            0, 0, 0, 0, 1, 0, 0, 0, 2, 0, 0, 0, 3, 0, 0, 0, 4, 0, 0, 0, 5, 0, 0, 0, 6, 0, 0, 0, 7,
            0, 0, 0,
        ];
        let mut rng = ChaChaRng::from_seed(seed);

        // Store the 17*i-th 32-bit word,
        // i.e., the i-th word of the i-th 16-word block
        let mut results = [0u32; 16];
        for i in results.iter_mut() {
            *i = rng.next_u32();
            for _ in 0..16 {
                rng.next_u32();
            }
        }
        let expected = [
            0xf225c81a, 0x6ab1be57, 0x04d42951, 0x70858036, 0x49884684, 0x64efec72, 0x4be2d186,
            0x3615b384, 0x11cfa18e, 0xd3c50049, 0x75c775f6, 0x434c6530, 0x2c5bad8f, 0x898881dc,
            0x5f1c86d9, 0xc1f8e7f4,
        ];
        assert_eq!(results, expected);
    }

    #[test]
    fn test_chacha_true_bytes() {
        let seed = [0u8; 32];
        let mut rng = ChaChaRng::from_seed(seed);
        let mut results = [0u8; 32];
        rng.fill_bytes(&mut results);
        let expected = [
            118, 184, 224, 173, 160, 241, 61, 144, 64, 93, 106, 229, 83, 134, 189, 40, 189, 210,
            25, 184, 160, 141, 237, 26, 168, 54, 239, 204, 139, 119, 13, 199,
        ];
        assert_eq!(results, expected);
    }

    #[test]
    fn test_chacha_nonce() {
        // Test vector 5 from
        // https://tools.ietf.org/html/draft-nir-cfrg-chacha20-poly1305-04
        // Although we do not support setting a nonce, we try it here anyway so
        // we can use this test vector.
        let seed = [0u8; 32];
        let mut rng = ChaChaRng::from_seed(seed);
        // 96-bit nonce in LE order is: 0,0,0,0, 0,0,0,0, 0,0,0,2
        rng.set_stream(2u64 << (24 + 32));

        let mut results = [0u32; 16];
        for i in results.iter_mut() {
            *i = rng.next_u32();
        }
        let expected = [
            0x374dc6c2, 0x3736d58c, 0xb904e24a, 0xcd3f93ef, 0x88228b1a, 0x96a4dfb3, 0x5b76ab72,
            0xc727ee54, 0x0e0e978a, 0xf3145c95, 0x1b748ea8, 0xf786c297, 0x99c28f5f, 0x628314e8,
            0x398a19fa, 0x6ded1b53,
        ];
        assert_eq!(results, expected);
    }

    #[test]
    fn test_chacha_clone_streams() {
        let seed = [
            0, 0, 0, 0, 1, 0, 0, 0, 2, 0, 0, 0, 3, 0, 0, 0, 4, 0, 0, 0, 5, 0, 0, 0, 6, 0, 0, 0, 7,
            0, 0, 0,
        ];
        let mut rng = ChaChaRng::from_seed(seed);
        let mut clone = rng.clone();
        for _ in 0..16 {
            assert_eq!(rng.next_u64(), clone.next_u64());
        }

        rng.set_stream(51);
        for _ in 0..7 {
            assert!(rng.next_u32() != clone.next_u32());
        }
        clone.set_stream(51); // switch part way through block
        for _ in 7..16 {
            assert_eq!(rng.next_u32(), clone.next_u32());
        }
    }

    #[test]
    fn test_chacha_word_pos_wrap_exact() {
        use super::{BUF_BLOCKS, BLOCK_WORDS};
        let mut rng = ChaChaRng::from_seed(Default::default());
        // refilling the buffer in set_word_pos will wrap the block counter to 0
        let last_block = (1 << 68) - u128::from(BUF_BLOCKS * BLOCK_WORDS);
        rng.set_word_pos(last_block);
        assert_eq!(rng.get_word_pos(), last_block);
    }

    #[test]
    fn test_chacha_word_pos_wrap_excess() {
        use super::BLOCK_WORDS;
        let mut rng = ChaChaRng::from_seed(Default::default());
        // refilling the buffer in set_word_pos will wrap the block counter past 0
        let last_block = (1 << 68) - u128::from(BLOCK_WORDS);
        rng.set_word_pos(last_block);
        assert_eq!(rng.get_word_pos(), last_block);
    }

    #[test]
    fn test_chacha_word_pos_zero() {
        let mut rng = ChaChaRng::from_seed(Default::default());
        assert_eq!(rng.get_word_pos(), 0);
        rng.set_word_pos(0);
        assert_eq!(rng.get_word_pos(), 0);
    }
}