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
//! HMAC — Hash-based Message Authentication Code (RFC 2104), generic over any
//! [`Digest`].
use super::{Digest, Mac};
use crate::ct::{Choice, ConstantTimeEq};
const IPAD: u8 = 0x36;
const OPAD: u8 = 0x5c;
/// HMAC keyed with a hash function `D`.
///
/// `HMAC(K, m) = H((K' ^ opad) || H((K' ^ ipad) || m))`, where `K'` is the key
/// reduced to a single block: hashed first if longer than the block size, then
/// zero-padded.
///
/// ```
/// use purecrypto::hash::HmacSha256;
/// let tag = HmacSha256::mac(b"key", b"message");
/// assert!(bool::from(HmacSha256::new(b"key").chain(b"message").verify(&tag)));
/// ```
#[derive(Clone)]
pub struct Hmac<D: Digest> {
/// Hasher fed `K' ^ ipad`, then the message.
inner: D,
/// Hasher fed `K' ^ opad`, finalized over the inner digest at the end.
outer: D,
}
impl<D: Digest> Hmac<D> {
/// Creates an HMAC instance keyed with `key`.
pub fn new(key: &[u8]) -> Self {
// Reduce the key to a single zero-padded block.
let mut block = D::zeroed_block();
let buf = block.as_mut();
if key.len() > buf.len() {
let hashed = D::digest(key);
let h = hashed.as_ref();
buf[..h.len()].copy_from_slice(h);
} else {
buf[..key.len()].copy_from_slice(key);
}
let mut ipad_block = block;
let mut opad_block = block;
for b in ipad_block.as_mut() {
*b ^= IPAD;
}
for b in opad_block.as_mut() {
*b ^= OPAD;
}
let mut inner = D::new();
inner.update(ipad_block.as_ref());
let mut outer = D::new();
outer.update(opad_block.as_ref());
Hmac { inner, outer }
}
/// Feeds `data` into the MAC. May be called any number of times.
#[inline]
pub fn update(&mut self, data: &[u8]) {
self.inner.update(data);
}
/// Feeds `data` and returns `self`, for call chaining.
#[inline]
pub fn chain(mut self, data: &[u8]) -> Self {
self.update(data);
self
}
/// Consumes the MAC and returns the authentication tag.
#[inline]
pub fn finalize(mut self) -> D::Output {
// Extract the hashers rather than moving them out of `self`, which the
// `Drop` impl forbids; the leftover fresh hashers are wiped on drop.
let inner = core::mem::replace(&mut self.inner, D::new()).finalize();
let mut outer = core::mem::replace(&mut self.outer, D::new());
outer.update(inner.as_ref());
outer.finalize()
}
/// Consumes the MAC and checks it against `expected` in constant time.
///
/// The comparison time depends only on the (public) tag length, not on
/// where a mismatch occurs — avoiding the timing leak of a byte-by-byte
/// `==`.
#[inline]
pub fn verify(self, expected: &[u8]) -> Choice {
let tag = self.finalize();
tag.as_ref().ct_eq(expected)
}
/// Computes the tag for `data` under `key` in one call.
#[inline]
pub fn mac(key: &[u8], data: &[u8]) -> D::Output {
let mut h = Self::new(key);
h.update(data);
h.finalize()
}
}
impl<D: Digest> Drop for Hmac<D> {
fn drop(&mut self) {
// Wipe the key-derived inner/outer hash state.
self.inner.zeroize();
self.outer.zeroize();
}
}
impl<D: Digest> Mac for Hmac<D> {
// HMAC is a fixed-output MAC: its tag is exactly the digest length. This
// makes the default `Mac::verify` length-strict (rejecting truncated tags)
// for code that reaches the MAC through the trait.
const OUTPUT_LEN: Option<usize> = Some(D::OUTPUT_LEN);
#[inline]
fn update(&mut self, data: &[u8]) {
Hmac::update(self, data);
}
/// Writes the full HMAC tag, truncated to `out.len()` if it is shorter than
/// the digest length.
#[inline]
fn finalize_into(self, out: &mut [u8]) {
let tag = self.finalize();
let t = tag.as_ref();
let n = out.len().min(t.len());
out[..n].copy_from_slice(&t[..n]);
}
#[inline]
fn verify(self, expected: &[u8]) -> Choice {
Hmac::verify(self, expected)
}
}
/// HMAC-SHA-224.
pub type HmacSha224 = Hmac<super::Sha224>;
/// HMAC-SHA-256.
pub type HmacSha256 = Hmac<super::Sha256>;
/// HMAC-SHA-384.
pub type HmacSha384 = Hmac<super::Sha384>;
/// HMAC-SHA-512.
pub type HmacSha512 = Hmac<super::Sha512>;
/// HMAC-SHA-512/224.
pub type HmacSha512_224 = Hmac<super::Sha512_224>;
/// HMAC-SHA-512/256.
pub type HmacSha512_256 = Hmac<super::Sha512_256>;
#[cfg(test)]
mod tests {
use super::*;
use crate::test_util::from_hex;
// RFC 4231 test vectors.
#[test]
fn rfc4231_tc1() {
// 20-byte key, short message.
let key = [0x0bu8; 20];
let data = b"Hi There";
assert_eq!(
HmacSha256::mac(&key, data),
from_hex::<32>("b0344c61d8db38535ca8afceaf0bf12b881dc200c9833da726e9376c2e32cff7")
);
assert_eq!(
HmacSha224::mac(&key, data),
from_hex::<28>("896fb1128abbdf196832107cd49df33f47b4b1169912ba4f53684b22")
);
assert_eq!(
HmacSha384::mac(&key, data),
from_hex::<48>(
"afd03944d84895626b0825f4ab46907f15f9dadbe4101ec682aa034c7cebc59c\
faea9ea9076ede7f4af152e8b2fa9cb6"
)
);
assert_eq!(
HmacSha512::mac(&key, data),
from_hex::<64>(
"87aa7cdea5ef619d4ff0b4241a1d6cb02379f4e2ce4ec2787ad0b30545e17cde\
daa833b7d6b8a702038b274eaea3f4e4be9d914eeb61f1702e696c203a126854"
)
);
}
#[test]
fn rfc4231_tc2() {
// 4-byte key.
let key = b"Jefe";
let data = b"what do ya want for nothing?";
assert_eq!(
HmacSha256::mac(key, data),
from_hex::<32>("5bdcc146bf60754e6a042426089575c75a003f089d2739839dec58b964ec3843")
);
assert_eq!(
HmacSha512::mac(key, data),
from_hex::<64>(
"164b7a7bfcf819e2e395fbe73b56e0a387bd64222e831fd610270cd7ea250554\
9758bf75c05a994a6d034f65f8f0e6fdcaeab1a34d4a6b4b636e070a38bce737"
)
);
}
#[test]
fn rfc4231_tc6_long_key() {
// 131-byte key (> 64-byte block) forces the hash-the-key path.
let key = [0xaau8; 131];
let data = b"Test Using Larger Than Block-Size Key - Hash Key First";
assert_eq!(
HmacSha256::mac(&key, data),
from_hex::<32>("60e431591ee0b67f0d8a26aacbf5b77f8e0bc6213728c5140546040f0ee37f54")
);
}
#[test]
fn streaming_matches_oneshot() {
let key = b"secret key";
let msg = b"The quick brown fox jumps over the lazy dog";
let oneshot = HmacSha256::mac(key, msg);
let mut h = HmacSha256::new(key);
for &byte in msg {
h.update(&[byte]);
}
assert_eq!(h.finalize(), oneshot);
}
#[test]
fn verify_constant_time() {
let key = b"k";
let msg = b"data";
let tag = HmacSha256::mac(key, msg);
assert!(bool::from(HmacSha256::new(key).chain(msg).verify(&tag)));
// A flipped bit must fail.
let mut bad = tag;
bad[0] ^= 1;
assert!(!bool::from(HmacSha256::new(key).chain(msg).verify(&bad)));
// Wrong length must fail.
assert!(!bool::from(
HmacSha256::new(key).chain(msg).verify(&tag[..31])
));
}
#[test]
fn trait_verify_rejects_truncated_tag() {
use crate::hash::Mac;
let key = b"k";
let msg = b"data";
let tag = HmacSha256::mac(key, msg);
// The full-length tag verifies through the trait path.
let m = HmacSha256::new(key).chain(msg);
assert!(bool::from(Mac::verify(m, &tag)));
// A truncated tag must be rejected via the trait path: the default
// `Mac::verify` is length-strict for fixed-output MACs, so checking
// only a prefix of the tag cannot forge a match.
for trunc in [16usize, 24, 31] {
let m = HmacSha256::new(key).chain(msg);
assert!(
!bool::from(Mac::verify(m, &tag[..trunc])),
"truncated tag of len {trunc} was accepted"
);
}
// A trailing-zero-padded over-length tag must also fail.
let mut over = [0u8; 40];
over[..tag.len()].copy_from_slice(&tag);
let m = HmacSha256::new(key).chain(msg);
assert!(!bool::from(Mac::verify(m, &over)));
}
}