      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

    // Copyright 2018 Developers of the Rand project.
    // Copyright 2014 The Rust Project Developers.
    //
    // 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.

    use core::fmt;
    use rand_core::{CryptoRng, RngCore, SeedableRng, Error, le};
    use rand_core::block::{BlockRngCore, BlockRng};

    const SEED_WORDS: usize = 8; // 8 words for the 256-bit key
    const STATE_WORDS: usize = 16;

    /// 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. Also it is very suitable for SIMD implementation.
    /// Here we do not provide a SIMD implementation yet, except for what is
    /// provided by auto-vectorisation.
    ///
    /// 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 20 rounds in this implementation, but hope to allow type-level
    /// configuration in the future.
    ///
    /// 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 [`set_word_pos`] and [`set_stream`].
    ///
    /// 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/)
    ///
    /// [`set_word_pos`]: #method.set_word_pos
    /// [`set_stream`]: #method.set_stream
    /// [`BlockRng`]: ../rand_core/block/struct.BlockRng.html
    /// [`RngCore`]: ../rand_core/trait.RngCore.html
    #[derive(Clone, Debug)]
    pub struct ChaChaRng(BlockRng<ChaChaCore>);

    impl RngCore for ChaChaRng {
        #[inline]
        fn next_u32(&mut self) -> u32 {
            self.0.next_u32()
        }

        #[inline]
        fn next_u64(&mut self) -> u64 {
            self.0.next_u64()
        }

        #[inline]
        fn fill_bytes(&mut self, dest: &mut [u8]) {
            self.0.fill_bytes(dest)
        }

        #[inline]
        fn try_fill_bytes(&mut self, dest: &mut [u8]) -> Result<(), Error> {
            self.0.try_fill_bytes(dest)
        }
    }

    impl SeedableRng for ChaChaRng {
        type Seed = <ChaChaCore as SeedableRng>::Seed;

        fn from_seed(seed: Self::Seed) -> Self {
            ChaChaRng(BlockRng::<ChaChaCore>::from_seed(seed))
        }

        fn from_rng<R: RngCore>(rng: R) -> Result<Self, Error> {
            BlockRng::<ChaChaCore>::from_rng(rng).map(ChaChaRng)
        }
    }

    impl CryptoRng for ChaChaRng {}

    impl ChaChaRng {
        /// 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.
        ///
        /// Note: this function is currently only available with Rust 1.26 or later.
        #[cfg(all(rustc_1_26, not(target_os = "emscripten")))]
        pub fn get_word_pos(&self) -> u128 {
            let mut c = (self.0.core.state[13] as u64) << 32
                      | (self.0.core.state[12] as u64);
            let mut index = self.0.index();
            // c is the end of the last block generated, unless index is at end
            if index >= STATE_WORDS {
                index = 0;
            } else {
                c = c.wrapping_sub(1);
            }
            ((c as u128) << 4) | (index as u128)
        }

        /// 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.
        ///
        /// Note: this function is currently only available with Rust 1.26 or later.
        #[cfg(all(rustc_1_26, not(target_os = "emscripten")))]
        pub fn set_word_pos(&mut self, word_offset: u128) {
            let index = (word_offset as usize) & 0xF;
            let counter = (word_offset >> 4) as u64;
            self.0.core.state[12] = counter as u32;
            self.0.core.state[13] = (counter >> 32) as u32;
            if index != 0 {
                self.0.generate_and_set(index); // also increments counter
            } else {
                self.0.reset();
            }
        }

        /// 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.
        pub fn set_stream(&mut self, stream: u64) {
            let index = self.0.index();
            self.0.core.state[14] = stream as u32;
            self.0.core.state[15] = (stream >> 32) as u32;
            if index < STATE_WORDS {
                // we need to regenerate a partial result buffer
                {
                    // reverse of counter adjustment in generate()
                    if self.0.core.state[12] == 0 {
                        self.0.core.state[13] = self.0.core.state[13].wrapping_sub(1);
                    }
                    self.0.core.state[12] = self.0.core.state[12].wrapping_sub(1);
                }
                self.0.generate_and_set(index);
            }
        }
    }

    /// The core of `ChaChaRng`, used with `BlockRng`.
    #[derive(Clone)]
    pub struct ChaChaCore {
        state: [u32; STATE_WORDS],
    }

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

    macro_rules! quarter_round{
        ($a: expr, $b: expr, $c: expr, $d: expr) => {{
            $a = $a.wrapping_add($b); $d ^= $a; $d = $d.rotate_left(16);
            $c = $c.wrapping_add($d); $b ^= $c; $b = $b.rotate_left(12);
            $a = $a.wrapping_add($b); $d ^= $a; $d = $d.rotate_left( 8);
            $c = $c.wrapping_add($d); $b ^= $c; $b = $b.rotate_left( 7);
        }}
    }

    macro_rules! double_round{
        ($x: expr) => {{
            // Column round
            quarter_round!($x[ 0], $x[ 4], $x[ 8], $x[12]);
            quarter_round!($x[ 1], $x[ 5], $x[ 9], $x[13]);
            quarter_round!($x[ 2], $x[ 6], $x[10], $x[14]);
            quarter_round!($x[ 3], $x[ 7], $x[11], $x[15]);
            // Diagonal round
            quarter_round!($x[ 0], $x[ 5], $x[10], $x[15]);
            quarter_round!($x[ 1], $x[ 6], $x[11], $x[12]);
            quarter_round!($x[ 2], $x[ 7], $x[ 8], $x[13]);
            quarter_round!($x[ 3], $x[ 4], $x[ 9], $x[14]);
        }}
    }

    impl BlockRngCore for ChaChaCore {
        type Item = u32;
        type Results = [u32; STATE_WORDS];

        fn generate(&mut self, results: &mut Self::Results) {
            // For some reason extracting this part into a separate function
            // improves performance by 50%.
            fn core(results: &mut [u32; STATE_WORDS],
                    state: &[u32; STATE_WORDS])
            {
                let mut tmp = *state;
                let rounds = 20;
                for _ in 0..rounds / 2 {
                    double_round!(tmp);
                }
                for i in 0..STATE_WORDS {
                    results[i] = tmp[i].wrapping_add(state[i]);
                }
            }

            core(results, &self.state);

            // update 64-bit counter
            self.state[12] = self.state[12].wrapping_add(1);
            if self.state[12] != 0 { return; };
            self.state[13] = self.state[13].wrapping_add(1);
        }
    }

    impl SeedableRng for ChaChaCore {
        type Seed = [u8; SEED_WORDS*4];

        fn from_seed(seed: Self::Seed) -> Self {
            let mut seed_le = [0u32; SEED_WORDS];
            le::read_u32_into(&seed, &mut seed_le);
            Self {
                state: [0x61707865, 0x3320646E, 0x79622D32, 0x6B206574, // constants
                        seed_le[0], seed_le[1], seed_le[2], seed_le[3], // seed
                        seed_le[4], seed_le[5], seed_le[6], seed_le[7], // seed
                        0, 0, 0, 0], // counter
             }
        }
    }

    impl CryptoRng for ChaChaCore {}

    impl From<ChaChaCore> for ChaChaRng {
        fn from(core: ChaChaCore) -> Self {
            ChaChaRng(BlockRng::new(core))
        }
    }

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

        #[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]
        #[cfg(all(rustc_1_26, not(target_os = "emscripten")))]
        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());
            }
        }
    }

# Help

## Keyboard Shortcuts

`?`
  Show this help dialog
`S`
  Focus the search field
`↑`
  Move up in search results
`↓`
  Move down in search results
`↹`
  Switch tab
`⏎`
  Go to active search result
`+`
  Expand all sections
`-`
  Collapse all sections

## Search Tricks

Prefix searches with a type followed by a colon (e.g., `fn:`) to restrict the search to a given type.

Accepted types are: `fn`, `mod`, `struct`, `enum`, `trait`, `type`, `macro`, and `const`.

Search functions by type signature (e.g., `vec -> usize` or `* -> vec`)

Search multiple things at once by splitting your query with comma (e.g., `str,u8` or `String,struct:Vec,test`)