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());
        }
    }
}