src/librand/chacha.rs

   1 // Copyright 2014 The Rust Project Developers. See the COPYRIGHT
   2 // file at the top-level directory of this distribution and at
   3 // http://rust-lang.org/COPYRIGHT.
   4 //
   5 // Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
   6 // http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
   7 // <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
   8 // option. This file may not be copied, modified, or distributed
   9 // except according to those terms.
  10
  11 //! The ChaCha random number generator.
  12
  13 use core::prelude::*;
  14 use {Rng, SeedableRng, Rand};
  15
  16 const KEY_WORDS    : usize =  8; // 8 words for the 256-bit key
  17 const STATE_WORDS  : usize = 16;
  18 const CHACHA_ROUNDS: usize = 20; // Cryptographically secure from 8 upwards as of this writing
  19
  20 /// A random number generator that uses the ChaCha20 algorithm [1].
  21 ///
  22 /// The ChaCha algorithm is widely accepted as suitable for
  23 /// cryptographic purposes, but this implementation has not been
  24 /// verified as such. Prefer a generator like `OsRng` that defers to
  25 /// the operating system for cases that need high security.
  26 ///
  27 /// [1]: D. J. Bernstein, [*ChaCha, a variant of
  28 /// Salsa20*](http://cr.yp.to/chacha.html)
  29 #[derive(Copy, Clone)]
  30 pub struct ChaChaRng {
  31     buffer:  [u32; STATE_WORDS], // Internal buffer of output
  32     state:   [u32; STATE_WORDS], // Initial state
  33     index:   usize,                 // Index into state
  34 }
  35
  36 static EMPTY: ChaChaRng = ChaChaRng {
  37     buffer:  [0; STATE_WORDS],
  38     state:   [0; STATE_WORDS],
  39     index:   STATE_WORDS
  40 };
  41
  42
  43 macro_rules! quarter_round{
  44     ($a: expr, $b: expr, $c: expr, $d: expr) => {{
  45         $a = $a.wrapping_add($b); $d = $d ^ $a; $d = $d.rotate_left(16);
  46         $c = $c.wrapping_add($d); $b = $b ^ $c; $b = $b.rotate_left(12);
  47         $a = $a.wrapping_add($b); $d = $d ^ $a; $d = $d.rotate_left( 8);
  48         $c = $c.wrapping_add($d); $b = $b ^ $c; $b = $b.rotate_left( 7);
  49     }}
  50 }
  51
  52 macro_rules! double_round{
  53     ($x: expr) => {{
  54         // Column round
  55         quarter_round!($x[ 0], $x[ 4], $x[ 8], $x[12]);
  56         quarter_round!($x[ 1], $x[ 5], $x[ 9], $x[13]);
  57         quarter_round!($x[ 2], $x[ 6], $x[10], $x[14]);
  58         quarter_round!($x[ 3], $x[ 7], $x[11], $x[15]);
  59         // Diagonal round
  60         quarter_round!($x[ 0], $x[ 5], $x[10], $x[15]);
  61         quarter_round!($x[ 1], $x[ 6], $x[11], $x[12]);
  62         quarter_round!($x[ 2], $x[ 7], $x[ 8], $x[13]);
  63         quarter_round!($x[ 3], $x[ 4], $x[ 9], $x[14]);
  64     }}
  65 }
  66
  67 #[inline]
  68 fn core(output: &mut [u32; STATE_WORDS], input: &[u32; STATE_WORDS]) {
  69     *output = *input;
  70
  71     for _ in 0..CHACHA_ROUNDS / 2 {
  72         double_round!(output);
  73     }
  74
  75     for i in 0..STATE_WORDS {
  76         output[i] = output[i].wrapping_add(input[i]);
  77     }
  78 }
  79
  80 impl ChaChaRng {
  81
  82     /// Create an ChaCha random number generator using the default
  83     /// fixed key of 8 zero words.
  84     pub fn new_unseeded() -> ChaChaRng {
  85         let mut rng = EMPTY;
  86         rng.init(&[0; KEY_WORDS]);
  87         rng
  88     }
  89
  90     /// Sets the internal 128-bit ChaCha counter to
  91     /// a user-provided value. This permits jumping
  92     /// arbitrarily ahead (or backwards) in the pseudorandom stream.
  93     ///
  94     /// Since the nonce words are used to extend the counter to 128 bits,
  95     /// users wishing to obtain the conventional ChaCha pseudorandom stream
  96     /// associated with a particular nonce can call this function with
  97     /// arguments `0, desired_nonce`.
  98     pub fn set_counter(&mut self, counter_low: u64, counter_high: u64) {
  99         self.state[12] = (counter_low >>  0) as u32;
 100         self.state[13] = (counter_low >> 32) as u32;
 101         self.state[14] = (counter_high >>  0) as u32;
 102         self.state[15] = (counter_high >> 32) as u32;
 103         self.index = STATE_WORDS; // force recomputation
 104     }
 105
 106     /// Initializes `self.state` with the appropriate key and constants
 107     ///
 108     /// We deviate slightly from the ChaCha specification regarding
 109     /// the nonce, which is used to extend the counter to 128 bits.
 110     /// This is provably as strong as the original cipher, though,
 111     /// since any distinguishing attack on our variant also works
 112     /// against ChaCha with a chosen-nonce. See the XSalsa20 [1]
 113     /// security proof for a more involved example of this.
 114     ///
 115     /// The modified word layout is:
 116     /// ```text
 117     /// constant constant constant constant
 118     /// key      key      key      key
 119     /// key      key      key      key
 120     /// counter  counter  counter  counter
 121     /// ```
 122     /// [1]: Daniel J. Bernstein. [*Extending the Salsa20
 123     /// nonce.*](http://cr.yp.to/papers.html#xsalsa)
 124     fn init(&mut self, key: &[u32; KEY_WORDS]) {
 125         self.state[0] = 0x61707865;
 126         self.state[1] = 0x3320646E;
 127         self.state[2] = 0x79622D32;
 128         self.state[3] = 0x6B206574;
 129
 130         for i in 0..KEY_WORDS {
 131             self.state[4+i] = key[i];
 132         }
 133
 134         self.state[12] = 0;
 135         self.state[13] = 0;
 136         self.state[14] = 0;
 137         self.state[15] = 0;
 138
 139         self.index = STATE_WORDS;
 140     }
 141
 142     /// Refill the internal output buffer (`self.buffer`)
 143     fn update(&mut self) {
 144         core(&mut self.buffer, &self.state);
 145         self.index = 0;
 146         // update 128-bit counter
 147         self.state[12] += 1;
 148         if self.state[12] != 0 { return };
 149         self.state[13] += 1;
 150         if self.state[13] != 0 { return };
 151         self.state[14] += 1;
 152         if self.state[14] != 0 { return };
 153         self.state[15] += 1;
 154     }
 155 }
 156
 157 impl Rng for ChaChaRng {
 158     #[inline]
 159     fn next_u32(&mut self) -> u32 {
 160         if self.index == STATE_WORDS {
 161             self.update();
 162         }
 163
 164         let value = self.buffer[self.index % STATE_WORDS];
 165         self.index += 1;
 166         value
 167     }
 168 }
 169
 170 impl<'a> SeedableRng<&'a [u32]> for ChaChaRng {
 171
 172     fn reseed(&mut self, seed: &'a [u32]) {
 173         // reset state
 174         self.init(&[0; KEY_WORDS]);
 175         // set key in place
 176         let key = &mut self.state[4 .. 4+KEY_WORDS];
 177         for (k, s) in key.iter_mut().zip(seed.iter()) {
 178             *k = *s;
 179         }
 180     }
 181
 182     /// Create a ChaCha generator from a seed,
 183     /// obtained from a variable-length u32 array.
 184     /// Only up to 8 words are used; if less than 8
 185     /// words are used, the remaining are set to zero.
 186     fn from_seed(seed: &'a [u32]) -> ChaChaRng {
 187         let mut rng = EMPTY;
 188         rng.reseed(seed);
 189         rng
 190     }
 191 }
 192
 193 impl Rand for ChaChaRng {
 194     fn rand<R: Rng>(other: &mut R) -> ChaChaRng {
 195         let mut key : [u32; KEY_WORDS] = [0; KEY_WORDS];
 196         for word in &mut key {
 197             *word = other.gen();
 198         }
 199         SeedableRng::from_seed(&key[..])
 200     }
 201 }
 202
 203
 204 #[cfg(test)]
 205 mod test {
 206     use std::prelude::v1::*;
 207
 208     use core::iter::order;
 209     use {Rng, SeedableRng};
 210     use super::ChaChaRng;
 211
 212     #[test]
 213     fn test_rng_rand_seeded() {
 214         let s = ::test::rng().gen_iter::<u32>().take(8).collect::<Vec<u32>>();
 215         let mut ra: ChaChaRng = SeedableRng::from_seed(&*s);
 216         let mut rb: ChaChaRng = SeedableRng::from_seed(&*s);
 217         assert!(order::equals(ra.gen_ascii_chars().take(100),
 218                               rb.gen_ascii_chars().take(100)));
 219     }
 220
 221     #[test]
 222     fn test_rng_seeded() {
 223         let seed : &[_] = &[0,1,2,3,4,5,6,7];
 224         let mut ra: ChaChaRng = SeedableRng::from_seed(seed);
 225         let mut rb: ChaChaRng = SeedableRng::from_seed(seed);
 226         assert!(order::equals(ra.gen_ascii_chars().take(100),
 227                               rb.gen_ascii_chars().take(100)));
 228     }
 229
 230     #[test]
 231     fn test_rng_reseed() {
 232         let s = ::test::rng().gen_iter::<u32>().take(8).collect::<Vec<u32>>();
 233         let mut r: ChaChaRng = SeedableRng::from_seed(&*s);
 234         let string1: String = r.gen_ascii_chars().take(100).collect();
 235
 236         r.reseed(&s);
 237
 238         let string2: String = r.gen_ascii_chars().take(100).collect();
 239         assert_eq!(string1, string2);
 240     }
 241
 242     #[test]
 243     fn test_rng_true_values() {
 244         // Test vectors 1 and 2 from
 245         // http://tools.ietf.org/html/draft-nir-cfrg-chacha20-poly1305-04
 246         let seed : &[_] = &[0; 8];
 247         let mut ra: ChaChaRng = SeedableRng::from_seed(seed);
 248
 249         let v = (0..16).map(|_| ra.next_u32()).collect::<Vec<_>>();
 250         assert_eq!(v,
 251                    vec!(0xade0b876, 0x903df1a0, 0xe56a5d40, 0x28bd8653,
 252                         0xb819d2bd, 0x1aed8da0, 0xccef36a8, 0xc70d778b,
 253                         0x7c5941da, 0x8d485751, 0x3fe02477, 0x374ad8b8,
 254                         0xf4b8436a, 0x1ca11815, 0x69b687c3, 0x8665eeb2));
 255
 256         let v = (0..16).map(|_| ra.next_u32()).collect::<Vec<_>>();
 257         assert_eq!(v,
 258                    vec!(0xbee7079f, 0x7a385155, 0x7c97ba98, 0x0d082d73,
 259                         0xa0290fcb, 0x6965e348, 0x3e53c612, 0xed7aee32,
 260                         0x7621b729, 0x434ee69c, 0xb03371d5, 0xd539d874,
 261                         0x281fed31, 0x45fb0a51, 0x1f0ae1ac, 0x6f4d794b));
 262
 263
 264         let seed : &[_] = &[0,1,2,3,4,5,6,7];
 265         let mut ra: ChaChaRng = SeedableRng::from_seed(seed);
 266
 267         // Store the 17*i-th 32-bit word,
 268         // i.e., the i-th word of the i-th 16-word block
 269         let mut v : Vec<u32> = Vec::new();
 270         for _ in 0..16 {
 271             v.push(ra.next_u32());
 272             for _ in 0..16 {
 273                 ra.next_u32();
 274             }
 275         }
 276
 277         assert_eq!(v,
 278                    vec!(0xf225c81a, 0x6ab1be57, 0x04d42951, 0x70858036,
 279                         0x49884684, 0x64efec72, 0x4be2d186, 0x3615b384,
 280                         0x11cfa18e, 0xd3c50049, 0x75c775f6, 0x434c6530,
 281                         0x2c5bad8f, 0x898881dc, 0x5f1c86d9, 0xc1f8e7f4));
 282     }
 283
 284     #[test]
 285     fn test_rng_clone() {
 286         let seed : &[_] = &[0; 8];
 287         let mut rng: ChaChaRng = SeedableRng::from_seed(seed);
 288         let mut clone = rng.clone();
 289         for _ in 0..16 {
 290             assert_eq!(rng.next_u64(), clone.next_u64());
 291         }
 292     }
 293 }