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+// 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(rust_1_26)]
+ 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(rust_1_26)]
+ 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(rust_1_26)]
+ 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());
+ }
+ }
+}
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