rand_chacha/src/chacha.rs - third_party/rust-mirrors/rand - Git at Google

 // Copyright 2018 Developers of the Rand project.
 //
 // 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.

 #[cfg(not(feature = "std"))] use core;
 #[cfg(feature = "std")] use std as core;

 use self::core::fmt;
 use crate::guts::ChaCha;
 use rand_core::block::{BlockRng, BlockRngCore};
 use rand_core::{CryptoRng, Error, RngCore, SeedableRng};

 const STREAM_PARAM_NONCE: u32 = 1;
 const STREAM_PARAM_BLOCK: u32 = 0;

 // NB. this must remain consistent with some currently hard-coded numbers in this module
 const BUF_BLOCKS: u8 = 4;
 // number of 32-bit words per ChaCha block (fixed by algorithm definition)
 const BLOCK_WORDS: u8 = 16;

 pub struct Array64<T>([T; 64]);
 impl<T> Default for Array64<T>
 where T: Default
 {
     #[rustfmt::skip]
     fn default() -> Self {
         Self([
             T::default(), T::default(), T::default(), T::default(), T::default(), T::default(), T::default(), T::default(),
             T::default(), T::default(), T::default(), T::default(), T::default(), T::default(), T::default(), T::default(),
             T::default(), T::default(), T::default(), T::default(), T::default(), T::default(), T::default(), T::default(),
             T::default(), T::default(), T::default(), T::default(), T::default(), T::default(), T::default(), T::default(),
             T::default(), T::default(), T::default(), T::default(), T::default(), T::default(), T::default(), T::default(),
             T::default(), T::default(), T::default(), T::default(), T::default(), T::default(), T::default(), T::default(),
             T::default(), T::default(), T::default(), T::default(), T::default(), T::default(), T::default(), T::default(),
             T::default(), T::default(), T::default(), T::default(), T::default(), T::default(), T::default(), T::default(),
         ])
     }
 }
 impl<T> AsRef<[T]> for Array64<T> {
     fn as_ref(&self) -> &[T] {
         &self.0
     }
 }
 impl<T> AsMut<[T]> for Array64<T> {
     fn as_mut(&mut self) -> &mut [T] {
         &mut self.0
     }
 }
 impl<T> Clone for Array64<T>
 where T: Copy + Default
 {
     fn clone(&self) -> Self {
         let mut new = Self::default();
         new.0.copy_from_slice(&self.0);
         new
     }
 }
 impl<T> fmt::Debug for Array64<T> {
     fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
         write!(f, "Array64 {{}}")
     }
 }

 macro_rules! chacha_impl {
     ($ChaChaXCore:ident, $ChaChaXRng:ident, $rounds:expr, $doc:expr) => {
         #[doc=$doc]
         #[derive(Clone, PartialEq, Eq)]
         pub struct $ChaChaXCore {
             state: ChaCha,
         }

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

         impl BlockRngCore for $ChaChaXCore {
             type Item = u32;
             type Results = Array64<u32>;
             #[inline]
             fn generate(&mut self, r: &mut Self::Results) {
                 // Fill slice of words by writing to equivalent slice of bytes, then fixing endianness.
                 self.state.refill4($rounds, unsafe {
                     &mut *(&mut *r as *mut Array64<u32> as *mut [u8; 256])
                 });
                 for x in r.as_mut() {
                     *x = x.to_le();
                 }
             }
         }

         impl SeedableRng for $ChaChaXCore {
             type Seed = [u8; 32];
             #[inline]
             fn from_seed(seed: Self::Seed) -> Self {
                 $ChaChaXCore { state: ChaCha::new(&seed, &[0u8; 8]) }
             }
         }

         impl CryptoRng for $ChaChaXCore {}

         /// 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. We provide a SIMD
         /// implementation to support high throughput on a variety of common hardware platforms.
         ///
         /// 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 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 the `set_word_pos` and `set_stream` methods.
         ///
         /// 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/)
         #[derive(Clone, Debug)]
         pub struct $ChaChaXRng {
             rng: BlockRng<$ChaChaXCore>,
         }

         impl SeedableRng for $ChaChaXRng {
             type Seed = [u8; 32];
             #[inline]
             fn from_seed(seed: Self::Seed) -> Self {
                 let core = $ChaChaXCore::from_seed(seed);
                 Self {
                     rng: BlockRng::new(core),
                 }
             }
         }

         impl RngCore for $ChaChaXRng {
             #[inline]
             fn next_u32(&mut self) -> u32 {
                 self.rng.next_u32()
             }
             #[inline]
             fn next_u64(&mut self) -> u64 {
                 self.rng.next_u64()
             }
             #[inline]
             fn fill_bytes(&mut self, bytes: &mut [u8]) {
                 self.rng.fill_bytes(bytes)
             }
             #[inline]
             fn try_fill_bytes(&mut self, bytes: &mut [u8]) -> Result<(), Error> {
                 self.rng.try_fill_bytes(bytes)
             }
         }

         impl $ChaChaXRng {
             // The buffer is a 4-block window, i.e. it is always at a block-aligned position in the
             // stream but if the stream has been seeked it may not be self-aligned.

             /// 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.
             #[inline]
             pub fn get_word_pos(&self) -> u128 {
                 let buf_start_block = {
                     let buf_end_block = self.rng.core.state.get_stream_param(STREAM_PARAM_BLOCK);
                     u64::wrapping_sub(buf_end_block, BUF_BLOCKS.into())
                 };
                 let (buf_offset_blocks, block_offset_words) = {
                     let buf_offset_words = self.rng.index() as u64;
                     let blocks_part = buf_offset_words / u64::from(BLOCK_WORDS);
                     let words_part = buf_offset_words % u64::from(BLOCK_WORDS);
                     (blocks_part, words_part)
                 };
                 let pos_block = u64::wrapping_add(buf_start_block, buf_offset_blocks);
                 let pos_block_words = u128::from(pos_block) * u128::from(BLOCK_WORDS);
                 pos_block_words + u128::from(block_offset_words)
             }

             /// 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.
             #[inline]
             pub fn set_word_pos(&mut self, word_offset: u128) {
                 let block = (word_offset / u128::from(BLOCK_WORDS)) as u64;
                 self.rng
                     .core
                     .state
                     .set_stream_param(STREAM_PARAM_BLOCK, block);
                 self.rng.generate_and_set((word_offset % u128::from(BLOCK_WORDS)) as usize);
             }

             /// 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.
             #[inline]
             pub fn set_stream(&mut self, stream: u64) {
                 self.rng
                     .core
                     .state
                     .set_stream_param(STREAM_PARAM_NONCE, stream);
                 if self.rng.index() != 64 {
                     let wp = self.get_word_pos();
                     self.set_word_pos(wp);
                 }
             }
         }

         impl CryptoRng for $ChaChaXRng {}

         impl From<$ChaChaXCore> for $ChaChaXRng {
             fn from(core: $ChaChaXCore) -> Self {
                 $ChaChaXRng {
                     rng: BlockRng::new(core),
                 }
             }
         }

         impl PartialEq<$ChaChaXRng> for $ChaChaXRng {
             fn eq(&self, rhs: &$ChaChaXRng) -> bool {
                 self.rng.core.state.stream64_eq(&rhs.rng.core.state)
                     && self.get_word_pos() == rhs.get_word_pos()
             }
         }
         impl Eq for $ChaChaXRng {}
     }
 }

 chacha_impl!(ChaCha20Core, ChaCha20Rng, 10, "ChaCha with 20 rounds");
 chacha_impl!(ChaCha12Core, ChaCha12Rng, 6, "ChaCha with 12 rounds");
 chacha_impl!(ChaCha8Core, ChaCha8Rng, 4, "ChaCha with 8 rounds");

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

     type ChaChaRng = super::ChaCha20Rng;

     #[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]
     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());
         }
     }

     #[test]
     fn test_chacha_word_pos_wrap_exact() {
         use super::{BUF_BLOCKS, BLOCK_WORDS};
         let mut rng = ChaChaRng::from_seed(Default::default());
         // refilling the buffer in set_word_pos will wrap the block counter to 0
         let last_block = (1 << 68) - u128::from(BUF_BLOCKS * BLOCK_WORDS);
         rng.set_word_pos(last_block);
         assert_eq!(rng.get_word_pos(), last_block);
     }

     #[test]
     fn test_chacha_word_pos_wrap_excess() {
         use super::BLOCK_WORDS;
         let mut rng = ChaChaRng::from_seed(Default::default());
         // refilling the buffer in set_word_pos will wrap the block counter past 0
         let last_block = (1 << 68) - u128::from(BLOCK_WORDS);
         rng.set_word_pos(last_block);
         assert_eq!(rng.get_word_pos(), last_block);
     }

     #[test]
     fn test_chacha_word_pos_zero() {
         let mut rng = ChaChaRng::from_seed(Default::default());
         assert_eq!(rng.get_word_pos(), 0);
         rng.set_word_pos(0);
         assert_eq!(rng.get_word_pos(), 0);
     }
 }
	// Copyright 2018 Developers of the Rand project.
	//
	// 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.

	#[cfg(not(feature = "std"))] use core;
	#[cfg(feature = "std")] use std as core;

	use self::core::fmt;
	use crate::guts::ChaCha;
	use rand_core::block::{BlockRng, BlockRngCore};
	use rand_core::{CryptoRng, Error, RngCore, SeedableRng};

	const STREAM_PARAM_NONCE: u32 = 1;
	const STREAM_PARAM_BLOCK: u32 = 0;

	// NB. this must remain consistent with some currently hard-coded numbers in this module
	const BUF_BLOCKS: u8 = 4;
	// number of 32-bit words per ChaCha block (fixed by algorithm definition)
	const BLOCK_WORDS: u8 = 16;

	pub struct Array64<T>([T; 64]);
	impl<T> Default for Array64<T>
	where T: Default
	{
	#[rustfmt::skip]
	fn default() -> Self {
	Self([
	T::default(), T::default(), T::default(), T::default(), T::default(), T::default(), T::default(), T::default(),
	T::default(), T::default(), T::default(), T::default(), T::default(), T::default(), T::default(), T::default(),
	T::default(), T::default(), T::default(), T::default(), T::default(), T::default(), T::default(), T::default(),
	T::default(), T::default(), T::default(), T::default(), T::default(), T::default(), T::default(), T::default(),
	T::default(), T::default(), T::default(), T::default(), T::default(), T::default(), T::default(), T::default(),
	T::default(), T::default(), T::default(), T::default(), T::default(), T::default(), T::default(), T::default(),
	T::default(), T::default(), T::default(), T::default(), T::default(), T::default(), T::default(), T::default(),
	T::default(), T::default(), T::default(), T::default(), T::default(), T::default(), T::default(), T::default(),
	])
	}
	}
	impl<T> AsRef<[T]> for Array64<T> {
	fn as_ref(&self) -> &[T] {
	&self.0
	}
	}
	impl<T> AsMut<[T]> for Array64<T> {
	fn as_mut(&mut self) -> &mut [T] {
	&mut self.0
	}
	}
	impl<T> Clone for Array64<T>
	where T: Copy + Default
	{
	fn clone(&self) -> Self {
	let mut new = Self::default();
	new.0.copy_from_slice(&self.0);
	new
	}
	}
	impl<T> fmt::Debug for Array64<T> {
	fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
	write!(f, "Array64 {{}}")
	}
	}

	macro_rules! chacha_impl {
	($ChaChaXCore:ident, $ChaChaXRng:ident, $rounds:expr, $doc:expr) => {
	#[doc=$doc]
	#[derive(Clone, PartialEq, Eq)]
	pub struct $ChaChaXCore {
	state: ChaCha,
	}

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

	impl BlockRngCore for $ChaChaXCore {
	type Item = u32;
	type Results = Array64<u32>;
	#[inline]
	fn generate(&mut self, r: &mut Self::Results) {
	// Fill slice of words by writing to equivalent slice of bytes, then fixing endianness.
	self.state.refill4($rounds, unsafe {
	&mut (&mut r as mut Array64<u32> as mut [u8; 256])
	});
	for x in r.as_mut() {
	*x = x.to_le();
	}
	}
	}

	impl SeedableRng for $ChaChaXCore {
	type Seed = [u8; 32];
	#[inline]
	fn from_seed(seed: Self::Seed) -> Self {
	$ChaChaXCore { state: ChaCha::new(&seed, &[0u8; 8]) }
	}
	}

	impl CryptoRng for $ChaChaXCore {}

	/// 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. We provide a SIMD
	/// implementation to support high throughput on a variety of common hardware platforms.
	///
	/// 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 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 the `set_word_pos` and `set_stream` methods.
	///
	/// 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/)
	#[derive(Clone, Debug)]
	pub struct $ChaChaXRng {
	rng: BlockRng<$ChaChaXCore>,
	}

	impl SeedableRng for $ChaChaXRng {
	type Seed = [u8; 32];
	#[inline]
	fn from_seed(seed: Self::Seed) -> Self {
	let core = $ChaChaXCore::from_seed(seed);
	Self {
	rng: BlockRng::new(core),
	}
	}
	}

	impl RngCore for $ChaChaXRng {
	#[inline]
	fn next_u32(&mut self) -> u32 {
	self.rng.next_u32()
	}
	#[inline]
	fn next_u64(&mut self) -> u64 {
	self.rng.next_u64()
	}
	#[inline]
	fn fill_bytes(&mut self, bytes: &mut [u8]) {
	self.rng.fill_bytes(bytes)
	}
	#[inline]
	fn try_fill_bytes(&mut self, bytes: &mut [u8]) -> Result<(), Error> {
	self.rng.try_fill_bytes(bytes)
	}
	}

	impl $ChaChaXRng {
	// The buffer is a 4-block window, i.e. it is always at a block-aligned position in the
	// stream but if the stream has been seeked it may not be self-aligned.

	/// 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.
	#[inline]
	pub fn get_word_pos(&self) -> u128 {
	let buf_start_block = {
	let buf_end_block = self.rng.core.state.get_stream_param(STREAM_PARAM_BLOCK);
	u64::wrapping_sub(buf_end_block, BUF_BLOCKS.into())
	};
	let (buf_offset_blocks, block_offset_words) = {
	let buf_offset_words = self.rng.index() as u64;
	let blocks_part = buf_offset_words / u64::from(BLOCK_WORDS);
	let words_part = buf_offset_words % u64::from(BLOCK_WORDS);
	(blocks_part, words_part)
	};
	let pos_block = u64::wrapping_add(buf_start_block, buf_offset_blocks);
	let pos_block_words = u128::from(pos_block) * u128::from(BLOCK_WORDS);
	pos_block_words + u128::from(block_offset_words)
	}

	/// 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.
	#[inline]
	pub fn set_word_pos(&mut self, word_offset: u128) {
	let block = (word_offset / u128::from(BLOCK_WORDS)) as u64;
	self.rng
	.core
	.state
	.set_stream_param(STREAM_PARAM_BLOCK, block);
	self.rng.generate_and_set((word_offset % u128::from(BLOCK_WORDS)) as usize);
	}

	/// 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.
	#[inline]
	pub fn set_stream(&mut self, stream: u64) {
	self.rng
	.core
	.state
	.set_stream_param(STREAM_PARAM_NONCE, stream);
	if self.rng.index() != 64 {
	let wp = self.get_word_pos();
	self.set_word_pos(wp);
	}
	}
	}

	impl CryptoRng for $ChaChaXRng {}

	impl From<$ChaChaXCore> for $ChaChaXRng {
	fn from(core: $ChaChaXCore) -> Self {
	$ChaChaXRng {
	rng: BlockRng::new(core),
	}
	}
	}

	impl PartialEq<$ChaChaXRng> for $ChaChaXRng {
	fn eq(&self, rhs: &$ChaChaXRng) -> bool {
	self.rng.core.state.stream64_eq(&rhs.rng.core.state)
	&& self.get_word_pos() == rhs.get_word_pos()
	}
	}
	impl Eq for $ChaChaXRng {}
	}
	}

	chacha_impl!(ChaCha20Core, ChaCha20Rng, 10, "ChaCha with 20 rounds");
	chacha_impl!(ChaCha12Core, ChaCha12Rng, 6, "ChaCha with 12 rounds");
	chacha_impl!(ChaCha8Core, ChaCha8Rng, 4, "ChaCha with 8 rounds");

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

	type ChaChaRng = super::ChaCha20Rng;

	#[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]
	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());
	}
	}

	#[test]
	fn test_chacha_word_pos_wrap_exact() {
	use super::{BUF_BLOCKS, BLOCK_WORDS};
	let mut rng = ChaChaRng::from_seed(Default::default());
	// refilling the buffer in set_word_pos will wrap the block counter to 0
	let last_block = (1 << 68) - u128::from(BUF_BLOCKS * BLOCK_WORDS);
	rng.set_word_pos(last_block);
	assert_eq!(rng.get_word_pos(), last_block);
	}

	#[test]
	fn test_chacha_word_pos_wrap_excess() {
	use super::BLOCK_WORDS;
	let mut rng = ChaChaRng::from_seed(Default::default());
	// refilling the buffer in set_word_pos will wrap the block counter past 0
	let last_block = (1 << 68) - u128::from(BLOCK_WORDS);
	rng.set_word_pos(last_block);
	assert_eq!(rng.get_word_pos(), last_block);
	}

	#[test]
	fn test_chacha_word_pos_zero() {
	let mut rng = ChaChaRng::from_seed(Default::default());
	assert_eq!(rng.get_word_pos(), 0);
	rng.set_word_pos(0);
	assert_eq!(rng.get_word_pos(), 0);
	}
	}