Re: [RFC PATCH v2 01/12] crypto: chacha20-generic - add HChaCha20 library function

[Ard Biesheuvel]

On 16 October 2018 at 01:54, Eric Biggers <ebiggers@xxxxxxxxxx> wrote:
> From: Eric Biggers <ebiggers@xxxxxxxxxx>
>
> Refactor the unkeyed permutation part of chacha20_block() into its own
> function, then add hchacha20_block() which is the ChaCha equivalent of
> HSalsa20 and is an intermediate step towards XChaCha20 (see
> https://cr.yp.to/snuffle/xsalsa-20081128.pdf).  HChaCha20 skips the
> final addition of the initial state, and outputs only certain words of
> the state.  It should not be used for streaming directly.
>
> Signed-off-by: Eric Biggers <ebiggers@xxxxxxxxxx>

Reviewed-by: Ard Biesheuvel <ard.biesheuvel@xxxxxxxxxx>

> ---
>  include/crypto/chacha20.h |  2 ++
>  lib/chacha20.c            | 50 ++++++++++++++++++++++++++++++++++-----
>  2 files changed, 46 insertions(+), 6 deletions(-)
>
> diff --git a/include/crypto/chacha20.h b/include/crypto/chacha20.h
> index f76302d99e2be..fbec4e6a87890 100644
> --- a/include/crypto/chacha20.h
> +++ b/include/crypto/chacha20.h
> @@ -19,6 +19,8 @@ struct chacha20_ctx {
>  };
>
>  void chacha20_block(u32 *state, u8 *stream);
> +void hchacha20_block(const u32 *in, u32 *out);
> +
>  void crypto_chacha20_init(u32 *state, struct chacha20_ctx *ctx, u8 *iv);
>  int crypto_chacha20_setkey(struct crypto_skcipher *tfm, const u8 *key,
>                            unsigned int keysize);
> diff --git a/lib/chacha20.c b/lib/chacha20.c
> index d907fec6a9ed1..6a484e16171d1 100644
> --- a/lib/chacha20.c
> +++ b/lib/chacha20.c
> @@ -1,5 +1,5 @@
>  /*
> - * ChaCha20 256-bit cipher algorithm, RFC7539
> + * The "hash function" used as the core of the ChaCha20 stream cipher (RFC7539)
>   *
>   * Copyright (C) 2015 Martin Willi
>   *
> @@ -16,14 +16,10 @@
>  #include <asm/unaligned.h>
>  #include <crypto/chacha20.h>
>
> -void chacha20_block(u32 *state, u8 *stream)
> +static void chacha20_permute(u32 *x)
>  {
> -       u32 x[16];
>         int i;
>
> -       for (i = 0; i < ARRAY_SIZE(x); i++)
> -               x[i] = state[i];
> -
>         for (i = 0; i < 20; i += 2) {
>                 x[0]  += x[4];    x[12] = rol32(x[12] ^ x[0],  16);
>                 x[1]  += x[5];    x[13] = rol32(x[13] ^ x[1],  16);
> @@ -65,6 +61,25 @@ void chacha20_block(u32 *state, u8 *stream)
>                 x[8]  += x[13];   x[7]  = rol32(x[7]  ^ x[8],   7);
>                 x[9]  += x[14];   x[4]  = rol32(x[4]  ^ x[9],   7);
>         }
> +}
> +
> +/**
> + * chacha20_block - generate one keystream block and increment block counter
> + * @state: input state matrix (16 32-bit words)
> + * @stream: output keystream block (64 bytes)
> + *
> + * This is the ChaCha20 core, a function from 64-byte strings to 64-byte
> + * strings.  The caller has already converted the endianness of the input.  This
> + * function also handles incrementing the block counter in the input matrix.
> + */
> +void chacha20_block(u32 *state, u8 *stream)
> +{
> +       u32 x[16];
> +       int i;
> +
> +       memcpy(x, state, 64);
> +
> +       chacha20_permute(x);
>
>         for (i = 0; i < ARRAY_SIZE(x); i++)
>                 put_unaligned_le32(x[i] + state[i], &stream[i * sizeof(u32)]);
> @@ -72,3 +87,26 @@ void chacha20_block(u32 *state, u8 *stream)
>         state[12]++;
>  }
>  EXPORT_SYMBOL(chacha20_block);
> +
> +/**
> + * hchacha20_block - abbreviated ChaCha20 core, for XChaCha20
> + * @in: input state matrix (16 32-bit words)
> + * @out: output (8 32-bit words)
> + *
> + * HChaCha20 is the ChaCha equivalent of HSalsa20 and is an intermediate step
> + * towards XChaCha20 (see https://cr.yp.to/snuffle/xsalsa-20081128.pdf).
> + * HChaCha20 skips the final addition of the initial state, and outputs only
> + * certain words of the state.  It should not be used for streaming directly.
> + */
> +void hchacha20_block(const u32 *in, u32 *out)
> +{
> +       u32 x[16];
> +
> +       memcpy(x, in, 64);
> +
> +       chacha20_permute(x);
> +
> +       memcpy(&out[0], &x[0], 16);
> +       memcpy(&out[4], &x[12], 16);
> +}
> +EXPORT_SYMBOL(hchacha20_block);
> --
> 2.19.1.331.ge82ca0e54c-goog
>
>
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