Re: [PATCH v6 02/13] crypto: ecdsa - Convert byte arrays with key coordinates to digits

["Jarkko Sakkinen" <jarkko@kernel.org>]

On Tue Mar 12, 2024 at 8:36 PM EET, Stefan Berger wrote:
> From: Stefan Berger <stefanb@linux.ibm.com>
>
> For NIST P192/256/384 the public key's x and y parameters could be copied
> directly from a given array since both parameters filled 'ndigits' of

We "could" do various things but a commit message should describe the
gain or motivation of doing something, even if you think it should be
obvious.


> digits (a 'digit' is a u64). For support of NIST P521 the key parameters
> need to have leading zeros prepended to the most significant digit since
> only 2 bytes of the most significant digit are provided.
>
> Therefore, implement ecc_digits_from_bytes to convert a byte array into an
> array of digits and use this function in ecdsa_set_pub_key where an input
> byte array needs to be converted into digits.

Please, dscribe the format of "array of digits" instead of waving hands
here.


>
> Suggested-by: Lukas Wunner <lukas@wunner.de>
> Signed-off-by: Stefan Berger <stefanb@linux.ibm.com>
> Tested-by: Lukas Wunner <lukas@wunner.de>
> ---
>  crypto/ecdsa.c                | 14 +++++++++-----
>  include/crypto/internal/ecc.h | 21 +++++++++++++++++++++
>  2 files changed, 30 insertions(+), 5 deletions(-)
>
> diff --git a/crypto/ecdsa.c b/crypto/ecdsa.c
> index fbd76498aba8..6653dec17327 100644
> --- a/crypto/ecdsa.c
> +++ b/crypto/ecdsa.c
> @@ -222,9 +222,8 @@ static int ecdsa_ecc_ctx_reset(struct ecc_ctx *ctx)
>  static int ecdsa_set_pub_key(struct crypto_akcipher *tfm, const void *key, unsigned int keylen)
>  {
>  	struct ecc_ctx *ctx = akcipher_tfm_ctx(tfm);
> +	unsigned int digitlen, ndigits;
>  	const unsigned char *d = key;
> -	const u64 *digits = (const u64 *)&d[1];
> -	unsigned int ndigits;
>  	int ret;
>  
>  	ret = ecdsa_ecc_ctx_reset(ctx);
> @@ -238,12 +237,17 @@ static int ecdsa_set_pub_key(struct crypto_akcipher *tfm, const void *key, unsig
>  		return -EINVAL;
>  
>  	keylen--;
> -	ndigits = (keylen >> 1) / sizeof(u64);
> +	digitlen = keylen >> 1;
> +
> +	ndigits = DIV_ROUND_UP(digitlen, sizeof(u64));
>  	if (ndigits != ctx->curve->g.ndigits)
>  		return -EINVAL;
>  
> -	ecc_swap_digits(digits, ctx->pub_key.x, ndigits);
> -	ecc_swap_digits(&digits[ndigits], ctx->pub_key.y, ndigits);
> +	d++;

This pointer increment is more confusing...

> +
> +	ecc_digits_from_bytes(d, digitlen, ctx->pub_key.x, ndigits);

...than "&d[1]" which would also match better the earlier code.


> +	ecc_digits_from_bytes(&d[digitlen], digitlen, ctx->pub_key.y, ndigits);
> +
>  	ret = ecc_is_pubkey_valid_full(ctx->curve, &ctx->pub_key);
>  
>  	ctx->pub_key_set = ret == 0;
> diff --git a/include/crypto/internal/ecc.h b/include/crypto/internal/ecc.h
> index 4f6c1a68882f..ab722a8986b7 100644
> --- a/include/crypto/internal/ecc.h
> +++ b/include/crypto/internal/ecc.h
> @@ -56,6 +56,27 @@ static inline void ecc_swap_digits(const void *in, u64 *out, unsigned int ndigit
>  		out[i] = get_unaligned_be64(&src[ndigits - 1 - i]);
>  }
>  
> +/**
> + * ecc_digits_from_bytes() - Create ndigits-sized digits array from byte array
> + * @in:       Input byte array
> + * @nbytes    Size of input byte array
> + * @out       Output digits array
> + * @ndigits:  Number of digits to create from byte array
> + */
> +static inline void ecc_digits_from_bytes(const u8 *in, unsigned int nbytes,
> +					 u64 *out, unsigned int ndigits)
> +{
> +	unsigned int o = nbytes & 7;
> +	__be64 msd = 0;
> +
> +	if (o) {
> +		memcpy((u8 *)&msd + sizeof(msd) - o, in, o);
> +		out[--ndigits] = be64_to_cpu(msd);
> +		in += o;
> +	}
> +	ecc_swap_digits(in, out, ndigits);
> +}
> +
>  /**
>   * ecc_is_key_valid() - Validate a given ECDH private key
>   *

BR, Jarkko