Re: [PATCH 1/2] crypto: ecdsa - Use ecc_digits_from_bytes to create hash digits array

[Jarkko Sakkinen]

On Thu May 30, 2024 at 2:08 AM EEST, Stefan Berger wrote:
> Since ecc_digits_from_bytes will provide zeros when an insufficient number
> of bytes are passed in the input byte array, use it to create the hash
> digits directly from the input byte array. This avoids going through an
> intermediate byte array (rawhash) that has the first few bytes filled with
> zeros.
>
> Signed-off-by: Stefan Berger <stefanb@xxxxxxxxxxxxx>
> ---
>  crypto/ecdsa.c | 17 ++++-------------
>  1 file changed, 4 insertions(+), 13 deletions(-)
>
> diff --git a/crypto/ecdsa.c b/crypto/ecdsa.c
> index 258fffbf623d..fa029f36110b 100644
> --- a/crypto/ecdsa.c
> +++ b/crypto/ecdsa.c
> @@ -142,10 +142,8 @@ static int ecdsa_verify(struct akcipher_request *req)
>  	struct ecdsa_signature_ctx sig_ctx = {
>  		.curve = ctx->curve,
>  	};
> -	u8 rawhash[ECC_MAX_BYTES];
>  	u64 hash[ECC_MAX_DIGITS];
>  	unsigned char *buffer;
> -	ssize_t diff;
>  	int ret;
>  
>  	if (unlikely(!ctx->pub_key_set))
> @@ -164,18 +162,11 @@ static int ecdsa_verify(struct akcipher_request *req)
>  	if (ret < 0)
>  		goto error;
>  
> -	/* if the hash is shorter then we will add leading zeros to fit to ndigits */
> -	diff = bufsize - req->dst_len;
> -	if (diff >= 0) {
> -		if (diff)
> -			memset(rawhash, 0, diff);
> -		memcpy(&rawhash[diff], buffer + req->src_len, req->dst_len);
> -	} else if (diff < 0) {
> -		/* given hash is longer, we take the left-most bytes */
> -		memcpy(&rawhash, buffer + req->src_len, bufsize);
> -	}
> +	if (bufsize > req->dst_len)
> +		bufsize = req->dst_len;
>  
> -	ecc_swap_digits((u64 *)rawhash, hash, ctx->curve->g.ndigits);
> +	ecc_digits_from_bytes(buffer + req->src_len, bufsize,
> +			      hash, ctx->curve->g.ndigits);
>  
>  	ret = _ecdsa_verify(ctx, hash, sig_ctx.r, sig_ctx.s);
>  

Reviewed-by: Jarkko Sakkinen <jarkko@xxxxxxxxxx>

I don't think it'd be even nit-picking to say that the function
called would really need kdoc. I had to spend about 20 minutes
to reacall ecc_digits_from_bytes().

Like something to remind what, how and why... So that you can
recap quickly. Once I got grip of it (for the 2nd time) the
code itself was just fine, no complains on that.

BR, Jarkko