Re: [PATCH] crypto: ecdsa: Fix the public key format description

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

On Mon May 27, 2024 at 11:28 PM EEST, Jarkko Sakkinen wrote:
> Public key blob is not just x and y concatenated. It follows RFC5480
> section 2.2. Address this by re-documenting the function with the
> correct description of the format.
>
> Link: https://datatracker.ietf.org/doc/html/rfc5480
> Fixes: 4e6602916bc6 ("crypto: ecdsa - Add support for ECDSA signature verification")
> Signed-off-by: Jarkko Sakkinen <jarkko@kernel.org>
> ---
> It is a bug fix that does not really need a stable backport. Still
> categorizes as a bug because by following the existing documentation
> you end up with an error code.
>  crypto/ecdsa.c | 5 ++---
>  1 file changed, 2 insertions(+), 3 deletions(-)
>
> diff --git a/crypto/ecdsa.c b/crypto/ecdsa.c
> index 258fffbf623d..55114146ff84 100644
> --- a/crypto/ecdsa.c
> +++ b/crypto/ecdsa.c
> @@ -215,9 +215,8 @@ static int ecdsa_ecc_ctx_reset(struct ecc_ctx *ctx)
>  }
>  
>  /*
> - * Set the public key given the raw uncompressed key data from an X509
> - * certificate. The key data contain the concatenated X and Y coordinates of
> - * the public key.
> + * Set the public ECC key as defined by RFC5480 section 2.2 "Subject Public
> + * Key". Only the uncompressed format is supported.
>   */
>  static int ecdsa_set_pub_key(struct crypto_akcipher *tfm, const void *key, unsigned int keylen)
>  {

Based on this, is this now along the lines of correct format":

       *ptr++ = 0x04; /* uncompressed */
       ptr = asn1_encode_octet_string(&ptr[0], &in[sizeof(in)], &x[0], x_size);
       ptr = asn1_encode_octet_string(&ptr[0], &in[sizeof(in)], &x[x_size + 2], x_size);
       in_len = ptr - in;
       ret = crypto_akcipher_set_pub_key(tfm, in, in_len);

Notes:

- ptr points to the beginning of in (initially)
- x: "u16, u8[x_size], u16, u8[x_size]"
- x_size is length of ecc i.e. 32 in the case of p256.

I actually did the snippet by reading the RFC so this is pretty good
test for the patch.

BR, Jarkko