Re: [PATCH v6 12/13] crypto: asymmetric_keys - Adjust signature size calculation for NIST P521

Jarkko Sakkinen
2024-03-18T21:12:14Z

On Tue Mar 12, 2024 at 8:36 PM EET, Stefan Berger wrote:
> From: Stefan Berger <stefanb@linux.ibm.com>
>
> Adjust the calculation of the maximum signature size for support of
> NIST P521. While existing curves may prepend a 0 byte to their coordinates
> (to make the number positive), NIST P521 will not do this since only the
> first bit in the most significant byte is used.
>
> If the encoding of the x & y coordinates requires at least 128 bytes then
> an additional byte is needed for the encoding of the length. Take this into
> account when calculating the maximum signature size.
>
> Signed-off-by: Stefan Berger <stefanb@linux.ibm.com>
> Reviewed-by: Lukas Wunner <lukas@wunner.de>
> Tested-by: Lukas Wunner <lukas@wunner.de>
> ---
> crypto/asymmetric_keys/public_key.c | 14 +++++++++++++-
> 1 file changed, 13 insertions(+), 1 deletion(-)
>
> diff --git a/crypto/asymmetric_keys/public_key.c b/crypto/asymmetric_keys/public_key.c
> index e5f22691febd..16cc0be28929 100644
> --- a/crypto/asymmetric_keys/public_key.c
> +++ b/crypto/asymmetric_keys/public_key.c
> @@ -233,6 +233,7 @@ static int software_key_query(const struct kernel_pkey_params *params,
> info->key_size = len * 8;
>
> if (strncmp(pkey->pkey_algo, "ecdsa", 5) == 0) {
> + int slen = len;
> /*
> * ECDSA key sizes are much smaller than RSA, and thus could
> * operate on (hashed) inputs that are larger than key size.
> @@ -246,8 +247,19 @@ static int software_key_query(const struct kernel_pkey_params *params,
> * Verify takes ECDSA-Sig (described in RFC 5480) as input,
> * which is actually 2 'key_size'-bit integers encoded in
> * ASN.1. Account for the ASN.1 encoding overhead here.
> + *
> + * NIST P192/256/384 may prepend a '0' to a coordinate to
> + * indicate a positive integer. NIST P521 never needs it.
> */
> - info->max_sig_size = 2 * (len + 3) + 2;
> + if (strcmp(pkey->pkey_algo, "ecdsa-nist-p521") != 0)
> + slen += 1;

Just wondering the logic of picking between these:

1. "strncmp"
2. "strcmp"

Now the "ecdsa" is matched with strncmp and "ecdsa-nist-p521" is
compared with strcmp.

So is there a good reason to use different function in these
cases?

I'd guess both could be using strcmp since comparing against
constant...

> + /* Length of encoding the x & y coordinates */
> + slen = 2 * (slen + 2);
> + /*
> + * If coordinate encoding takes at least 128 bytes then an
> + * additional byte for length encoding is needed.
> + */
> + info->max_sig_size = 1 + (slen >= 128) + 1 + slen;
> } else {
> info->max_data_size = len;
> info->max_sig_size = len;

BR, Jarkko