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

[Jarkko Sakkinen]

On Tue Mar 19, 2024 at 12:42 AM EET, Stefan Berger wrote:
>
>
> On 3/18/24 17:12, Jarkko Sakkinen wrote:
> > On Tue Mar 12, 2024 at 8:36 PM EET, Stefan Berger wrote:
> >> From: Stefan Berger <stefanb@xxxxxxxxxxxxx>
> >>
> >> 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@xxxxxxxxxxxxx>
> >> Reviewed-by: Lukas Wunner <lukas@xxxxxxxxx>
> >> Tested-by: Lukas Wunner <lukas@xxxxxxxxx>
> >> ---
> >>   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"
> > 
>
> strncmp: prefix-matching
> strcmp: full string matching

Right, in first case is necessary because strcmp() would return "-1" for
the substring.

BR, Jarkko