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

Stefan Berger
2024-03-18T22:46:00Z

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@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"
>

strncmp: prefix-matching
strcmp: full string matching

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

That's prefix matching vs. full string match.

.. and indeed 'ecdsa' is a prefix of 'ecdsa-nist-p521'.

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

Yes, there is.

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

No, prefix versus full string matching requires different function calls.

>
>> + /* 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
>