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[v5,11/12] crypto: asymmetric_keys - Adjust signature size calculation for NIST P521

Message ID 20240306222257.979304-12-stefanb@linux.ibm.com (mailing list archive)
State Superseded
Delegated to: Herbert Xu
Headers show
Series Add support for NIST P521 to ecdsa | expand

Commit Message

Stefan Berger March 6, 2024, 10:22 p.m. UTC 
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 mbox series

Patch

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;
+		/* 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;