crypto: ecdsa - Move X9.62 signature size calculation into template

software_key_query() returns the maximum signature and digest size for a
given key to user space.  When it only supported RSA keys, calculating
those sizes was trivial as they were always equivalent to the key size.

However when ECDSA was added, the function grew somewhat complicated
calculations which take the ASN.1 encoding and curve into account.
This doesn't scale well and adjusting the calculations is easily
forgotten when adding support for new encodings or curves.  In fact,
when NIST P521 support was recently added, the function was initially
not amended:

https://lore.kernel.org/all/b749d5ee-c3b8-4cbd-b252-7773e4536e07@linux.ibm.com/

Introduce a ->max_size() callback to struct sig_alg and take advantage
of it to move the signature size calculations to ecdsa-x962.c.

Introduce a ->digest_size() callback to struct sig_alg and move the
maximum ECDSA digest size to ecdsa.c.  It is common across ecdsa-x962.c
and the upcoming ecdsa-p1363.c and thus inherited by both of them.

For all other algorithms, continue using the key size as maximum
signature and digest size.

Signed-off-by: Lukas Wunner <lukas@wunner.de>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>

1 files changed, 4 insertions, 34 deletions

diff --git a/crypto/asymmetric_keys/public_key.c b/crypto/asymmetric_keys/public_key.c
index 8bf5aa329c26..ec2c0e009b49 100644
--- a/crypto/asymmetric_keys/public_key.c
+++ b/crypto/asymmetric_keys/public_key.c
@@ -202,6 +202,8 @@ static int software_key_query(const struct kernel_pkey_params *params,
 			goto error_free_tfm;
 
 		len = crypto_sig_keysize(sig);
+		info->max_sig_size = crypto_sig_maxsize(sig);
+		info->max_data_size = crypto_sig_digestsize(sig);
 
 		info->supported_ops = KEYCTL_SUPPORTS_VERIFY;
 		if (pkey->key_is_private)
@@ -227,6 +229,8 @@ static int software_key_query(const struct kernel_pkey_params *params,
 			goto error_free_tfm;
 
 		len = crypto_akcipher_maxsize(tfm);
+		info->max_sig_size = len;
+		info->max_data_size = len;
 
 		info->supported_ops = KEYCTL_SUPPORTS_ENCRYPT;
 		if (pkey->key_is_private)
@@ -234,40 +238,6 @@ 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.
-		 * For example SHA384-hashed input used with secp256r1
-		 * based keys.  Set max_data_size to be at least as large as
-		 * the largest supported hash size (SHA512)
-		 */
-		info->max_data_size = 64;
-
-		/*
-		 * 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.
-		 */
-		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;
-	}
-
 	info->max_enc_size = len;
 	info->max_dec_size = len;