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, 36 insertions, 0 deletions

diff --git a/crypto/ecdsa-x962.c b/crypto/ecdsa-x962.c
index 8a15232dfa77..6a77c13e192b 100644
--- a/crypto/ecdsa-x962.c
+++ b/crypto/ecdsa-x962.c
@@ -99,6 +99,40 @@ static unsigned int ecdsa_x962_key_size(struct crypto_sig *tfm)
 	return crypto_sig_keysize(ctx->child);
 }
 
+static unsigned int ecdsa_x962_max_size(struct crypto_sig *tfm)
+{
+	struct ecdsa_x962_ctx *ctx = crypto_sig_ctx(tfm);
+	struct sig_alg *alg = crypto_sig_alg(ctx->child);
+	int slen = crypto_sig_keysize(ctx->child);
+
+	/*
+	 * Verify takes ECDSA-Sig-Value (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(alg->base.cra_name, "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.
+	 */
+	return 1 + (slen >= 128) + 1 + slen;
+}
+
+static unsigned int ecdsa_x962_digest_size(struct crypto_sig *tfm)
+{
+	struct ecdsa_x962_ctx *ctx = crypto_sig_ctx(tfm);
+
+	return crypto_sig_digestsize(ctx->child);
+}
+
 static int ecdsa_x962_set_pub_key(struct crypto_sig *tfm,
 				  const void *key, unsigned int keylen)
 {
@@ -180,6 +214,8 @@ static int ecdsa_x962_create(struct crypto_template *tmpl, struct rtattr **tb)
 
 	inst->alg.verify = ecdsa_x962_verify;
 	inst->alg.key_size = ecdsa_x962_key_size;
+	inst->alg.max_size = ecdsa_x962_max_size;
+	inst->alg.digest_size = ecdsa_x962_digest_size;
 	inst->alg.set_pub_key = ecdsa_x962_set_pub_key;
 
 	inst->free = ecdsa_x962_free;