Change ECDSA_METHOD's size() to group_order_size()

The |size| method was documented to return the same as |ECDSA_size| -
the max size of an ECDSA signature. However, this involves some ASN.1
calculations which is best done once. What custom implementations want
to give is the size of the group order on which the ASN.1 computations
are based.

This change switches the |size| method to allow that.

Change-Id: I95b6e0c2b52bfcd0d74850c2c4e9bc01269255e2
Reviewed-on: https://boringssl-review.googlesource.com/1200
Reviewed-by: David Benjamin <davidben@chromium.org>
Reviewed-by: Adam Langley <agl@google.com>

1 files changed, 24 insertions, 20 deletions

diff --git a/crypto/ecdsa/ecdsa_asn1.c b/crypto/ecdsa/ecdsa_asn1.c
index 9d683617..e54dcca0 100644
--- a/crypto/ecdsa/ecdsa_asn1.c
+++ b/crypto/ecdsa/ecdsa_asn1.c
@@ -69,35 +69,39 @@ DECLARE_ASN1_ENCODE_FUNCTIONS_const(ECDSA_SIG, ECDSA_SIG);
 IMPLEMENT_ASN1_FUNCTIONS_const(ECDSA_SIG);
 
 size_t ECDSA_size(const EC_KEY *key) {
-  size_t ret, i;
+  size_t ret, i, group_order_size;
   ASN1_INTEGER bs;
   BIGNUM *order = NULL;
   unsigned char buf[4];
   const EC_GROUP *group;
 
-  if (key->ecdsa_meth && key->ecdsa_meth->size) {
-    return key->ecdsa_meth->size(key);
-  }
+  if (key->ecdsa_meth && key->ecdsa_meth->group_order_size) {
+    group_order_size = key->ecdsa_meth->group_order_size(key);
+  } else {
+    size_t num_bits;
 
-  if (key == NULL) {
-    return 0;
-  }
-  group = EC_KEY_get0_group(key);
-  if (group == NULL) {
-    return 0;
-  }
+    if (key == NULL) {
+      return 0;
+    }
+    group = EC_KEY_get0_group(key);
+    if (group == NULL) {
+      return 0;
+    }
 
-  order = BN_new();
-  if (order == NULL) {
-    return 0;
-  }
-  if (!EC_GROUP_get_order(group, order, NULL)) {
-    BN_clear_free(order);
-    return 0;
+    order = BN_new();
+    if (order == NULL) {
+      return 0;
+    }
+    if (!EC_GROUP_get_order(group, order, NULL)) {
+      BN_clear_free(order);
+      return 0;
+    }
+
+    num_bits = BN_num_bits(order);
+    group_order_size = (num_bits + 7) / 8;
   }
 
-  i = BN_num_bits(order);
-  bs.length = (i + 7) / 8;
+  bs.length = group_order_size;
   bs.data = buf;
   bs.type = V_ASN1_INTEGER;
   /* If the top bit is set the ASN.1 encoding is 1 larger. */