Merge pull request #281 from cose-wg/ecdsa-format

Fixed length ECDSA format

Author: emanjon

draft-ietf-cose-cbor-encoded-cert.md

@@ -357,7 +357,7 @@ For elliptic curve public keys in Weierstraß form (id-ecPublicKey), keys may be
 
 ### Encoding of issuerSignatureValue
 
-For ECDSA signatures, the SEQUENCE and INTEGER type and length fields as well as any leading 0x00 byte (to indicate that the number is not negative) are omitted. If the two INTEGER value fields have different lengths, the shorter INTEGER value field is padded with zeroes so that the two fields have the same length. The resulting byte string is encoded as a CBOR byte string.
+For ECDSA signatures, the SEQUENCE and INTEGER type and length fields as well as any leading 0x00 byte (to indicate that the number is not negative) are omitted. Each of the two INTEGER value fields are then padded with leading zeroes to the same fixed length, given by the number of bytes needed to represent the order n of the cyclic subgroup used with the algorithm. For example, for P-256, the number of bytes for each integer is 32. The resulting byte string is encoded as a CBOR byte string.
 
 ## Encoding of Extensions {#ext-encoding}