Hi
@Phil: The 64-byte bc::ec_signature type represents R, S values, each as 32 bytes. When such a signature is DER encoded, the leading null bytes are omitted, so that the DER encoded R&S values may vary in length.
-Consider
bc::der_signature in Libbitcoin
-DER encoding in BIP66:
https://github.com/bitcoin/bips/blob/master/bip-0066.mediawiki.