ECDSA sign with OpenSSL, verify with Crypto++

Question

I create an ECDSA keypair (secp128r1) in my application with Wei Dai's Crypto++. Signing and verifying works as expected. I do not add the message itself to the signature to minimize the signature length (which is exactly 32 Bytes).

However, when I create the signature with openssl:

$ cat test.txt | openssl dgst -ecdsa-with-SHA1 -sign sample.key -keyform DER > act.bin

OpenSSL obviously puts the message itself to the signature resulting in a larger signature (e.g. 39 Bytes). I can verify the signature with Crypto++ if I set CryptoPP::SignatureVerificationFilter::PUT_MESSAGE.

Can I tell OpenSSL to sign a message with NOT putting the message to the signature such that the resulting signature is 32 Byte exactly?

Answer 1

CodesInChaos is correct. The extra bytes in the signature are from the ASN.1 encoding, and not the original message being signed. For example, here is a 39 byte signature generated with an ECDSA key with curve secp128r1:

30 25 02 10 4E 32 32 90 CA D9 BD D2 5F 8B BE 3B
F2 BF E9 7F 02 11 00 A7 83 A6 68 AD 74 7E 1A 0E
8F 73 BD DF 7A E8 B5

The 30 indicates that a Sequence follows. The 25 tells you that the Sequence is 0x25 bytes long. The 02 indicates that the first item in the Sequence is an Integer. The 10 tells you that the first Integer is 0x10 bytes long. The following 0x10 (16) bytes are the "r" value of the ECDSA signature. Following the first integer is the byte 02. This tells you that the 2nd Integer of the Sequence is about to begin. 11 tells you that the next 0x11 (17) bytes make up the 2nd Integer, which is the "s" value of the ECDSA signature. It's 11 bytes because the first byte of the Integer is 00. "00" is inserted whenever the first byte of an integer is >= 0x80. This is to avoid the most significant bit being a 1, which would indicate a negative integer.

So after all that, the real signature values are:

r: 4E 32 32 90 CA D9 BD D2 5F 8B BE 3B F2 BF E9 7F
s: A7 83 A6 68 AD 74 7E 1A 0E 8F 73 BD DF 7A E8 B5

The "extra" bytes are for ASN.1 formatting.