Errors using python's ECDSA lib to sign AWS messages

Question

I'm signing my messages using my code below:

def sign_msg_hash(self, msg_hash: HexBytes):
        signature = self._kms_client.sign(
            KeyId=self._key_id,
            Message=msg_hash,
            MessageType="DIGEST",
            SigningAlgorithm="ECDSA_SHA_256",
        )
        act_signature = signature["Signature"]
        
        return (act_signature)

However, when trying to do the following:

 sign = kms.sign_msg_hash(transaction)
vks = ecdsa.VerifyingKey.from_public_key_recovery_with_digest(
                sign, transaction, curve=ecdsa.SECP256k1, hashfunc=sha256
            )

I get the following error:

ecdsa.util.MalformedSignature: Invalid length of signature, expected 64 bytes long, provided string is 72 bytes long

Now, when trying to use sign[:64] instead, it sometimes works but at other times gives me the following error:

raise SquareRootError("%d has no square root modulo %d" % (a, p))
ecdsa.numbertheory.SquareRootError: 6631794589973073742270549970789085262483305971731159368608959895351281521222 has no square root modulo 115792089237316195423570985008687907853269984665640564039457584007908834671663

I'm not sure if this has anything to do with the encoding of the signature from the KMS or not.

score 1 · Accepted Answer · edited Sep 18 '22 at 02:47

So after some digging, it turned it out it was a matter of encodings, as the kms sign function returns the signature in DER format.

Hence, what I did was the following:

I took the signature and split it, and got the r and s values as follows:

decoding=ecdsa.util.sigdecode_der(sign,int("fffffffffffffffffffffffffffffffebaaedce6af48a03bbfd25e8cd0364141",16))

I then concatenated the result (note that r and s should be in big-endian order):

new_sign = decoding[0].to_bytes(32, byteorder= 'big') + decoding[1].to_bytes(32, byteorder= 'big')

I then used the new sign:

vks = ecdsa.VerifyingKey.from_public_key_recovery_with_digest(
            new_sign, transaction, curve=ecdsa.SECP256k1, hashfunc=sha256
      )

Another thing I encountered was with kms_client.get_public_key()

First call the method:

kms_pub_key_bytes = kms_client.get_public_key(KeyId=key_id)["PublicKey"]

Then get the last 128 chars which are essentially the r and s values
```
kms_pre_sha3 = kms_pub_key_bytes.hex()[-128:]
```

Then do what you want with the public key.

Errors using python's ECDSA lib to sign AWS messages

1 Answers1