Signing and Verifying messages with Pycardano and COSE

pycardano_cose.py

# Signing and Verifying messages with COSE
from binascii import hexlify, unhexlify
from typing import Tuple, Union

import pycardano
from cose.algorithms import EdDSA
from cose.headers import KID, Algorithm
from cose.keys import CoseKey
from cose.keys.curves import Ed25519
from cose.keys.keyops import SignOp, VerifyOp
from cose.keys.keyparam import KpKeyOps, KpKty, OKPKpCurve, OKPKpD, OKPKpX
from cose.keys.keytype import KtyOKP
from cose.messages import CoseMessage, Sign1Message


# some examping outputs from Nami and CCvault to play around with
# both contain the same message and are signed with the same address

# nami provides a dict object
# the key is the entire COSE key needed to decode the signature, including the wallet verification key
nami_signature = {
    "key": "a401010327200621582079464729926aeba53960944f155aed9a8526a74e0ca688f9988d9e00a4568be2",
    "signature": "845846a20127676164647265737358390152ac4d56d3fa81fd1db7c0a793d27751249cc5d8a1bcf4e3c71bbcd522b1748fb1534a6b8eba3a3a21068e63c2a01f9051d8b2ca511aa5dba166686173686564f44f707963617264616e6f20726f636b7358409728af5a71092c3027386b275879abc1396845beab1386c73108e506847d00b1a0bb0135e02028beb60b133b758c97c32e12588bbf35b07dbfb93d5eb4593f0a",
}

# CCvault provides just the signature, the key is implied to be a standard template
# the `sign_message()` function below provides output in the same format
# in this case, the wallet verification key is attached to the headers of the signature
ccvault_signature = "845869a3012704582079464729926aeba53960944f155aed9a8526a74e0ca688f9988d9e00a4568be2676164647265737358390152ac4d56d3fa81fd1db7c0a793d27751249cc5d8a1bcf4e3c71bbcd522b1748fb1534a6b8eba3a3a21068e63c2a01f9051d8b2ca511aa5dba166686173686564f44f707963617264616e6f20726f636b73584053144591651830a0f724c6934323e667bd444415a7bc28f6567bc961c8c752df577ef09fe6dd628e0d366ee7b86dba8d40194e004d45c5bbcd123acdb5c19a0b"


def sign_message(
    message: str,
    payment_signing_key: pycardano.key.PaymentSigningKey,
    payment_verification_key: pycardano.key.PaymentVerificationKey,
) -> str:
    """
    Sign a message with your payment verificaiton key.

        Parameters:
            message (str): Message to be signed
            payment_signing_key (pycardano.key.PaymentSigningKey): Key which is used to sign the message
            payment_verification_key (pycardano.key.PaymentVerificationKey): Key which will be attached so others can verify the signature

        Returns:
            signed_message (str): a hex-encoded string containing the signed message and verification key
    """

    # create the message object, attach verification key to the header
    msg = Sign1Message(
        phdr={Algorithm: EdDSA, KID: payment_verification_key.payload},
        payload=message.encode("utf-8"),
    )

    # build the CoseSign1 signing key from a dictionary
    cose_key = {
        KpKty: KtyOKP,
        OKPKpCurve: Ed25519,
        KpKeyOps: [SignOp, VerifyOp],
        OKPKpD: payment_signing_key.payload,  # private key
        OKPKpX: payment_verification_key.payload,  # public key
    }

    cose_key = CoseKey.from_dict(cose_key)

    msg.key = cose_key  # attach the key to the message

    encoded = msg.encode()

    # turn the enocded message into a hex string and remove the first byte
    # which is always "d2"
    signed_message = hexlify(encoded).decode("utf-8")[2:]

    return signed_message


def verify_signed_message(
    signed_payload: Union[str, dict]
) -> Tuple[bool, bool, str, pycardano.Address]:
    """
    Verify the signature of a COSESign1 message and decode.
    Supports messages signed by Nami, CCVault, or `sign_message()`.

       Parameters:
            signed_payload (str): A hex-encoded string

       Returns :
            verified (bool): If the signature is verified
            addresses_match (bool): Whether the address provided belongs to the verification key used to sign the message
            message (str): The contents of the signed message
            address (pycardano.Address): The address to which the signing keys belong

        Note: Both `verified` and `address_match` should be True.
    """

    # nami prvoides us with a dictionary, and ccvault with a string

    if isinstance(signed_payload, dict):
        # this is nami, the cose key is attached as a dict object which contains the verification key
        # the headers of the signature are emtpy
        key = signed_payload.get("key")
        signed_message = signed_payload.get("signature")

    else:
        # CCVault and `sign_message()`
        key = ""  # key will be extracted later from the payload headers
        signed_message = signed_payload

    # Add back the "D2" header byte and decode
    decoded_message = CoseMessage.decode(unhexlify("d2" + signed_message))

    # in `sign_message()` and CCVault the verification key is
    # attached to the header, and the key is generated by us from a template

    # generate/extract the cose key
    if not key:
        # i.e. not Nami

        # get the verification key from the headers
        verification_key = decoded_message.phdr[KID]

        # generate the COSE key
        cose_key = {
            KpKty: KtyOKP,
            OKPKpCurve: Ed25519,
            KpKeyOps: [SignOp, VerifyOp],
            OKPKpX: verification_key,  # public key
        }

        cose_key = CoseKey.from_dict(cose_key)

    else:
        # nami, key is attached
        cose_key = CoseKey.decode(unhexlify(key))
        verification_key = cose_key[OKPKpX]

    # attach the key to the decoded message
    decoded_message.key = cose_key

    verified = decoded_message.verify_signature()

    data = decoded_message.payload.decode("utf-8")

    address = pycardano.Address.from_primitive(decoded_message.phdr["address"])

    # check that the address atatched in the headers matches the one of the verification key used to sign the message
    addresses_match = (
        address.payment_part
        == pycardano.PaymentVerificationKey.from_primitive(verification_key).hash()
    )
    
    # normally you want to check both that the message is verified and that the addresses match

    return verified, addresses_match, data, address