util.py

from cryptography import x509
from cryptography.hazmat.backends import default_backend
from cryptography.hazmat.primitives import serialization

from cryptography.hazmat.primitives import hashes
from cryptography.hazmat.primitives.asymmetric import ec
from cryptography.hazmat.primitives.serialization import load_pem_public_key
from cryptography.exceptions import InvalidSignature


# extracts and returns public key from a given cert (in pem format)
def extract_public_key(cert):
# read the certificate
#    with open("cert.pem", "rb") as cert_file:
#        cert_data = cert_file.read()

# load the certificate
    certificate = x509.load_pem_x509_certificate(cert, default_backend())

# extract the public key
    public_key = certificate.public_key()

# save the public key to a PEM file
#    with open("cert_public.pem", "wb") as pub_key_file:
#        pub_key_file.write(public_key.public_bytes(
#            encoding=serialization.Encoding.PEM,
#            format=serialization.PublicFormat.SubjectPublicKeyInfo
#        ))
    pem_public_key = public_key.public_bytes(
        encoding=serialization.Encoding.PEM,
        format=serialization.PublicFormat.SubjectPublicKeyInfo
    )

    return pem_public_key

def verify_artifact_signature(signature, public_key, artifact_filename):
    # load the public key
    # with open("cert_public.pem", "rb") as pub_key_file:
    #    public_key = load_pem_public_key(pub_key_file.read())

        # load the signature
    #    with open("hello.sig", "rb") as sig_file:
    #        signature = sig_file.read()

    public_key = load_pem_public_key(public_key)
    # load the data to be verified
    with open(artifact_filename, "rb") as data_file:
        data = data_file.read()

    # verify the signature
    try:
        public_key.verify(
            signature,
            data,
            ec.ECDSA(hashes.SHA256())
        )
    except InvalidSignature as e:
        print("Signature is invalid")
    except Exception as e:
        print("Exception in verifying artifact signature:", e)