3.2 Decode Python Example

This is a Python script example for verifying the signed entries and decoding the contents. The script is tested on Python 2.7 and Python 3.7. Required packages can be installed with the Python package manager pip:

pip install python-jose[cryptography]

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import json
from base64 import b64decode, b16encode
from argparse import ArgumentParser
import jose.jws
from jose.utils import base64url_decode, base64url_encode
from cryptography import x509
from cryptography.hazmat.backends import default_backend
from cryptography.hazmat.primitives import hashes, serialization
from cryptography.hazmat.primitives.asymmetric import ec

parser = ArgumentParser(
    description='Verify and decode secure element manifest'
)
parser.add_argument(
    '--manifest',
    help='Manifest file to process',
    nargs=1,
    type=str,
    required=True,
    metavar='file'
)
parser.add_argument(
    '--cert',
    help='Verification certificate file in PEM format',
    nargs=1,
    type=str,
    required=True,
    metavar='file'
)
args = parser.parse_args()

# List out allowed verification algorithms for the JWS. Only allows
# public-key based ones.
verification_algorithms = [
    'RS256', 'RS384', 'RS512', 'ES256', 'ES384', 'ES512'
]

# Load manifest as JSON
with open(args.manifest[0], 'rb') as f:
    manifest = json.load(f)

# Load verification certificate in PEM format
with open(args.cert[0], 'rb') as f:
    verification_cert = x509.load_pem_x509_certificate(
        data=f.read(),
        backend=default_backend()
    )

# Convert verification certificate public key to PEM format
verification_public_key_pem = verification_cert.public_key().public_bytes(
    encoding=serialization.Encoding.PEM,
    format=serialization.PublicFormat.SubjectPublicKeyInfo
).decode('ascii')

# Get the base64url encoded subject key identifier for the verification cert
ski_ext = verification_cert.extensions.get_extension_for_class(
    extclass=x509.SubjectKeyIdentifier
)
verification_cert_kid_b64 = base64url_encode(
    ski_ext.value.digest
).decode('ascii')

# Get the base64url encoded sha-256 thumbprint for the verification cert
verification_cert_x5t_s256_b64 = base64url_encode(
    verification_cert.fingerprint(hashes.SHA256())
).decode('ascii')

# Process all the entries in the manifest
for i, signed_se in enumerate(manifest):
    print('')
    print('Processing entry {} of {}:'.format(i+1, len(manifest)))
    print('uniqueId: {}'.format(
        signed_se['header']['uniqueId']
    ))
    # Decode the protected header
    protected = json.loads(
        base64url_decode(
            signed_se['protected'].encode('ascii')
        )
    )
    if protected['kid'] != verification_cert_kid_b64:
        raise ValueError('kid does not match certificate value')
    if protected['x5t#S256'] != verification_cert_x5t_s256_b64:
        raise ValueError('x5t#S256 does not match certificate value')

    # Convert JWS to compact form as required by python-jose
    jws_compact = '.'.join([
        signed_se['protected'],
        signed_se['payload'],
        signed_se['signature']
    ])

    # Verify and decode the payload. If verification fails an exception will
    # be raised.
    se = json.loads(
        jose.jws.verify(
            token=jws_compact,
            key=verification_public_key_pem,
            algorithms=verification_algorithms
        )
    )
    if se['uniqueId'] != signed_se['header']['uniqueId']:
        raise ValueError(
            (
                'uniqueId in header "{}" does not match version in' +
                ' payload "{}"'
            ).format(
                signed_se['header']['uniqueId'],
                se['uniqueId']
            )
        )
    print('Verified')

    print('SecureElement = ')
    print(json.dumps(se, indent=2))

    # Decode public keys and certificates
    try:
        public_keys = se['publicKeySet']['keys']
    except KeyError:
        public_keys = []
    for jwk in public_keys:
        print('Public key in slot {}:'.format(int(jwk['kid'])))
        if jwk['kty'] != 'EC':
            raise ValueError(
                'Unsupported {}'.format(json.dumps({'kty': jwk['kty']}))
            )
        if jwk['crv'] != 'P-256':
            raise ValueError(
                'Unsupported {}'.format(json.dumps({'crv': jwk['crv']}))
            )
        # Decode x and y integers
        # Using int.from_bytes() would be more efficient in python 3
        x = int(
            b16encode(base64url_decode(jwk['x'].encode('utf8'))),
            16
        )
        y = int(
            b16encode(base64url_decode(jwk['y'].encode('utf8'))),
            16
        )
        public_key = ec.EllipticCurvePublicNumbers(
            curve=ec.SECP256R1(),
            x=x,
            y=y
        ).public_key(default_backend())
        print(public_key.public_bytes(
            encoding=serialization.Encoding.PEM,
            format=serialization.PublicFormat.SubjectPublicKeyInfo
        ).decode('ascii'))

        # Decode any available certificates
        for cert_b64 in jwk.get('x5c', []):
            cert = x509.load_der_x509_certificate(
                data=b64decode(cert_b64),
                backend=default_backend()
            )
            print(cert.public_bytes(
                encoding=serialization.Encoding.PEM
            ).decode('ascii'))