pycose:snake: --- CBOR Object Signing and Encryption

This project is a Python implementation of the IETF CBOR Encoded Message Syntax (COSE). COSE has reached RFC status and is now available at RFC 8152.

Installation

$ pip install cose

:warning:WARNING:warning:: There is package on PyPI called pycose which contains old code from this repository. Since I am not the maintainer I cannot update that package or remove it.

What is COSE ?

CBOR Encoded Message Syntax (COSE) is a data format for concise representation of small messages RFC 8152. COSE is optimized for low power devices. The messages can be encrypted, MAC'ed and signed. There are 6 different types of COSE messages:

• Encrypt0: An encrypted COSE message with a single recipient. The payload and AAD are protected by a shared CEK (Content Encryption Keys)
• Encrypt: An encrypted COSE message can have multiple recipients. For each recipient the CEK is encrypted with a KEK (Key Encryption Key) - using AES key wrap - and added to the message.
• MAC0: An authenticated COSE message with one recipient.
• MAC: An authenticated COSE message that can have multiple recipients. For each recipient, the authentication key is encrypted with a KEK and added to the message.
• Sign1: A signed COSE message with a single signature.
• Sign: A COSE message that has been signed by multiple entities (each signature is carried in a COSE signature structure, added to the message).

A basic COSE message consists of 2 information buckets and the payload:

• Protected header: This message field contains information that needs to be protected. This information is taken into account during the encryption, calculation of the MAC or the signature.
• Unprotected header: The information contained in the unprotected header is not protected by the cryptographic algorithms.
• Payload: Contains the payload of the message, protected (mac'ed, signed or encrypted) by the cryptographic algorithms.

Additionally, based on the message type, other message fields can be added:

• MAC or signature (for MAC0 or Sign1 messages)
• COSE recipients or COSE signatures (for MAC, Encrypt, and Sign messages)

Examples

Encoding

from binascii import unhexlify
from cose.messages import Enc0Message
from cose.keys import SymmetricKey

# Create a COSE Encrypt0 Message
msg = Enc0Message(
    phdr={'ALG': 'A128GCM', 'IV': unhexlify(b'01010101010101010101010101010101')},
    uhdr={'KID': b'meriadoc.brandybuck@buckland.example'},
    payload='a secret message'.encode('utf-8')
)

# Create a COSE Symmetric Key
cose_key = SymmetricKey(key=unhexlify(b'000102030405060708090a0b0c0d0e0f'))
msg.key = cose_key

# Performs encryption and CBOR serialization
msg.encode()
b'\xd0\x83U\xa2\x01\x01\x05P\x01\x01\x01\x01\x01\x01\x01\x01\x01\x01\x01\x01\x01\x01\x01\x01\xa1\x04X$meriadoc.brandybuck@buckland.exampleX \xc4\xaf\x85\xacJQ4\x93\x19\x93\xec\n\x18c\xa6\xe8\xc6n\xf4\xc9\xac\x161^\xe6\xfe\xcd\x9b.\x1cy\xa1'

Decoding

from binascii import unhexlify
from cose.messages import CoseMessage
from cose.keys import SymmetricKey

# message bytes (CBOR encoded)
msg =  b'\xd0\x83U\xa2\x01\x01\x05P\x01\x01\x01\x01\x01\x01\x01\x01\x01\x01\x01\x01\x01\x01\x01\x01\xa1\x04X$meriadoc.brandybuck@buckland.exampleX \xc4\xaf\x85\xacJQ4\x93\x19\x93\xec\n\x18c\xa6\xe8\xc6n\xf4\xc9\xac\x161^\xe6\xfe\xcd\x9b.\x1cy\xa1'

cose_msg = CoseMessage.decode(msg)

# Create a COSE Symmetric Key
cose_key = SymmetricKey(key=unhexlify(b'000102030405060708090a0b0c0d0e0f'))
cose_msg.key = cose_key

cose_msg.decrypt()
b'a secret message'

More examples

More examples can be found here

Testing

To run the test suite you need pytest:

$ pip install pytest

Move to the root of the repository and type:

$ pytest

Cryptography

The project depends on pyca/cryptography for all cryptographic operations, except the deterministic ECDSA algorithm. For deterministic ECDSA cose uses python-ecdsa.

Documentation

More documentation on COSE and the cose API can be found at: https://pycose.readthedocs.io