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pycose --- CBOR Object Signing and Encryption

Python package Documentation Status

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 pycose

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 pycose.messages import Enc0Message
from pycose.keys import SymmetricKey

# Create a COSE Encrypt0 Message
msg = Enc0Message(
    phdr={'ALG': 'A128GCM', 'IV': unhexlify(b'01010101010101010101010101010101')},
    uhdr={'KID': b'[email protected]'},
    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\x04[email protected] \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 pycose.messages import Enc0Message
from pycose.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\x04[email protected] \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 = Enc0Message.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