cwt 0.5.0

A Python implementation of CBOR Web Token (CWT) and CBOR Object Signing and Encryption (COSE).

Python CWT

A Python implementation of CBOR Web Token (CWT) and CBOR Object Signing and Encryption (COSE).

See Document for details:

• Usage Examples
• API Reference
• Supported CWT Claims
• Supported COSE Algorithms
• Referenced Specifications

Installing

Install with pip:

pip install cwt

Usase

Python CWT is an easy-to-use CWT/COSE library a little bit inspired by PyJWT. If you already know about JSON Web Token (JWT), little knowledge of CBOR, COSE and CWT is required to use this library.

>>> import cwt
>>> from cwt import claims, cose_key
>>> key = cose_key.from_symmetric_key(alg="HS256")
>>> token = cwt.encode({"iss": "coaps://as.example", "sub": "dajiaji", "cti": "123"}, key)
>>> token.hex()
'd18443a10105a05835a60172636f6170733a2f2f61732e6578616d706c65026764616a69616a69'
'0743313233041a609097b7051a609089a7061a609089a758201fad9b0a76803194bd11ca9b9b3c'
'bbf1028005e15321665a768994f38c7127f7'
>>> decoded = cwt.decode(token, key)
>>> decoded
{1: 'coaps://as.example', 2: 'dajiaji', 7: b'123',
 4: 1620088759, 5: 1620085159, 6: 1620085159}
>>> readable = claims.from_dict(decoded)
>>> readable.iss
'coaps://as.example'
>>> readable.sub
'dajiaji'
>>> readable.exp
1620088759

Followings are typical and basic examples which create CWT, verify and decode it:

• MACed CWT
• Signed CWT
• Encrypted CWT
• Nested CWT
• CWT with PoP key

See Usage Examples for details.

MACed CWT

Create a MACed CWT with HS256, verify and decode it as follows:

import cwt
from cwt import cose_key

key = cose_key.from_symmetric_key(alg="HS256")
token = cwt.encode({"iss": "coaps://as.example", "sub": "dajiaji", "cti": "123"}, key)
decoded = cwt.decode(token, key)

CBOR-like structure (Dict[int, Any]) can also be used as follows:

import cwt
from cwt import cose_key

key = cose_key.from_symmetric_key(alg="HS256")
token = cwt.encode({1: "coaps://as.example", 2: "dajiaji", 7: b"123"}, key)
decoded = cwt.decode(token, key)

MAC algorithms other than HS256 are listed in Supported COSE Algorithms.

Signed CWT

Create an Ed25519 (Ed25519 for use w/ EdDSA only) key pair:

$ openssl genpkey -algorithm ed25519 -out private_key.pem
$ openssl pkey -in private_key.pem -pubout -out public_key.pem

Create a Signed CWT with Ed25519, verify and decode it with the key pair as follows:

import cwt
from cwt import cose_key

with open("./private_key.pem") as key_file:
    private_key = cose_key.from_pem(key_file.read())
with open("./public_key.pem") as key_file:
    public_key = cose_key.from_pem(key_file.read())


token = cwt.encode(
    {"iss": "coaps://as.example", "sub": "dajiaji", "cti": "123"}, private_key
)

decoded = cwt.decode(token, public_key)

JWKs can also be used instead of the PEM-formatted keys as follows:

import cwt
from cwt import cose_key

private_key = cose_key.from_jwk({
    "kty": "OKP",
    "d": "L8JS08VsFZoZxGa9JvzYmCWOwg7zaKcei3KZmYsj7dc",
    "use": "sig",
    "crv": "Ed25519",
    "kid": "01",
    "x": "2E6dX83gqD_D0eAmqnaHe1TC1xuld6iAKXfw2OVATr0",
    "alg": "EdDSA",
})
public_key = cose_key.from_jwk({
    "kty": "OKP",
    "use": "sig",
    "crv": "Ed25519",
    "kid": "01",
    "x": "2E6dX83gqD_D0eAmqnaHe1TC1xuld6iAKXfw2OVATr0",
})

token = cwt.encode(
    {"iss": "coaps://as.example", "sub": "dajiaji", "cti": "123"}, private_key
)
decoded = cwt.decode(token, public_key)

Algorithms other than Ed25519 are listed in Supported COSE Algorithms.

Encrypted CWT

Create an encrypted CWT with ChaCha20/Poly1305 (ChaCha20/Poly1305 w/ 256-bit key, 128-bit tag), and decrypt it as follows:

import cwt
from cwt import cose_key

enc_key = cose_key.from_symmetric_key(alg="ChaCha20/Poly1305")
token = cwt.encode({"iss": "coaps://as.example", "sub": "dajiaji", "cti": "123"}, enc_key)
decoded = cwt.decode(token, enc_key)

Algorithms other than ChaCha20/Poly1305 are listed in Supported COSE Algorithms.

Nested CWT

Create a signed CWT and encrypt it, and then decrypt and verify the nested CWT as follows.

import cwt
from cwt import cose_key

with open("./private_key.pem") as key_file:
    private_key = cose_key.from_pem(key_file.read())
with open("./public_key.pem") as key_file:
    public_key = cose_key.from_pem(key_file.read())

# Creates a CWT with ES256 signing.
token = cwt.encode(
    {"iss": "coaps://as.example", "sub": "dajiaji", "cti": "123"}, private_key
)

# Encrypts the signed CWT.
enc_key = cose_key.from_symmetric_key(alg="ChaCha20/Poly1305")
nested = cwt.encode(token, enc_key)

# Decrypts and verifies the nested CWT.
decoded = cwt.decode(nested, [enc_key, public_key])

CWT with PoP key

This library supports Proof-of-Possession Key Semantics for CBOR Web Tokens (CWTs). A CWT can include a PoP key as follows:

On the issuer side:

import cwt
from cwt import cose_key

# Prepares a signing key for CWT in advance.
with open("./private_key_of_issuer.pem") as key_file:
    private_key = cose_key.from_pem(key_file.read())

# Sets the PoP key to a CWT for the presenter.
token = cwt.encode(
    {
        "iss": "coaps://as.example",
        "sub": "dajiaji",
        "cti": "123",
        "cnf": {
            "jwk": {  # Provided by the CWT presenter.
                "kty": "OKP",
                "use": "sig",
                "crv": "Ed25519",
                "kid": "01",
                "x": "2E6dX83gqD_D0eAmqnaHe1TC1xuld6iAKXfw2OVATr0",
                "alg": "EdDSA",
            },
        },
    },
    private_key,
)

# Issues the token to the presenter.

On the CWT presenter side:

import cwt
from cwt import cose_key

# Prepares a private PoP key in advance.
with open("./private_pop_key.pem") as key_file:
    pop_key_private = cose_key.from_pem(key_file.read())

# Receives a message (e.g., nonce)  from the recipient.
msg = b"could-you-sign-this-message?"  # Provided by recipient.

# Signs the message with the private PoP key.
sig = pop_key_private.sign(msg)

# Sends the msg and the sig with the CWT to the recipient.

On the CWT recipient side:

import cwt
from cwt import claims, cose_key

# Prepares the public key of the issuer in advance.
with open("./public_key_of_issuer.pem") as key_file:
    public_key = cose_key.from_pem(key_file.read())

# Verifies and decodes the CWT received from the presenter.
raw = cwt.decode(token, public_key)
decoded = claims.from_dict(raw)

# Extracts the PoP key from the CWT.
extracted_pop_key = cose_key.from_dict(decoded.cnf)  # = raw[8][1]

# Then, verifies the message sent by the presenter
# with the signature which is also sent by the presenter as follows:
extracted_pop_key.verify(msg, sig)

Usage Examples shows other examples which use other confirmation methods for PoP keys.

Tests

You can run tests from the project root after cloning with:

$ tox