Python CWT - A Python implementation of CWT/COSE
Python CWT is a CBOR Web Token (CWT) and CBOR Object Signing and Encryption (COSE) implementation compliant with:
- RFC8392: CWT (CBOR Web Token)
- RFC8152: COSE (CBOR Object Signing and Encryption)
- and related various specifications. See Referenced Specifications.
It is designed to make users who already know about JWS/JWE/JWT be able to use it in ease. Little knowledge of CBOR/COSE/CWT is required to use it.
You can install Python CWT with pip:
$ pip install cwt
And then, you can use it as follows:
>>> import cwt
>>> from cwt import COSEKey
>>> key = COSEKey.from_symmetric_key(alg="HS256", kid="01")
>>> token = cwt.encode({"iss": "coaps://as.example", "sub": "dajiaji", "cti": "123"}, key)
>>> token.hex()
'd18443a10105a05835a60172636f6170733a2f2f61732e657861'...
>>> cwt.decode(token, key)
{1: 'coaps://as.example', 2: 'dajiaji', 7: b'123', 4: 1620088759, 5: 1620085159, 6: 1620085159}
See Documentation for details.
Index
- Installation
- CWT Usage Examples
- COSE Usage Examples
- API Reference
- Supported CWT Claims
- Supported COSE Algorithms
- Referenced Specifications
- Tests
- Contributing
Installation
Install with pip:
pip install cwt
CWT Usage Examples
Followings are typical and basic examples which create various types of CWTs, verify and decode them.
CWT API in the examples are built on top of COSE API.
See API Reference and CWT Usage Examples on document for more details.
MACed CWT
Create a MACed CWT with HS256, verify and decode it as follows:
import cwt
from cwt import Claims, COSEKey
try:
key = COSEKey.from_symmetric_key(alg="HS256", kid="01")
token = cwt.encode({"iss": "coaps://as.example", "sub": "dajiaji", "cti": "123"}, key)
decoded = cwt.decode(token, key)
# If you want to treat the result like a JWT;
readable = Claims.new(decoded)
assert readable.iss == 'coaps://as.example'
assert readable.sub == 'dajiaji'
assert readable.cti == '123'
# readable.exp == 1620088759
# readable.nbf == 1620085159
# readable.iat == 1620085159
except Exception as err:
# All the other examples in this document omit error handling but this CWT library
# can throw following errors:
# ValueError: Invalid arguments.
# EncodeError: Failed to encode.
# VerifyError: Failed to verify.
# DecodeError: Failed to decode.
print(err)
A raw CWT structure (Dict[int, Any]) can also be used as follows:
import cwt
from cwt import COSEKey
key = COSEKey.from_symmetric_key(alg="HS256", kid="01")
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 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 COSEKey
# The sender side:
with open("./private_key.pem") as key_file:
private_key = COSEKey.from_pem(key_file.read(), kid="01")
token = cwt.encode(
{"iss": "coaps://as.example", "sub": "dajiaji", "cti": "123"}, private_key
)
# The recipient side:
with open("./public_key.pem") as key_file:
public_key = COSEKey.from_pem(key_file.read(), kid="01")
decoded = cwt.decode(token, public_key)
JWKs can also be used instead of the PEM-formatted keys as follows:
import cwt
from cwt import COSEKey
# The sender side:
private_key = COSEKey.from_jwk({
"kid": "01",
"kty": "OKP",
"key_ops": ["sign"],
"alg": "EdDSA",
"crv": "Ed25519",
"x": "2E6dX83gqD_D0eAmqnaHe1TC1xuld6iAKXfw2OVATr0",
"d": "L8JS08VsFZoZxGa9JvzYmCWOwg7zaKcei3KZmYsj7dc",
})
token = cwt.encode(
{"iss": "coaps://as.example", "sub": "dajiaji", "cti": "123"}, private_key
)
# The recipient side:
public_key = COSEKey.from_jwk({
"kid": "01",
"kty": "OKP",
"key_ops": ["verify"],
"crv": "Ed25519",
"x": "2E6dX83gqD_D0eAmqnaHe1TC1xuld6iAKXfw2OVATr0",
})
decoded = cwt.decode(token, public_key)
Signing algorithms other than Ed25519 are listed in Supported COSE Algorithms.
Encrypted CWT
Create an encrypted CWT with ChaCha20/Poly1305 and decrypt it as follows:
import cwt
from cwt import COSEKey
enc_key = COSEKey.from_symmetric_key(alg="ChaCha20/Poly1305", kid="01")
token = cwt.encode({"iss": "coaps://as.example", "sub": "dajiaji", "cti": "123"}, enc_key)
decoded = cwt.decode(token, enc_key)
Encryption 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 COSEKey
# A shared encryption key.
enc_key = COSEKey.from_symmetric_key(alg="ChaCha20/Poly1305", kid="enc-01")
# Creates a CWT with ES256 signing.
with open("./private_key.pem") as key_file:
private_key = COSEKey.from_pem(key_file.read(), kid="sig-01")
token = cwt.encode(
{"iss": "coaps://as.example", "sub": "dajiaji", "cti": "123"}, private_key
)
# Encrypts the signed CWT.
nested = cwt.encode(token, enc_key)
# Decrypts and verifies the nested CWT.
with open("./public_key.pem") as key_file:
public_key = COSEKey.from_pem(key_file.read(), kid="sig-01")
decoded = cwt.decode(nested, [enc_key, public_key])
CWT with User Settings
The cwt in cwt.encode() and cwt.decode() above is a global CWT class instance created with default settings in advance. The default settings are as follows:
expires_in:3600seconds. This is the default lifetime in seconds of CWTs.leeway:60seconds. This is the default leeway in seconds for validatingexpandnbf.
If you want to change the settings, you can create your own CWT class instance as follows:
from cwt import COSEKey, CWT
key = COSEKey.from_symmetric_key(alg="HS256", kid="01")
mycwt = CWT.new(expires_in=3600*24, leeway=10)
token = mycwt.encode({"iss": "coaps://as.example", "sub": "dajiaji", "cti": "123"}, key)
decoded = mycwt.decode(token, key)
CWT with User-Defined Claims
You can use your own claims as follows:
Note that such user-defined claim's key should be less than -65536.
import cwt
from cwt import COSEKey
# The sender side:
with open("./private_key.pem") as key_file:
private_key = COSEKey.from_pem(key_file.read(), kid="01")
token = cwt.encode(
{
1: "coaps://as.example", # iss
2: "dajiaji", # sub
7: b"123", # cti
-70001: "foo",
-70002: ["bar"],
-70003: {"baz": "qux"},
-70004: 123,
},
private_key,
)
# The recipient side:
with open("./public_key.pem") as key_file:
public_key = COSEKey.from_pem(key_file.read(), kid="01")
raw = cwt.decode(token, public_key)
assert raw[-70001] == "foo"
assert raw[-70002][0] == "bar"
assert raw[-70003]["baz"] == "qux"
assert raw[-70004] == 123
readable = Claims.new(raw)
assert readable.get(-70001) == "foo"
assert readable.get(-70002)[0] == "bar"
assert readable.get(-70003)["baz"] == "qux"
assert readable.get(-70004) == 123
User-defined claims can also be used with JSON-based claims as follows:
import cwt
from cwt import Claims, COSEKey
with open("./private_key.pem") as key_file:
private_key = COSEKey.from_pem(key_file.read(), kid="01")
my_claim_names = {
"ext_1": -70001,
"ext_2": -70002,
"ext_3": -70003,
"ext_4": -70004,
}
cwt.set_private_claim_names(my_claim_names)
token = cwt.encode(
{
"iss": "coaps://as.example",
"sub": "dajiaji",
"cti": b"123",
"ext_1": "foo",
"ext_2": ["bar"],
"ext_3": {"baz": "qux"},
"ext_4": 123,
},
private_key,
)
with open("./public_key.pem") as key_file:
public_key = COSEKey.from_pem(key_file.read(), kid="01")
raw = cwt.decode(token, public_key)
readable = Claims.new(
raw,
private_claims_names=my_claim_names,
)
assert readable.get("ext_1") == "foo"
assert readable.get("ext_2")[0] == "bar"
assert readable.get("ext_3")["baz"] == "qux"
assert readable.get("ext_4") == 123
CWT with PoP Key
Python CWT 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 COSEKey
# Prepares a signing key for CWT in advance.
with open("./private_key_of_issuer.pem") as key_file:
private_key = COSEKey.from_pem(key_file.read(), kid="issuer-01")
# 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": "presenter-01",
"x": "2E6dX83gqD_D0eAmqnaHe1TC1xuld6iAKXfw2OVATr0",
"alg": "EdDSA",
},
},
},
private_key,
)
# Issues the token to the presenter.
On the CWT presenter side:
import cwt
from cwt import COSEKey
# Prepares a private PoP key in advance.
with open("./private_pop_key.pem") as key_file:
pop_key_private = COSEKey.from_pem(key_file.read(), kid="presenter-01")
# 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, COSEKey
# Prepares the public key of the issuer in advance.
with open("./public_key_of_issuer.pem") as key_file:
public_key = COSEKey.from_pem(key_file.read(), kid="issuer-01")
# Verifies and decodes the CWT received from the presenter.
raw = cwt.decode(token, public_key)
decoded = Claims.new(raw)
# Extracts the PoP key from the CWT.
extracted_pop_key = COSEKey.new(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.
CWT with Private CA
Python CWT supports the case of using an arbitrary private CA as a root of trust. In this case, a COSE message sender needs to specify the trust relationship chaining up to the root CA by using x5chain header parameter. On the other hand, a COSE message receiver needs to specify trusted root CAs by using ca_certs parameter of CWT/COSE constructor (CWT.new() or COSE.new()).
import cwt
from cwt import Claims, COSEKey
# The sernder side:
with open("./private_key_of_cert.pem")) as f:
private_key = COSEKey.from_pem(f.read(), kid="01")
token = cwt.encode(
{"iss": "coaps://as.example", "sub": "dajiaji", "cti": "123"}, private_key
)
# The recipient side:
public_key = COSEKey.from_jwk(
{
"kty": "EC",
"use": "sig",
"crv": "P-256",
"kid": "P-256-01",
"x": "oONCv1QoiajIbcW21Dqy6EnGvBTuF26GU7dy6JzOfXk",
"y": "sl6k77K0TS36FW-TyEGLHY14ovZfdZ9DZWsbA8BTHGc",
"x5c": [
# The DER formatted X509 certificate which pairs with the private_key_of_cert.pem above.
"MIIClDCCAXygAwIBAgIBBDANBgkqhkiG9w0BAQsFADBmMQswCQYDVQQGEwJKUDEOMAwGA1UECAwFVG9reW8xEDAOBgNVBAoMB2RhamlhamkxEzARBgNVBAMMCnB5dGhvbi1jd3QxIDAeBgkqhkiG9w0BCQEWEWRhamlhamlAZ21haWwuY29tMB4XDTIxMTAwMzEzMDE1MFoXDTMxMTAwMTEzMDE1MFowZDELMAkGA1UEBhMCSlAxDjAMBgNVBAgMBVRva3lvMQ0wCwYDVQQKDAR0ZXN0MRUwEwYDVQQDDAx0ZXN0LmV4YW1wbGUxHzAdBgkqhkiG9w0BCQEWEHRlc3RAZXhhbXBsZS5jb20wWTATBgcqhkjOPQIBBggqhkjOPQMBBwNCAASg40K_VCiJqMhtxbbUOrLoSca8FO4XboZTt3LonM59ebJepO-ytE0t-hVvk8hBix2NeKL2X3WfQ2VrGwPAUxxnoxowGDAJBgNVHRMEAjAAMAsGA1UdDwQEAwIE8DANBgkqhkiG9w0BAQsFAAOCAQEAZFfvFbaDk_DmG2cPGTwqwnFok1QnH2Tzkjk7p4vs1ycWzEDltkhyzcJxTSHoQGdykf7fG8NCrEqfi1G3hOyAtGxVIVcqsI-KIJCESp43zrNz5HsbwEY8l5rvcwohKGlE_idIFt5IuDTv7vsg_FaCIDeruw0NrXAACnLTwksawsxaCvtY12U0wsI2aC2Sb6V3HL-OLgcN6ZWzZ054L88JllckYnqJB8wCVBzzX2K2sZH3yeS39oRWZOVG6fwXsX4k0fHFx-Fn6KlrBU15pbjMLMn0ow0X3Y8e7FOgfkkph-N7e2SxceXNjrLiumOdclPm9yGSWoGsOJdId53dPvqAsQ",
# The root certificate which is used for signing the above certificate (optional).
"MIIDrzCCApegAwIBAgIUIK_CYzdq4BLLVXqSclNBgXy6mgswDQYJKoZIhvcNAQELBQAwZjELMAkGA1UEBhMCSlAxDjAMBgNVBAgMBVRva3lvMRAwDgYDVQQKDAdkYWppYWppMRMwEQYDVQQDDApweXRob24tY3d0MSAwHgYJKoZIhvcNAQkBFhFkYWppYWppQGdtYWlsLmNvbTAgFw0yMTEwMDIyMzU0NTZaGA8yMDcxMDkyMDIzNTQ1NlowZjELMAkGA1UEBhMCSlAxDjAMBgNVBAgMBVRva3lvMRAwDgYDVQQKDAdkYWppYWppMRMwEQYDVQQDDApweXRob24tY3d0MSAwHgYJKoZIhvcNAQkBFhFkYWppYWppQGdtYWlsLmNvbTCCASIwDQYJKoZIhvcNAQEBBQADggEPADCCAQoCggEBANFg4sw-uPWbPBbkJuohXc89O0gaqG1H2i1wzxxka32XNKIdwrxOJvsB2eALo3q7dTqLKCgzrjdd5N07gi0KzqjoIXIXqKpV5tm0fP5gCzEOWgxySCfBJOJyyvO6WvYXdvukEBnL-48D8RSjQH9fQEju5RG0taFZE-0nQ7n3P0J-Q-OfBUEoRiHvCd8oUx0s-fBpKdfhMAbD1sGAQ9CokUFeWc49em8inNqia5xljBtSYo6_2Zx9eb7B53wvBC0EmtS4SRyksR2emlr6GxMj_EZW7hcTfZCM4V2JYXliuAEdxA0sB7q-WqLg4OvltBQxCBgTTEXRCzxj3XXZy7QyUacCAwEAAaNTMFEwHQYDVR0OBBYEFA9id2cL_Chjv6liRN3HD849TARsMB8GA1UdIwQYMBaAFA9id2cL_Chjv6liRN3HD849TARsMA8GA1UdEwEB_wQFMAMBAf8wDQYJKoZIhvcNAQELBQADggEBAArIej5eJN1OmD3l3ef9QzosCxKThNwqNY55CoSSC3IRl-IAXy9Lvx7cgiliwBgCv99RbXZ1ZnptTHC_1kzMzPhPg9pGKDowFP-rywaB9-NTuHTWQ4hkKDsru5dpf75ILNI5PTUi1iiBM7TdgSerpEVroUWZiOpGAdlKkmE1h4gkR6eQY9Q0IvVXwagy_PPoQ1XO1i5Hyg3aXeDZBgkE7AuW9uxtYQHzg8JG2TNko_yp497yf_Ew4t6KzGDhSa8L1euMPtclALDWFhgl6WmYsHOqAOsyZOLwpsifWa533wI9mtTvLEg8TFKMOdU0sbAoQSbrrI9m4QS7mzDLchngj3E"
],
"alg": "ES256"
})
# The recipient can specify trusted CAs as follows:
decoder = CWT.new(ca_certs="/path/to/cacerts.pem")
decoded = decoder.decode(token, public_key)
assert 1 in decoded and decoded[1] == "coaps://as.example"
CWT for EUDCC (EU Digital COVID Certificate)
Python CWT supports Electronic Health Certificate Specification and EUDCC (EU Digital COVID Certificate) compliant with Technical Specifications for Digital Green Certificates Volume 1
A following example shows how to verify an EUDCC:
import cwt
from cwt import load_pem_hcert_dsc
# A DSC(Document Signing Certificate) issued by a CSCA
# (Certificate Signing Certificate Authority) quoted from:
# https://github.com/eu-digital-green-certificates/dgc-testdata/blob/main/AT/2DCode/raw/1.json
dsc = "-----BEGIN CERTIFICATE-----\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\n-----END CERTIFICATE-----"
# An EUDCC (EU Digital COVID Certificate) quoted from:
# https://github.com/eu-digital-green-certificates/dgc-testdata/blob/main/AT/2DCode/raw/1.json
eudcc = bytes.fromhex(
"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"
)
public_key = load_pem_hcert_dsc(dsc)
decoded = cwt.decode(eudcc, keys=[public_key])
claims = Claims.new(decoded)
# claims.hcert[1] ==
# {
# 'v': [
# {
# 'dn': 1,
# 'ma': 'ORG-100030215',
# 'vp': '1119349007',
# 'dt': '2021-02-18',
# 'co': 'AT',
# 'ci': 'URN:UVCI:01:AT:10807843F94AEE0EE5093FBC254BD813#B',
# 'mp': 'EU/1/20/1528',
# 'is': 'Ministry of Health, Austria',
# 'sd': 2,
# 'tg': '840539006',
# }
# ],
# 'nam': {
# 'fnt': 'MUSTERFRAU<GOESSINGER',
# 'fn': 'Musterfrau-Gößinger',
# 'gnt': 'GABRIELE',
# 'gn': 'Gabriele',
# },
# 'ver': '1.0.0',
# 'dob': '1998-02-26',
# }
COSE Usage Examples
Followings are typical and basic examples which create various types of COSE messages, verify and decode them.
See API Reference and COSE Usage Examples on document for more details.
COSE MAC0
Create a COSE MAC0 message, verify and decode it as follows:
from cwt import COSE, COSEKey
mac_key = COSEKey.from_symmetric_key(alg="HS256", kid="01")
ctx = COSE.new(alg_auto_inclusion=True, kid_auto_inclusion=True)
encoded = ctx.encode_and_mac(b"Hello world!", mac_key)
assert b"Hello world!" == ctx.decode(encoded, mac_key)
Following two samples are other ways of writing the above example:
from cwt import COSE, COSEKey
mac_key = COSEKey.from_symmetric_key(alg="HS256", kid="01")
ctx = COSE.new()
encoded = ctx.encode_and_mac(
b"Hello world!",
mac_key,
protected={"alg": "HS256"},
unprotected={"kid": "01"},
)
assert b"Hello world!" == ctx.decode(encoded, mac_key)
from cwt import COSE, COSEKey
mac_key = COSEKey.from_symmetric_key(alg="HS256", kid="01")
ctx = COSE.new()
encoded = ctx.encode_and_mac(
b"Hello world!",
mac_key,
protected={1: 5},
unprotected={4: b"01"},
)
assert b"Hello world!" == ctx.decode(encoded, mac_key)
COSE MAC
Direct Key Distribution for MAC
The direct key distribution shares a MAC key between the sender and the recipient that is used directly. The follwing example shows the simplest way to make a COSE MAC message, verify and decode it with the direct key distribution method.
from cwt import COSE, COSEKey, Recipient
# The sender makes a COSE MAC message as follows:
mac_key = COSEKey.from_symmetric_key(alg="HS512", kid="01")
r = Recipient.from_jwk({"alg": "direct"})
r.apply(mac_key)
ctx = COSE.new()
encoded = ctx.encode_and_mac(b"Hello world!", mac_key, recipients=[r])
# The recipient has the same MAC key and can verify and decode it:
assert b"Hello world!" == ctx.decode(encoded, mac_key)
Direct Key with KDF for MAC
from secrets import token_bytes
from cwt import COSE, COSEKey, Recipient
shared_material = token_bytes(32)
shared_key = COSEKey.from_symmetric_key(shared_material, kid="01")
# The sender side:
r = Recipient.from_jwk(
{
"kty": "oct",
"alg": "direct+HKDF-SHA-256",
},
)
mac_key = r.apply(shared_key, context={"alg": "HS256"})
ctx = COSE.new(alg_auto_inclusion=True)
encoded = ctx.encode_and_mac(
b"Hello world!",
key=mac_key,
recipients=[r],
)
# The recipient side:
assert b"Hello world!" == ctx.decode(encoded, shared_key, context={"alg": "HS256"})
AES Key Wrap for MAC
The AES key wrap algorithm can be used to wrap a MAC key as follows:
from cwt import COSE, COSEKey, Recipient
# The sender side:
mac_key = COSEKey.from_symmetric_key(alg="HS512")
r = Recipient.from_jwk(
{
"kid": "01",
"alg": "A128KW",
"k": "hJtXIZ2uSN5kbQfbtTNWbg", # A shared wrapping key
},
)
r.apply(mac_key)
ctx = COSE.new(alg_auto_inclusion=True)
encoded = ctx.encode_and_mac(b"Hello world!", key=mac_key, recipients=[r])
# The recipient side:
shared_key = COSEKey.from_jwk(
{
"kid": "01",
"kty": "oct",
"alg": "A128KW",
"k": "hJtXIZ2uSN5kbQfbtTNWbg",
},
)
assert b"Hello world!" == ctx.decode(encoded, shared_key)
Direct Key Agreement for MAC
The direct key agreement methods can be used to create a shared secret. A KDF (Key Distribution Function) is then applied to the shared secret to derive a key to be used to protect the data. The follwing example shows a simple way to make a COSE Encrypt message, verify and decode it with the direct key agreement methods (ECDH-ES+HKDF-256 with various curves).
from cwt import COSE, COSEKey, Recipient
# The sender side:
r = Recipient.from_jwk(
{
"kty": "EC",
"alg": "ECDH-ES+HKDF-256",
"crv": "P-256",
},
)
# The following key is provided by the recipient in advance.
pub_key = COSEKey.from_jwk(
{
"kid": "01",
"kty": "EC",
"alg": "ECDH-ES+HKDF-256",
"crv": "P-256",
"x": "Ze2loSV3wrroKUN_4zhwGhCqo3Xhu1td4QjeQ5wIVR0",
"y": "HlLtdXARY_f55A3fnzQbPcm6hgr34Mp8p-nuzQCE0Zw",
}
)
mac_key = r.apply(recipient_key=pub_key, context={"alg": "HS256"})
ctx = COSE.new(alg_auto_inclusion=True)
encoded = ctx.encode_and_mac(
b"Hello world!",
key=mac_key,
recipients=[r],
)
# The recipient side:
# The following key is the private key of the above pub_key.
priv_key = COSEKey.from_jwk(
{
"kid": "01",
"kty": "EC",
"alg": "ECDH-ES+HKDF-256",
"crv": "P-256",
"x": "Ze2loSV3wrroKUN_4zhwGhCqo3Xhu1td4QjeQ5wIVR0",
"y": "HlLtdXARY_f55A3fnzQbPcm6hgr34Mp8p-nuzQCE0Zw",
"d": "r_kHyZ-a06rmxM3yESK84r1otSg-aQcVStkRhA-iCM8",
}
)
# The enc_key will be derived in decode() with priv_key and
# the sender's public key which is conveyed as the recipient
# information structure in the COSE Encrypt message (encoded).
assert b"Hello world!" == ctx.decode(encoded, priv_key, context={"alg": "HS256"})
Key Agreement with Key Wrap for MAC
from cwt import COSE, COSEKey, Recipient
# The sender side:
mac_key = COSEKey.from_symmetric_key(alg="HS256")
r = Recipient.from_jwk(
{
"kty": "EC",
"alg": "ECDH-SS+A128KW",
"crv": "P-256",
"x": "7cvYCcdU22WCwW1tZXR8iuzJLWGcd46xfxO1XJs-SPU",
"y": "DzhJXgz9RI6TseNmwEfLoNVns8UmvONsPzQDop2dKoo",
"d": "Uqr4fay_qYQykwcNCB2efj_NFaQRRQ-6fHZm763jt5w",
}
)
pub_key = COSEKey.from_jwk(
{
"kid": "[email protected]",
"kty": "EC",
"crv": "P-256",
"x": "Ze2loSV3wrroKUN_4zhwGhCqo3Xhu1td4QjeQ5wIVR0",
"y": "HlLtdXARY_f55A3fnzQbPcm6hgr34Mp8p-nuzQCE0Zw",
}
)
r.apply(mac_key, recipient_key=pub_key, context={"alg": "HS256"})
ctx = COSE.new(alg_auto_inclusion=True)
encoded = ctx.encode_and_mac(
b"Hello world!",
key=mac_key,
recipients=[r],
)
# The recipient side:
priv_key = COSEKey.from_jwk(
{
"kid": "[email protected]",
"kty": "EC",
"crv": "P-256",
"alg": "ECDH-SS+A128KW",
"x": "Ze2loSV3wrroKUN_4zhwGhCqo3Xhu1td4QjeQ5wIVR0",
"y": "HlLtdXARY_f55A3fnzQbPcm6hgr34Mp8p-nuzQCE0Zw",
"d": "r_kHyZ-a06rmxM3yESK84r1otSg-aQcVStkRhA-iCM8",
}
)
assert b"Hello world!" == ctx.decode(encoded, priv_key, context={"alg": "HS256"})
COSE Encrypt0
Create a COSE Encrypt0 message, verify and decode it as follows:
from cwt import COSE, COSEKey
enc_key = COSEKey.from_symmetric_key(alg="ChaCha20/Poly1305", kid="01")
ctx = COSE.new(alg_auto_inclusion=True, kid_auto_inclusion=True)
encoded = ctx.encode_and_encrypt(b"Hello world!", enc_key)
decoded = ctx.decode(encoded, enc_key)
COSE Encrypt
Direct Key Distribution for encryption
The direct key distribution shares a MAC key between the sender and the recipient that is used directly. The follwing example shows the simplest way to make a COSE MAC message, verify and decode it with the direct key distribution method.
from cwt import COSE, COSEKey, Recipient
enc_key = COSEKey.from_symmetric_key(alg="ChaCha20/Poly1305", kid="01")
# The sender side:
nonce = enc_key.generate_nonce()
r = Recipient.from_jwk({"alg": "direct"})
r.apply(enc_key)
ctx = COSE.new()
encoded = ctx.encode_and_encrypt(
b"Hello world!",
enc_key,
nonce=nonce,
recipients=[r],
)
# The recipient side:
assert b"Hello world!" == ctx.decode(encoded, enc_key)
Direct Key with KDF for encryption
from cwt import COSE, COSEKey, Recipient
shared_material = token_bytes(32)
shared_key = COSEKey.from_symmetric_key(shared_material, kid="01")
# The sender side:
r = Recipient.from_jwk(
{
"kty": "oct",
"alg": "direct+HKDF-SHA-256",
},
)
enc_key = r.apply(shared_key, context={"alg": "A256GCM"})
ctx = COSE.new(alg_auto_inclusion=True)
encoded = ctx.encode_and_encrypt(
b"Hello world!",
key=enc_key,
recipients=[r],
)
# The recipient side:
assert b"Hello world!" == ctx.decode(encoded, shared_key, context={"alg": "A256GCM"})
AES Key Wrap for encryption
The AES key wrap algorithm can be used to wrap a MAC key as follows:
from cwt import COSE, COSEKey, Recipient
# The sender side:
r = Recipient.from_jwk(
{
"kid": "01",
"kty": "oct",
"alg": "A128KW",
"k": "hJtXIZ2uSN5kbQfbtTNWbg", # A shared wrapping key
},
)
enc_key = COSEKey.from_symmetric_key(alg="ChaCha20/Poly1305")
r.apply(enc_key)
ctx = COSE.new(alg_auto_inclusion=True)
encoded = ctx.encode_and_encrypt(b"Hello world!", key=enc_key, recipients=[r])
# The recipient side:
shared_key = COSEKey.from_jwk(
{
"kid": "01",
"kty": "oct",
"alg": "A128KW",
"k": "hJtXIZ2uSN5kbQfbtTNWbg",
},
)
assert b"Hello world!" == ctx.decode(encoded, shared_key)
Direct Key Agreement for encryption
The direct key agreement methods can be used to create a shared secret. A KDF (Key Distribution Function) is then applied to the shared secret to derive a key to be used to protect the data. The follwing example shows a simple way to make a COSE Encrypt message, verify and decode it with the direct key agreement methods (ECDH-ES+HKDF-256 with various curves).
from cwt import COSE, COSEKey, Recipient
# The sender side:
r = Recipient.from_jwk(
{
"kty": "OKP",
"alg": "ECDH-ES+HKDF-256",
"crv": "X25519",
},
)
pub_key = COSEKey.from_jwk(
{
"kid": "01",
"kty": "OKP",
"alg": "ECDH-ES+HKDF-256",
"crv": "X25519",
"x": "y3wJq3uXPHeoCO4FubvTc7VcBuqpvUrSvU6ZMbHDTCI",
}
)
enc_key = r.apply(recipient_key=pub_key, context={"alg": "A128GCM"})
ctx = COSE.new(alg_auto_inclusion=True)
encoded = ctx.encode_and_encrypt(
b"Hello world!",
key=enc_key,
recipients=[r],
)
# The recipient side:
priv_key = COSEKey.from_jwk(
{
"kid": "01",
"kty": "OKP",
"alg": "ECDH-ES+HKDF-256",
"crv": "X25519",
"x": "y3wJq3uXPHeoCO4FubvTc7VcBuqpvUrSvU6ZMbHDTCI",
"d": "vsJ1oX5NNi0IGdwGldiac75r-Utmq3Jq4LGv48Q_Qc4",
}
)
assert b"Hello world!" == ctx.decode(encoded, priv_key, context={"alg": "A128GCM"})
Key Agreement with Key Wrap for encryption
from cwt import COSE, COSEKey, Recipient
# The sender side:
enc_key = COSEKey.from_symmetric_key(alg="A128GCM")
nonce = enc_key.generate_nonce()
r = Recipient.from_jwk(
{
"kty": "EC",
"alg": "ECDH-SS+A128KW",
"crv": "P-256",
"x": "7cvYCcdU22WCwW1tZXR8iuzJLWGcd46xfxO1XJs-SPU",
"y": "DzhJXgz9RI6TseNmwEfLoNVns8UmvONsPzQDop2dKoo",
"d": "Uqr4fay_qYQykwcNCB2efj_NFaQRRQ-6fHZm763jt5w",
}
)
pub_key = COSEKey.from_jwk(
{
"kid": "[email protected]",
"kty": "EC",
"crv": "P-256",
"x": "Ze2loSV3wrroKUN_4zhwGhCqo3Xhu1td4QjeQ5wIVR0",
"y": "HlLtdXARY_f55A3fnzQbPcm6hgr34Mp8p-nuzQCE0Zw",
}
)
r.apply(enc_key, recipient_key=pub_key, context={"alg": "A128GCM"})
ctx = COSE.new(alg_auto_inclusion=True)
encoded = ctx.encode_and_encrypt(
b"Hello world!",
key=enc_key,
nonce=nonce,
recipients=[r],
)
# The recipient side:
priv_key = COSEKey.from_jwk(
{
"kid": "[email protected]",
"kty": "EC",
"alg": "ECDH-SS+A128KW",
"crv": "P-256",
"x": "Ze2loSV3wrroKUN_4zhwGhCqo3Xhu1td4QjeQ5wIVR0",
"y": "HlLtdXARY_f55A3fnzQbPcm6hgr34Mp8p-nuzQCE0Zw",
"d": "r_kHyZ-a06rmxM3yESK84r1otSg-aQcVStkRhA-iCM8",
}
)
assert b"Hello world!" == ctx.decode(encoded, priv_key, context={"alg": "A128GCM"})
COSE Signature1
Create a COSE Signature1 message, verify and decode it as follows:
from cwt import COSE, COSEKey, Signer
# The sender side:
signer = Signer.new(
cose_key=COSEKey.from_jwk(
{
"kid": "01",
"kty": "EC",
"crv": "P-256",
"x": "usWxHK2PmfnHKwXPS54m0kTcGJ90UiglWiGahtagnv8",
"y": "IBOL-C3BttVivg-lSreASjpkttcsz-1rb7btKLv8EX4",
"d": "V8kgd2ZBRuh2dgyVINBUqpPDr7BOMGcF22CQMIUHtNM",
}
),
protected={"alg": "ES256"},
unprotected={"kid": "01"},
)
ctx = COSE.new()
encoded = ctx.encode_and_sign(b"Hello world!", signers=[signer])
# The recipient side:
pub_key = COSEKey.from_jwk(
{
"kid": "01",
"kty": "EC",
"crv": "P-256",
"x": "usWxHK2PmfnHKwXPS54m0kTcGJ90UiglWiGahtagnv8",
"y": "IBOL-C3BttVivg-lSreASjpkttcsz-1rb7btKLv8EX4",
}
)
assert b"Hello world!" == ctx.decode(encoded, pub_key)
COSE Signature
Create a COSE Signature message, verify and decode it as follows:
from cwt import COSE, COSEKey, Signer
# The sender side:
signer = Signer.new(
cose_key=COSEKey.from_jwk(
{
"kid": "01",
"kty": "EC",
"crv": "P-256",
"x": "usWxHK2PmfnHKwXPS54m0kTcGJ90UiglWiGahtagnv8",
"y": "IBOL-C3BttVivg-lSreASjpkttcsz-1rb7btKLv8EX4",
"d": "V8kgd2ZBRuh2dgyVINBUqpPDr7BOMGcF22CQMIUHtNM",
}
),
protected={1: -7},
unprotected={4: b"01"},
)
ctx = COSE.new()
encoded = ctx.encode_and_sign(b"Hello world!", signers=[signer])
# The recipient side:
pub_key = COSEKey.from_jwk(
{
"kid": "01",
"kty": "EC",
"crv": "P-256",
"x": "usWxHK2PmfnHKwXPS54m0kTcGJ90UiglWiGahtagnv8",
"y": "IBOL-C3BttVivg-lSreASjpkttcsz-1rb7btKLv8EX4",
}
)
assert b"Hello world!" == ctx.decode(encoded, pub_key)
API Reference
See Documentation.
Supported CWT Claims
See Documentation.
Supported COSE Algorithms
See Documentation.
Referenced Specifications
Python CWT is (partially) compliant with following specifications:
- RFC8152: CBOR Object Signing and Encryption (COSE)
- RFC8230: Using RSA Algorithms with COSE Messages
- RFC8392: CBOR Web Token (CWT)
- RFC8747: Proof-of-Possession Key Semantics for CBOR Web Tokens (CWTs)
- RFC8812: COSE and JOSE Registrations for Web Authentication (WebAuthn) Algorithms
- Electronic Health Certificate Specification
- Technical Specifications for Digital Green Certificates Volume 1
Tests
You can run tests from the project root after cloning with:
$ tox
Contributing
We welcome all kind of contributions, filing issues, suggesting new features or sending PRs.
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