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JSON Web Token implementation in Python
Python

Merge pull request #174 from gabrielg/master

Fail on encode and decode of bad JWS header values
latest commit c5ee34e86b
@mark-adams mark-adams authored

README.md

PyJWT

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A Python implementation of JSON Web Token draft 32. Original implementation was written by @progrium.

Installing

$ pip install PyJWT

A Note on Dependencies:

RSA and ECDSA signatures depend on the recommended cryptography package (0.8+). If you plan on using any of those algorithms, you'll need to install it as well.

$ pip install cryptography

If your system doesn't allow installing cryptography like on Google App Engine, you can install PyCrypto for RSA signatures and ecdsa for ECDSA signatures.

Usage

>>> import jwt
>>> encoded = jwt.encode({'some': 'payload'}, 'secret', algorithm='HS256')
'eyJhbGciOiJIUzI1NiIsInR5cCI6IkpXVCJ9.eyJzb21lIjoicGF5bG9hZCJ9.4twFt5NiznN84AWoo1d7KO1T_yoc0Z6XOpOVswacPZg'

Additional headers may also be specified.

>>> jwt.encode({'some': 'payload'}, 'secret', algorithm='HS256', headers={'kid': '230498151c214b788dd97f22b85410a5'})
'eyJhbGciOiJIUzI1NiIsInR5cCI6IkpXVCIsImtpZCI6IjIzMDQ5ODE1MWMyMTRiNzg4ZGQ5N2YyMmI4NTQxMGE1In0.eyJzb21lIjoicGF5bG9hZCJ9.DogbDGmMHgA_bU05TAB-R6geQ2nMU2BRM-LnYEtefwg'

Note the resulting JWT will not be encrypted, but verifiable with a secret key.

>>> jwt.decode(encoded, 'secret', algorithms=['HS256'])
{u'some': u'payload'}

If the secret is wrong, it will raise a jwt.DecodeError telling you as such. You can still get the payload by setting the verify argument to False.

>>> jwt.decode(encoded, verify=False)
{u'some': u'payload'}

Validation

Exceptions can be raised during decode() for other errors besides an invalid signature (e.g. for invalid issuer or audience (see below). All exceptions that signify that the token is invalid extend from the base InvalidTokenError exception class, so applications can use this approach to catch any issues relating to invalid tokens:

try:
     payload = jwt.decode(encoded)
except jwt.InvalidTokenError:
     pass  # do something sensible here, e.g. return HTTP 403 status code

Skipping Claim Verification

You may also override claim verification via the options dictionary. The default options are:

options = {
   'verify_signature': True,
   'verify_exp': True,
   'verify_nbf': True,
   'verify_iat': True,
   'verify_aud': True
   'require_exp': False,
   'require_iat': False,
   'require_nbf': False
}

You can skip validation of individual claims by passing an options dictionary with the "verify_" key set to False when you call jwt.decode(). For example, if you want to verify the signature of a JWT that has already expired, you could do so by setting verify_exp to False.

>>> options = {
>>>    'verify_exp': False,
>>> }

>>> encoded = '...' # JWT with an expired exp claim
>>> jwt.decode(encoded, 'secret', options=options)
{u'some': u'payload'}

NOTE: Changing the default behavior is done at your own risk, and almost certainly will make your application less secure. Doing so should only be done with a very clear understanding of what you are doing.

Requiring Optional Claims

In addition to skipping certain validations, you may also specify that certain optional claims are required by setting the appropriate require_<claim_name> option to True. If the claim is not present, PyJWT will raise a jwt.exceptions.MissingRequiredClaimError.

For instance, the following code would require that the token has a 'exp' claim and raise an error if it is not present:

>>> options = {
>>>     'require_exp': True
>>> }

>>> encoded =  '...' # JWT without an exp claim
>>> jwt.decode(encoded, 'secret', options=options)
jwt.exceptions.MissingRequiredClaimError: Token is missing the "exp" claim

Tests

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

$ python setup.py test

Algorithms

The JWT spec supports several algorithms for cryptographic signing. This library currently supports:

  • HS256 - HMAC using SHA-256 hash algorithm (default)
  • HS384 - HMAC using SHA-384 hash algorithm
  • HS512 - HMAC using SHA-512 hash algorithm
  • ES256 - ECDSA signature algorithm using SHA-256 hash algorithm
  • ES384 - ECDSA signature algorithm using SHA-384 hash algorithm
  • ES512 - ECDSA signature algorithm using SHA-512 hash algorithm
  • RS256 - RSASSA-PKCS1-v1_5 signature algorithm using SHA-256 hash algorithm
  • RS384 - RSASSA-PKCS1-v1_5 signature algorithm using SHA-384 hash algorithm
  • RS512 - RSASSA-PKCS1-v1_5 signature algorithm using SHA-512 hash algorithm
  • PS256 - RSASSA-PSS signature using SHA-256 and MGF1 padding with SHA-256
  • PS384 - RSASSA-PSS signature using SHA-384 and MGF1 padding with SHA-384
  • PS512 - RSASSA-PSS signature using SHA-512 and MGF1 padding with SHA-512

Encoding

You can specify which algorithm you would like to use to sign the JWT by using the algorithm parameter:

>>> encoded = jwt.encode({'some': 'payload'}, 'secret', algorithm='HS512')
'eyJhbGciOiJIUzUxMiIsInR5cCI6IkpXVCJ9.eyJzb21lIjoicGF5bG9hZCJ9.WTzLzFO079PduJiFIyzrOah54YaM8qoxH9fLMQoQhKtw3_fMGjImIOokijDkXVbyfBqhMo2GCNu4w9v7UXvnpA'

Decoding

When decoding, you can specify which algorithms you would like to permit when validating the JWT by using the algorithms parameter which takes a list of allowed algorithms:

>>> jwt.decode(encoded, 'secret', algorithms=['HS512', 'HS256'])
{u'some': u'payload'}

In the above case, if the JWT has any value for its alg header other than HS512 or HS256, the claim will be rejected with an InvalidAlgorithmError.

Asymmetric (Public-key) Algorithms

Usage of RSA (RS*) and EC (EC*) algorithms require a basic understanding of how public-key cryptography is used with regards to digital signatures. If you are unfamiliar, you may want to read this article.

When using the RSASSA-PKCS1-v1_5 algorithms, the key argument in both jwt.encode() and jwt.decode() ("secret" in the examples) is expected to be either an RSA public or private key in PEM or SSH format. The type of key (private or public) depends on whether you are signing or verifying.

When using the ECDSA algorithms, the key argument is expected to be an Elliptic Curve public or private key in PEM format. The type of key (private or public) depends on whether you are signing or verifying.

Support of registered claim names

JSON Web Token defines some registered claim names and defines how they should be used. PyJWT supports these registered claim names:

  • "exp" (Expiration Time) Claim
  • "nbf" (Not Before Time) Claim
  • "iss" (Issuer) Claim
  • "aud" (Audience) Claim
  • "iat" (Issued At) Claim

Expiration Time Claim

From the JWT spec:

The "exp" (expiration time) claim identifies the expiration time on or after which the JWT MUST NOT be accepted for processing. The processing of the "exp" claim requires that the current date/time MUST be before the expiration date/time listed in the "exp" claim. Implementers MAY provide for some small leeway, usually no more than a few minutes, to account for clock skew. Its value MUST be a number containing a NumericDate value. Use of this claim is OPTIONAL.

You can pass the expiration time as a UTC UNIX timestamp (an int) or as a datetime, which will be converted into an int. For example:

jwt.encode({'exp': 1371720939}, 'secret')

jwt.encode({'exp': datetime.utcnow()}, 'secret')

Expiration time is automatically verified in jwt.decode() and raises jwt.ExpiredSignatureError if the expiration time is in the past:

import jwt

try:
    jwt.decode('JWT_STRING', 'secret')
except jwt.ExpiredSignatureError:
    # Signature has expired

Expiration time will be compared to the current UTC time (as given by timegm(datetime.utcnow().utctimetuple())), so be sure to use a UTC timestamp or datetime in encoding.

You can turn off expiration time verification with the verify_exp parameter in the options argument.

PyJWT also supports the leeway part of the expiration time definition, which means you can validate a expiration time which is in the past but not very far. For example, if you have a JWT payload with a expiration time set to 30 seconds after creation but you know that sometimes you will process it after 30 seconds, you can set a leeway of 10 seconds in order to have some margin:

import datetime
import time
import jwt

jwt_payload = jwt.encode({
    'exp': datetime.datetime.utcnow() + datetime.timedelta(seconds=30)
}, 'secret')

time.sleep(32)

# JWT payload is now expired
# But with some leeway, it will still validate
jwt.decode(jwt_payload, 'secret', leeway=10)

Instead of specifying the leeway as a number of seconds, a datetime.timedelta instance can be used. The last line in the example above is equivalent to:

jwt.decode(jwt_payload, 'secret', leeway=datetime.timedelta(seconds=10))

Not Before Time Claim

The "nbf" (not before) claim identifies the time before which the JWT MUST NOT be accepted for processing. The processing of the "nbf" claim requires that the current date/time MUST be after or equal to the not-before date/time listed in the "nbf" claim. Implementers MAY provide for some small leeway, usually no more than a few minutes, to account for clock skew. Its value MUST be a number containing a NumericDate value. Use of this claim is OPTIONAL.

The nbf claim works similarly to the exp claim above.

jwt.encode({'nbf': 1371720939}, 'secret')

jwt.encode({'nbf': datetime.utcnow()}, 'secret')

Issuer Claim

The "iss" (issuer) claim identifies the principal that issued the JWT. The processing of this claim is generally application specific. The "iss" value is a case-sensitive string containing a StringOrURI value. Use of this claim is OPTIONAL.

import jwt


payload = {
    'some': 'payload',
    'iss': 'urn:foo'
}

token = jwt.encode(payload, 'secret')
decoded = jwt.decode(token, 'secret', issuer='urn:foo')

If the issuer claim is incorrect, jwt.InvalidIssuerError will be raised.

Audience Claim

The "aud" (audience) claim identifies the recipients that the JWT is intended for. Each principal intended to process the JWT MUST identify itself with a value in the audience claim. If the principal processing the claim does not identify itself with a value in the "aud" claim when this claim is present, then the JWT MUST be rejected. In the general case, the "aud" value is an array of case- sensitive strings, each containing a StringOrURI value. In the special case when the JWT has one audience, the "aud" value MAY be a single case-sensitive string containing a StringOrURI value. The interpretation of audience values is generally application specific. Use of this claim is OPTIONAL.

import jwt


payload = {
    'some': 'payload',
    'aud': 'urn:foo'
}

token = jwt.encode(payload, 'secret')
decoded = jwt.decode(token, 'secret', audience='urn:foo')

If the audience claim is incorrect, jwt.InvalidAudienceError will be raised.

Issued At Claim

The iat (issued at) claim identifies the time at which the JWT was issued. This claim can be used to determine the age of the JWT. Its value MUST be a number containing a NumericDate value. Use of this claim is OPTIONAL.

If the iat claim is in the future, an jwt.InvalidIssuedAtError exception will be raised.

jwt.encode({'iat': 1371720939}, 'secret')

jwt.encode({'iat': datetime.utcnow()}, 'secret')

Frequently Asked Questions

How can I extract a public / private key from a x509 certificate?

The load_pem_x509_certificate() function from cryptography can be used to extract the public or private keys from a x509 certificate in PEM format.

from cryptography.x509 import load_pem_x509_certificate
from cryptography.hazmat.backends import default_backend

cert_str = "-----BEGIN CERTIFICATE-----MIIDETCCAfm..."
cert_obj = load_pem_x509_certificate(cert_str, default_backend())
public_key = cert_obj.public_key()
private_key = cert_obj.private_key()
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