How to verify a JWT using python PyJWT with a public PEM cert?

Question

I recently upgraded from PyJWT 0.4.1 to 1.0.1 and I can't figure out how to verify a JWT signed with a public key.

My code looks like this:

import jwt
cert_string = "-----BEGIN CERTIFICATE-----\nMIICITCCAYqgAwIBAgIIBEsUSxL..."
token_string = "eyJhbGciOiJSUzI1NiIsImtpZCI6I..."
jwt.decode(token_string, cert_string, algorithms=['RS256'])

The error I get is:

File "<stdin>", line 1, in <module>
File "~/.virtualenvs/project/lib/python2.7/site-packages/jwt/api.py", line 117, in decode                            
key, algorithms, **kwargs)                                                                                         
File "~/.virtualenvs/project/lib/python2.7/site-packages/jwt/api.py", line 176, in _verify_signature                 
key = alg_obj.prepare_key(key)                                                                                     
File "~/.virtualenvs/project/lib/python2.7/site-packages/jwt/algorithms.py", line 165, in prepare_key                
key = load_pem_public_key(key, backend=default_backend())                                                          
File "~/.virtualenvs/project/lib/python2.7/site-packages/cryptography/hazmat/primitives/serialization.py", line 24, in load_pem_public_key
return backend.load_pem_public_key(data)                                                                           
File "~/.virtualenvs/project/lib/python2.7/site-packages/cryptography/hazmat/backends/multibackend.py", line 285, in load_pem_public_key
return b.load_pem_public_key(data)
File "~/.virtualenvs/project/lib/python2.7/site-packages/cryptography/hazmat/backends/openssl/backend.py", line 716, in load_pem_public_key
self._handle_key_loading_error()
File "~/.virtualenvs/project/lib/python2.7/site-packages/cryptography/hazmat/backends/openssl/backend.py", line 912, in _handle_key_loading_error
raise ValueError("Could not unserialize key data.")                                                                
ValueError: Could not unserialize key data.

I'm confident my cert_string and token are good. The following code runs OK:

from cryptography.x509 import load_pem_x509_certificate
from cryptography.hazmat.backends import default_backend
load_pem_x509_certificate(cert_string, default_backend())

My code that used to work with 0.4.1 looked like this:

cert_string = "".join(cert_string.strip().split("\n")[1:-1])
der = a2b_base64(cert_string)
cert = DerSequence()
cert.decode(der)
tbsCertificate = DerSequence()
tbsCertificate.decode(cert[0])
subjectPublicKeyInfo = tbsCertificate[6]
pub_key = RSA.importKey(subjectPublicKeyInfo)
jwt.decode(token_string, pub_key)

Any help would be appreciated.

score 20 · Accepted Answer · answered Apr 10 '15 at 19:32

20

You need to pass the public key instead of the full certificate to the decode method. So extract the key from the certificate in order to use it as in:

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()

and then:

token_string = "eyJhbGciOiJSUzI1NiIsImtpZCI6I..."
jwt.decode(token_string, public_key, algorithms=['RS256'])

answered Apr 10 '15 at 19:32

Hans Z.

50,496
12
102
115

4

getting error `TypeError: initializer for ctype 'char[]' must be a bytes or list or tuple, not str` – Aman Agarwal Jun 25 '18 at 12:25
This worked for me but my certificate needed the \n in the certificate. e.g. `cert_str = "-----BEGIN CERTIFICATE-----\nMIIDHDCCAgSgAwIBAgIIUAu/wmcX4dQwDQYJKoZIhvcNAQEFBQAwMTEvMC0GA1UE\nAxMmc2...` – vincent31337 May 19 '20 at 20:27
1

In Python 3 `load_pem_x509_certificate` is expecting `cert_str` to be bytes rather than a string. If you pass it a string you get `TypeError: from_buffer() cannot return the address of a unicode object` – Jason Heiss Sep 21 '20 at 15:28
In Python3, the fix is to suffix encode to your keys, i.e. cert_obj.encode('utf-8') – J. Gwinner Jul 16 '21 at 19:16

score 3 · Answer 2 · answered Feb 02 '17 at 15:05

If the key file is generated using ssh-keygen -t rsa (RFC4716), you can use the file directly.

Encode:

import jwt

pemfile = open("id_rsa", 'r')
keystring = pemfile.read()
pemfile.close()
token = jwt.encode(payload, keystring, algorithm='RS256')

Decode:

import jwt

pemfile = open("id_rsa.pub", 'r')
keystring = pemfile.read()
pemfile.close()
payload = jwt.decode(toekn, keystring,  verify=True)

dont forget to catch errors like jwt.ExpiredSignatureError etc

score 1 · Answer 3 · answered Jan 16 '19 at 00:27

1

I am using python 3.6

import jwt
from cryptography.hazmat.backends import default_backend
from cryptography.hazmat.primitives import serialization
key = b"""-----BEGIN PUBLIC KEY-----
MIIBI....<<Your public key basically>>
-----END PUBLIC KEY-----"""
public_key = serialization.load_pem_public_key(key, backend=default_backend())
print(jwt.decode(token, public_key))

This worked like a charm for me.

answered Jan 16 '19 at 00:27

Nagalakshmi Srirama

973
11
22

Unfortunately, with Python 3.6 (don't ask) I get Message=from_buffer() cannot return the address of a unicode object – J. Gwinner Jul 16 '21 at 18:54

How to verify a JWT using python PyJWT with a public PEM cert?

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