JWT decoding in python backend

[poshak-rialtes]

Summary

I am trying to extract access token from the session of the user in the python backend, which is present in the cookie "__Secure-next-auth.session-token". My issue is the code doesn't work. I have even tried hashing the secret key and decoding the cookie.

Additional information

Adding the code snippet. 
Note:- I am able to get the proper cookie which is present in the header.

 def calculate_sha256(self,secret):
        """
        Calculate the SHA-256 hash of a secret string.

        Args:
            secret (str): The input secret string.

        Returns:
            str: The SHA-256 hash in hexadecimal format.
        """
        sha256_hash = hashlib.sha256()
        sha256_hash.update(secret.encode('utf-8'))
        return sha256_hash.hexdigest()
    def _token_decode(self) -> dict:
        """
        Decodes the contents of the reauthentication cookie.

        Returns
        -------
        str
            The decoded JWT cookie for passwordless reauthentication.
        """
        try:
            #cookie = self._get_cookie()
            cookie = self.get_cookie_value(self.cookie_name )
            #jwt = decode(cookie,NEXTAUTH_SECRET,algorithms=['HS256'])
            try:
                hashedsecret = self.calculate_sha256(NEXTAUTH_SECRET)
                # Decode the JWT without verification
                decoded_token = pyjwt.decode(cookie, hashedsecret, algorithms=['None'],verify=False)
                print("Decoded JWT data:")
                print(decoded_token)
            except pyjwt.ExpiredSignatureError:
                # Handle token expiration if needed
                print("Token has expired.")
            except pyjwt.DecodeError:
                # Handle decoding errors (e.g., invalid token)
                print("Token decoding failed or token is invalid.")
        except:
            return False

Example

No response

[jmugicagonz]

I got into the same issue, did you solve it?

[poshak-rialtes]

reverse engineer next-auth code and it works fine now.
Sharing the code :

import json
from typing import Any, Dict

from hkdf import Hkdf
from jose.jwe import decrypt, encrypt


def __encryption_key(secret: str):
    return Hkdf("", bytes(secret, "utf-8")).expand(b"NextAuth.js Generated Encryption Key", 32)


def encode_jwe(payload: Dict[str, Any], secret: str):
    data = bytes(json.dumps(payload), "utf-8")
    key = __encryption_key(secret)
    return bytes.decode(encrypt(data, key), "utf-8")


def decode_jwe(token: str, secret: str):
    decrypted = decrypt(token, __encryption_key(secret))

    if decrypted:
        return json.loads(bytes.decode(decrypted, "utf-8"))
    else:
        return None

[poshak-rialtes]

Let me know if this works for you. Have a nice day.

[sokenny]

@poshak-rialtes This worked right away. Thanks man. I still can not believe one has to dig for a workaround like this.

[jmugicagonz]

thank you!
I get this error: jose.exceptions.JWEError: Invalid JWE Auth Tag

Might it be due to changes in the new implementation of next-auth, AuthJS?

[poshak-rialtes]

i am using following versions and it works, try downgrading once.
"next": "14.0.2",
"next-auth": "^4.23.1",

[gajendar-work]

Auth JS (V5) - "next-auth": "^5.0.0-beta.16"

spent 3 hour to figure it out , here is the code to save your time .

For Production (https)
the salt would be : "__Secure-authjs.session-token"

For Development (http)
the salt would be : "authjs.session-token"

below example is for http

import json
from typing import Any, Dict

from hkdf import Hkdf
from jose.jwe import decrypt, encrypt


def __encryption_key(secret: str):
    return Hkdf(bytes("authjs.session-token","utf-8"), bytes(secret, "utf-8")).expand(b"Auth.js Generated Encryption Key (authjs.session-token)", 64)


def encode_jwe(payload: Dict[str, Any], secret: str):
    data = bytes(json.dumps(payload), "utf-8")
    key = __encryption_key(secret)
    return bytes.decode(encrypt(data, key), "utf-8")


def decode_jwe(token: str, secret: str):
    e_key=  __encryption_key(secret)
    decrypted = decrypt(token,e_key)

    if decrypted:
        return json.loads(bytes.decode(decrypted, "utf-8"))
    else:
        return None

[kewldan]

Thanks ! Your comment must be in official documention

[pimpadev]

Thank you for share it, it works as a charm

[cblberlin]

i get

raise ValueError("Invalid padding bytes.")
ValueError: Invalid padding bytes.

During handling of the above exception, another exception occurred:

raise JWEError(e)
jose.exceptions.JWEError: Invalid padding bytes.

i downgraded to "next-auth": "^5.0.0-beta.16"

and i copied my token from console authjs.session-token

and i tried to decode it with my AUTH_SECRET which generated by auth.js

is there anything go wrong?

[gajendar-work]

@cblberlin

i use following class to decode it and it works with "next-auth": "^5.0.0-beta.16"

i hope this helps.

import json
from typing import Any, Dict
from hkdf import Hkdf #pip install hkdf
from jose.jwe import decrypt, encrypt #pip install python-jose
from typing import Optional
from pydantic import BaseModel


class UserJWEModel(BaseModel):
    name : Optional[str] = None
    email : str
    sub : str
    role : str
    domain : Optional[str] = None
    tenant_id : Optional[str] = None

class AuthJSDecoder:
    def __init__(self,secret:str,secure_cookies:bool) -> None:
        self.secret = secret

        self.salt = None

        if secure_cookies == True:
            self.salt = "__Secure-authjs.session-token"
        else:
            self.salt = "authjs.session-token"

    def __encryption_key(self):
        return Hkdf(bytes(self.salt,"utf-8"), bytes(self.secret, "utf-8")).expand(f"Auth.js Generated Encryption Key ({self.salt})".encode("utf-8"), 64)
    
    def encode_jwe(self,payload: Dict[str, Any]):
        data = bytes(json.dumps(payload), "utf-8")
        key = self.__encryption_key()
        return bytes.decode(encrypt(data, key), "utf-8")

    def decode_jwe(self,token: str):
        e_key=  self.__encryption_key()
        decrypted = decrypt(token,e_key)
        if decrypted:
            return UserJWEModel.model_validate(json.loads(bytes.decode(decrypted, "utf-8")))
        else:
            return None