How to sign and verify JWT RS256 in Python

import json
import time
import base64
import hmac
import hashlib
from cryptography.hazmat.primitives.asymmetric import rsa
from cryptography.hazmat.primitives import serialization, hashes
from cryptography.hazmat.primitives.asymmetric import padding
from cryptography.hazmat.primitives.asymmetric.utils import encode_dss_signature, decode_dss_signature

# Generate RSA key pair
private_key = rsa.generate_private_key(public_exponent=65537, key_size=2048)
public_key = private_key.public_key()

# Encode private/public keys to PEM for realistic usage
private_pem = private_key.private_bytes(
    encoding=serialization.Encoding.PEM,
    format=serialization.PrivateFormat.PKCS8,
    encryption_algorithm=serialization.NoEncryption()
)
public_pem = public_key.public_bytes(
    encoding=serialization.Encoding.PEM,
    format=serialization.PublicFormat.SubjectPublicKeyInfo
)

def b64url_encode(data):
    return base64.urlsafe_b64encode(data).rstrip(b'=').decode('utf-8')

def b64url_decode(data):
    padding = '=' * (4 - len(data) % 4)
    return base64.urlsafe_b64decode(data + padding)

def create_jwt(payload, private_pem):
    header = {"alg": "RS256", "typ": "JWT"}
    header_b64 = b64url_encode(json.dumps(header).encode())
    payload_b64 = b64url_encode(json.dumps(payload).encode())
    signing_input = f"{header_b64}.{payload_b64}".encode()
    
    key = serialization.load_pem_private_key(private_pem, password=None)
    signature = key.sign(signing_input, padding.PKCS1v15(), hashes.SHA256())
    return f"{header_b64}.{payload_b64}.{b64url_encode(signature)}"

def verify_jwt(token, public_pem):
    header_b64, payload_b64, signature_b64 = token.split('.')
    signing_input = f"{header_b64}.{payload_b64}".encode()
    
    key = serialization.load_pem_public_key(public_pem)
    try:
        key.verify(b64url_decode(signature_b64), signing_input, padding.PKCS1v15(), hashes.SHA256())
        payload = json.loads(b64url_decode(payload_b64))
        return True, payload
    except Exception:
        return False, None

if __name__ == "__main__":
    payload = {"sub": "1234567890", "name": "Alice", "iat": int(time.time())}
    token = create_jwt(payload, private_pem)
    print(f"JWT: {token}")
    
    valid, decoded = verify_jwt(token, public_pem)
    print(f"Valid: {valid}")
    print(f"Decoded payload: {decoded}")
    
    # Test tampered token
    tampered = token[:-4] + "xxxx"
    valid, decoded = verify_jwt(tampered, public_pem)
    print(f"Tampered valid: {valid}")