pem_to_jwk.py

#!/usr/bin/env python3
# -*- coding: utf-8 -*-
"""
Convert a PEM encoded X509 certificate into a JWK RSA public key.
"""
import base64
import json

from cryptography import x509
from cryptography.hazmat.primitives import serialization
from cryptography.hazmat.backends import default_backend

file_name = "instance/cert.pem"

pem_data = open(file_name, "rb").read()
cert = x509.load_pem_x509_certificate(pem_data, default_backend())

public_key = cert.public_key()
public_numbers = public_key.public_numbers()

e = public_numbers.e.to_bytes(3, byteorder="big")
n = public_numbers.n.to_bytes(256, byteorder="big")
x5c = cert.public_bytes(serialization.Encoding.DER)

e = base64.urlsafe_b64encode(e).rstrip(b"=").decode("ascii")
n = base64.urlsafe_b64encode(n).rstrip(b"=").decode("ascii")
x5c = base64.b64encode(x5c).decode("ascii")

jwk = {
    "kty": "RSA",
    "use": "sig",
    "alg": "RS256",
    "e": e,
    "n": n,
    "x5c": x5c,
}

print(json.dumps(jwk, indent=4))