jwk-to-pem.py

import argparse
import base64
import binascii
import fractions
import json
import os
import random
import subprocess
import sys
import tempfile
import textwrap

from pyasn1.type import univ, namedtype
from pyasn1.codec.der import encoder as der_encoder

# via: http://stackoverflow.com/questions/5486204/fast-modulo-calculations-in-python-and-ruby
def modexp ( g, u, p ):
    """computes s = (g ^ u) mod p
        args are base, exponent, modulus
        (see Bruce Schneier's book, _Applied Cryptography_ p. 244)"""
    s = 1
    while u != 0:
        if u & 1:
            s = (s * g)%p
        u >>= 1
        g = (g * g)%p;
    return s


# 2 functions via https://en.wikibooks.org/wiki/Algorithm_Implementation/Mathematics/Extended_Euclidean_algorithm
def egcd(a, b):
    if a == 0:
        return (b, 0, 1)
    else:
        g, y, x = egcd(b % a, a)
        return (g, x - (b // a) * y, y)


def modinv(a, m):
    g, x, y = egcd(a, m)
    if g != 1:
        raise Exception('modular inverse does not exist')
    else:
        return x % m


# implemented after: http://www.di-mgt.com.au/rsa_factorize_n.html
def factorRsa(n, e, d):
    # rsa sanity check:
    m = random.randint(0, n)
    me = modexp(m, e, n)
    md = modexp(me, d, n)
    assert md == m, ValueError("Not a valid RSA key!")

    k = d*e - 1
    while True:
        g = random.randint(2, n)
        t = k
        
        while t % 2 == 0 and t > 0:
            t = t//2
            x = modexp(g, t, n)
            y = fractions.gcd(x-1, n)
            if x > 1 and y > 1:
                p = y
                q = n/y
                return (p, q)
            else:
                print ".",


def asn1sequence(*elems):
    seq = univ.Sequence(componentType=namedtype.NamedTypes())
    for i,item in enumerate(elems):
        seq.setComponentByPosition(i, item)
    return seq


def jbase2int(s):
    s64 = s.replace("-", "+").replace("_", "/")
    s64 += "=" * (4 - (len(s64) % 4))
    bin = base64.b64decode(s64)
    hex = binascii.hexlify(bin)
    num = int(hex, 16)
    return num


def jwkrsa2pem(j, type='PEM'):
    if not hasattr(j, '__getitem__') or not hasattr(j, 'items'):
        # not a dict. probably json.
        try:
            j = json.loads(j)
        except Exception as e:
            # not json.
            raise
            raise ValueError("The parameter is neither a dictionary-like object, nor a JSON string.")

    conditions = (
        lambda d: "kty" in d,
        lambda d: d["kty"].lower().strip() == "rsa",
        lambda d: all(map(lambda k: k in d, ("n", "e", "d"))),
    )
    if not all(map(lambda f: f(j), conditions)):
        raise ValueError("The parameter is not a valid JWK RSA key")

    n = jbase2int(j["n"])
    assert n % 2 == 1, "n should not be an even number..."
    e = jbase2int(j["e"])
    d = jbase2int(j["d"])
    if "p" in j and "q" in j:
        p = jbase2int(j["p"])
        q = jbase2int(j["q"])
    else:
        p, q = factorRsa(n, e, d)
    if "dp" in j and "dq" in j:
        dp = jbase2int(j["dp"])
        dq = jbase2int(j["dq"])
    else:
        dp = modexp(d, 1, p-1)
        dq = modexp(d, 1, q-1)
    if "qi" in j:
        qi = jbase2int(j["qi"])
    else:
        qi = modinv(q, p)

    inner = der_encoder.encode(asn1sequence(
        univ.Integer(0),
        univ.Integer(n),
        univ.Integer(e),
        univ.Integer(d),
        univ.Integer(p),
        univ.Integer(q),
        univ.Integer(dp),
        univ.Integer(dq),
        univ.Integer(qi),
    ))

    if type == 'PEM':
        wrapper = textwrap.TextWrapper(width=64, break_on_hyphens=False)
        pem_b64 = wrapper.fill(base64.b64encode(inner))
        pem_tpl = """-----BEGIN RSA PRIVATE KEY-----
{}
-----END RSA PRIVATE KEY-----"""
        return pem_tpl.format(pem_b64)
    elif type == 'DER':
        return inner
    else:
        raise ValueError("Output type can only be 'DER' or 'PEM'")


if __name__ == "__main__":
    parser = argparse.ArgumentParser(description='Convert JWK private key to PEM or DER')
    parser.add_argument('-P', '--pem', dest='outform', action='store_const', const='PEM', default='PEM', help='Output in PEM format')
    parser.add_argument('-D', '--der', dest='outform', action='store_const', const='DER', help='Output in DER format')
    parser.add_argument('-e', '--encoding', dest='encoding', default='utf-8', help='Encoding of the JWK file')
    parser.add_argument('-in', dest='infile', required=True, help='Destination file, can be \'-\' for stdout')
    parser.add_argument('-out', dest='outfile', default='-', help='Input file')
    args = parser.parse_args()

    jwk_key = None
    try:
        with open(args.infile, "rb") as f:
            jwk_bytes = f.read()
            jwk_key = jwk_bytes.decode(args.encoding)
    except IOError:
        sys.stderr.write("ERROR: Cannot read file {}".format(args.infile))
        sys.exit(1)

    out = jwkrsa2pem(jwk_key, type=args.outform)

    if args.outfile == '-':
        print(out)
    else:
        with open(args.outfile, "wb") as f:
            f.write(out.encode('ascii'))