rsapq

#!/usr/bin/python3
#
# Check for "unusual" RSA keys with p<q
#
# RSA implementations usually create private keys with the first prime p
# larger than the second prime q (p>q).
#
# There is, however, no requirement, and keys encoded with p<q usually
# work, they are "just" unusual. They may break in some implementations.
#
# See also: https://bugzilla.mozilla.org/show_bug.cgi?id=1049435

import argparse

from cryptography.hazmat.primitives.asymmetric import rsa
from cryptography.hazmat.primitives.serialization import load_pem_private_key

ap = argparse.ArgumentParser()
ap.add_argument("files", nargs="+")
ap.add_argument("-q", "--quiet", action="store_true", help="only output unusual keys")
args = ap.parse_args()

for fn in args.files:
    with open(fn, "rb") as f:
        pem = f.read()
    try:
        key = load_pem_private_key(pem, password=None)
    except Exception:
        if not args.quiet:
            print(f"{fn}: cannot import key")
        continue

    if not isinstance(key, rsa.RSAPrivateKey):
        if not args.quiet:
            print(f"{fn}: No RSA key")
        continue

    p = key.private_numbers().p
    q = key.private_numbers().q

    if p == q:
        print(f"{fn}: p=q (square, broken key)")
    if p < q:
        print(f"{fn}: p<q (unusual)")
    elif not args.quiet:
        print(f"{fn}: p>q (common)")