tamper-armor.py - mozsearch

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#!/usr/bin/env python3

# This Source Code Form is subject to the terms of the Mozilla Public

# License, v. 2.0. If a copy of the MPL was not distributed with this

# file, You can obtain one at http://mozilla.org/MPL/2.0/.

"""Corrupt an ASCII-armored OpenPGP message so its MDC integrity check fails.

Flips one byte deep in the OpenPGP packet data and re-armors WITHOUT a CRC ("=")

line (the CRC is optional per RFC 9580), so the change lands in the ciphertext

rather than in the armor checksum. Reads armored input from a file argument (or

stdin) and writes the tampered armor (CRLF line endings) to stdout.

Only used when regenerating the remote-content test corpus; see

generate-remote-content-eml.txt.

Usage:

    python3 tamper-armor.py mdc.asc > mdc-tampered.asc

"""

import base64

import sys

def tamper(armored):

    lines = armored.replace("\r\n", "\n").split("\n")

    i = next(

        k for k, l in enumerate(lines) if l.startswith("-----BEGIN PGP MESSAGE-----")

    ) + 1

    while lines[i].strip():  # skip armor headers up to the blank separator

        i += 1

    i += 1

    body = []

    while not (lines[i].startswith("=") or lines[i].startswith("-----END")):

        body.append(lines[i].strip())

        i += 1

    raw = bytearray(base64.b64decode("".join(body)))

    raw[int(len(raw) * 0.66)] ^= 0xFF

    b64 = base64.b64encode(bytes(raw)).decode()

    wrapped = "\r\n".join(b64[j : j + 64] for j in range(0, len(b64), 64))

    return (

        "-----BEGIN PGP MESSAGE-----\r\n\r\n"

        f"{wrapped}\r\n"

        "-----END PGP MESSAGE-----\r\n"

def main():

    data = open(sys.argv[1]).read() if len(sys.argv) > 1 else sys.stdin.read()

    sys.stdout.write(tamper(data))

if __name__ == "__main__":

    sys.exit(main())