gpgmymail

#!/usr/bin/python3
# Copyright 2019 Julian Andres Klode <jak@jak-linux.org>
#
# Licensed under the Apache License, Version 2.0 (the "License");
# you may not use this file except in compliance with the License.
# You may obtain a copy of the License at
#
#     http://www.apache.org/licenses/LICENSE-2.0
#
# Unless required by applicable law or agreed to in writing, software
# distributed under the License is distributed on an "AS IS" BASIS,
# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
# See the License for the specific language governing permissions and
# limitations under the License.

"""Encrypt/Decrypt GPG/MIME messages.

This tool can encrypt and decrypt emails using PGP/MIME. Decryption only
works well for emails created with this tool. When encrypting, the tool
preserves all headers in the original email in the encrypted part, and
copies relevant headers to the output. When decrypting, any headers are
ignored, and only the encrypted headers are restored.
"""

import argparse
import sys
import email.encoders
import email.message
import email.mime.application
import email.mime.multipart
import email.mime.message
import typing

import gpg  # type: ignore


def encrypt(message: email.message.Message, recipients: typing.List[str]) -> str:
    """Encrypt given message"""
    with gpg.Context(armor=True) as c:
        keys = []
        for r in recipients:
            keys += list(c.keylist(r))

        encrypted_content, _res, _ = c.encrypt(message.as_bytes(), keys, sign=False)
    if not encrypted_content:
        raise ValueError(encrypted_content.status)

    # Build the parts
    enc = email.mime.application.MIMEApplication(
        _data=encrypted_content,
        _subtype="octet-stream",
        _encoder=email.encoders.encode_7or8bit,
    )

    control = email.mime.application.MIMEApplication(
        _data=b"Version: 1\n",
        _subtype='pgp-encrypted; name="msg.asc"',
        _encoder=email.encoders.encode_7or8bit,
    )
    control["Content-Disposition"] = 'inline; filename="msg.asc"'

    # Put the parts together
    encmsg = email.mime.multipart.MIMEMultipart(
        "encrypted", protocol="application/pgp-encrypted"
    )
    encmsg.attach(control)
    encmsg.attach(enc)

    # Copy headers
    headers_not_to_override = {key.lower() for key in encmsg.keys()}

    for key, value in message.items():
        if key.lower() not in headers_not_to_override:
            encmsg[key] = value

    return encmsg.as_bytes()


def decrypt(message: email.message.Message) -> str:
    """Decrypt the given message"""
    with gpg.Context(armor=True) as c:
        content, _decrypt_res, _verify_res = c.decrypt(message.as_bytes())
        return content


def main() -> None:
    """Program entry"""
    parser = argparse.ArgumentParser(description="Encrypt/Decrypt mail using GPG/MIME")
    parser.add_argument(
        "-d", "--decrypt", action="store_true", help="Decrypt rather than encrypt"
    )
    parser.add_argument(
        "recipient", nargs="*", help="key id or email of keys to encrypt for"
    )
    args = parser.parse_args()
    msg = email.message_from_binary_file(sys.stdin.buffer)

    if args.decrypt:
        sys.stdout.buffer.write(decrypt(msg))
    else:
        sys.stdout.buffer.write(encrypt(msg, args.recipient))


if __name__ == "__main__":
    main()