#!/usr/bin/python3

# Copyright (C) 2019 Daniel Kahn Gillmor
#
# This program is free software: you can redistribute it and/or modify
# it under the terms of the GNU General Public License as published by
# the Free Software Foundation, either version 3 of the License, or (at
# your option) any later version.
#
# This program is distributed in the hope that it will be useful, but
# WITHOUT ANY WARRANTY; without even the implied warranty of
# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
# General Public License for more details.
#
# You should have received a copy of the GNU General Public License
# along with this program.  If not, see <https://www.gnu.org/licenses/>.

'''Extract all OpenPGP certificates from an e-mail message

This is a simple script that is designed to take an e-mail
(rfc822/message) on standard input, and produces a series of
ASCII-armored OpenPGP certificates on standard output.

It currently tries to find OpenPGP certificates based on MIME types of
attachments (application/pgp-keys), and by pulling out anything that
looks like an Autocrypt: or Autocrypt-Gossip: header (see
https://autocrypt.org).

'''

import email
import sys
import base64
import binascii
import codecs
from typing import Optional, Generator

# parse email from stdin
message = email.message_from_binary_file(sys.stdin.buffer)

def openpgp_ascii_armor_checksum(data: bytes) -> bytearray:
    '''OpenPGP ASCII-armor checksum

(see https://tools.ietf.org/html/rfc4880#section-6.1)'''

    init = 0xB704CE
    poly = 0x1864CFB
    crc = init
    for b in data:
        crc ^= b << 16
        for i in range(8):
            crc <<= 1
            if crc & 0x1000000:
                crc ^= poly
    val = crc & 0xFFFFFF
    out = bytearray(3)
    out[0] = (val >> 16) & 0xFF
    out[1] = (val >> 8) & 0xFF
    out[2] = val & 0xFF
    return out

def enarmor_certificate(data: bytes) -> str:
    '''OpenPGP ASCII-armor

(see https://tools.ietf.org/html/rfc4880#section-6.2)'''

    cksum = openpgp_ascii_armor_checksum(data)
    key = codecs.decode(base64.b64encode(data), 'ascii')
    linelen = 64
    key = '\n'.join([key[i:i+linelen] for i in range(0, len(key), linelen)])
    return '-----BEGIN PGP PUBLIC KEY BLOCK-----\n\n' +\
        key + \
        '\n=' + codecs.decode(base64.b64encode(cksum), 'ascii') +\
        '\n-----END PGP PUBLIC KEY BLOCK-----\n'

def get_autocrypt_keys(m: email.message.Message) -> Generator[str, None, None]:
    '''Extract all Autocrypt headers from message

Note that we ignore the addr= property.
'''
    hdrs = m.get_all('Autocrypt')
    if hdrs is None: # the email.get_all() api is kindn of sad.
        hdrs = []
    ghdrs = m.get_all('Autocrypt-Gossip')
    if ghdrs is None: # the email.get_all() api is kindn of sad.
        ghdrs = []
    for ac in hdrs + ghdrs:
        # parse the base64 part
        try:
            keydata = str(ac).split('keydata=')[1].strip()
            keydata = keydata.replace(' ', '').replace('\t', '')
            keydatabin = base64.b64decode(keydata)
            yield enarmor_certificate(keydatabin)
        except (binascii.Error, IndexError) as e:
            print("failure to parse Autocrypt header: %s" % e,
                  file=sys.stderr)

def extract_attached_keys(m: email.message.Message) -> Generator[str, None, None]:
    for part in m.walk():
        if part.get_content_type() == 'application/pgp-keys':
            p = part.get_payload(decode=True)
            if not isinstance(p, bytes):
                raise TypeError('Expected part payload to be bytes')
            if p.startswith(b'-----BEGIN PGP PUBLIC KEY BLOCK-----\n'):
                yield codecs.decode(p, 'ascii')
            else: # this is probably binary-encoded, let's pretend that it is!
                yield enarmor_certificate(p)

# FIXME: should we try to decrypt encrypted messages as well?

for a in get_autocrypt_keys(message):
    print(a, end='')
for a in extract_attached_keys(message):
    print(a, end='')