#! /usr/bin/env python3
# -*- coding: utf-8 -*-
"""*********************************************************************
* Edward is free software: you can redistribute it and/or modify       *
* it under the terms of the GNU Affero Public License as published by  *
* the Free Software Foundation, either version 3 of the License, or    *
* (at your option) any later version.                                  *
*                                                                      *
* Edward 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 Affero Public License for more details.                          *
*                                                                      *
* You should have received a copy of the GNU Affero Public License     *
* along with Edward.  If not, see <http://www.gnu.org/licenses/>.      *
*                                                                      *
* Copyright (C) 2014-2015 Andrew Engelbrecht                (AGPLv3+)  *
* Copyright (C) 2014      Josh Drake                        (AGPLv3+)  *
* Copyright (C) 2014      Lisa Marie Maginnis               (AGPLv3+)  *
* Copyright (C) 2009-2015 Tails developers <tails@boum.org> ( GPLv3+)  *
* Copyright (C) 2009      W. Trevor King <wking@drexel.edu> ( GPLv2+)  *
*                                                                      *
* Special thanks to Josh Drake for writing the original edward bot! :) *
*                                                                      *
************************************************************************

Code sourced from these projects:

  * http://agpl.fsf.org/emailselfdefense.fsf.org/edward/CURRENT/edward.tar.gz
  * https://git-tails.immerda.ch/whisperback/tree/whisperBack/encryption.py?h=feature/python3
  * http://www.physics.drexel.edu/~wking/code/python/send_pgp_mime

"""

import sys
import gpgme
import re
import io
import os

import email.parser
import email.message
import email.encoders

from email.mime.multipart       import MIMEMultipart
from email.mime.application     import MIMEApplication
from email.mime.nonmultipart    import MIMENonMultipart

import edward_config

def main ():

    handle_args()

    gpgme_ctx = get_gpg_context(edward_config.gnupghome,
                              edward_config.sign_with_key)

    email_text = sys.stdin.read()
    email_from, email_subject = email_from_subject(email_text)

    plaintext, keys = email_decode_flatten(email_text, gpgme_ctx, False)
    encrypt_to_key = choose_reply_encryption_key(keys)

    reply_message = generate_reply(plaintext, email_from, \
                                   email_subject, encrypt_to_key,
                                   gpgme_ctx)

    print(reply_message)


def get_gpg_context (gnupghome, sign_with_key_fp):

    os.environ['GNUPGHOME'] = gnupghome

    gpgme_ctx = gpgme.Context()
    gpgme_ctx.armor = True

    try:
        sign_with_key = gpgme_ctx.get_key(sign_with_key_fp)
    except:
        error("unable to load signing key. is the gnupghome "
                + "and signing key properly set in the edward_config.py?")
        exit(1)

    gpgme_ctx.signers = [sign_with_key]

    return gpgme_ctx


def email_decode_flatten (email_text, gpgme_ctx, from_decryption):

    body = ""
    keys = []

    email_struct = email.parser.Parser().parsestr(email_text)

    for subpart in email_struct.walk():

        payload, description, filename, content_type \
                = get_email_subpart_info(subpart)

        if payload == "":
            continue

        if content_type == "multipart":
            continue

        if content_type == "application/pgp-encrypted":
            if ((description == "PGP/MIME version identification")
                and (payload.strip() != "Version: 1")):
                    debug("Warning: unknown " + description
                          + ": " + payload.strip())
            # ignore the version number
            continue

        if (filename == "encrypted.asc") or (content_type == "pgp/mime"):
            plaintext, more_keys = decrypt_text(payload, gpgme_ctx)

            body += plaintext
            keys += more_keys

        elif content_type == "application/pgp-keys":
            keys += add_gpg_keys(payload, gpgme_ctx)

        elif content_type == "text/plain":
            if from_decryption == True:
                body += payload + "\n"

                keys += add_gpg_keys(payload, gpgme_ctx)

            else:
                plaintext, more_keys = decrypt_text(payload, gpgme_ctx)

                body += plaintext
                keys += more_keys

                keys += add_gpg_keys(payload, gpgme_ctx)

    return body, keys


def email_from_subject (email_text):

    email_struct = email.parser.Parser().parsestr(email_text)

    email_from      = email_struct['From']
    email_subject   = email_struct['Subject']

    return email_from, email_subject


def get_email_subpart_info (part):

    charset             = part.get_content_charset()
    payload_bytes       = part.get_payload(decode=True)

    filename            = part.get_filename()
    content_type        = part.get_content_type()
    description_list    = part.get_params(header='content-description')

    if charset == None:
        charset = 'utf-8'

    if payload_bytes != None:
        payload = payload_bytes.decode(charset)
    else:
        payload = ""

    if description_list != None:
        description = description_list[0][0]
    else:
        description = ""

    return payload, description, filename, content_type


def add_gpg_keys (text, gpgme_ctx):

    key_blocks = scan_and_grab(text,
                               '-----BEGIN PGP PUBLIC KEY BLOCK-----',
                               '-----END PGP PUBLIC KEY BLOCK-----')

    keys = []
    for key_block in key_blocks:
        fp = io.BytesIO(key_block.encode('ascii'))

        result = gpgme_ctx.import_(fp)
        imports = result.imports

        if imports != []:
            fingerprint = imports[0][0]
            keys += [gpgme_ctx.get_key(fingerprint)]

            debug("added gpg key: " + fingerprint)

    return keys


def decrypt_text (gpg_text, gpgme_ctx):

    body = ""
    keys = []

    msg_blocks = scan_and_grab(gpg_text,
                               '-----BEGIN PGP MESSAGE-----',
                               '-----END PGP MESSAGE-----')

    plaintexts_and_sigs = decrypt_blocks(msg_blocks, gpgme_ctx)

    for pair in plaintexts_and_sigs:
        plaintext   = pair[0]
        sigs        = pair[1]

        for sig in sigs:
            keys += [gpgme_ctx.get_key(sig.fpr)]

        # recursive for nested layers of mime and/or gpg
        plaintext, more_keys = email_decode_flatten(plaintext, gpgme_ctx, True)

        body += plaintext
        keys += more_keys

    return body, keys


def scan_and_grab (text, start_text, end_text):

    matches = re.search('(' + start_text + '.*' + end_text + ')',
                        text, flags=re.DOTALL)

    if matches != None:
        match_tuple = matches.groups()
    else:
        match_tuple = ()

    return match_tuple


def decrypt_blocks (msg_blocks, gpgme_ctx):

    return [decrypt_block(block, gpgme_ctx) for block in msg_blocks]


def decrypt_block (msg_block, gpgme_ctx):

    block_b = io.BytesIO(msg_block.encode('ascii'))
    plain_b = io.BytesIO()

    try:
        sigs = gpgme_ctx.decrypt_verify(block_b, plain_b)
    except:
        return ("",[])

    plaintext = plain_b.getvalue().decode('utf-8')
    return (plaintext, sigs)


def choose_reply_encryption_key (keys):

    reply_key = None
    for key in keys:
        if (key.can_encrypt == True):
            reply_key = key
            break

    return reply_key


def generate_reply (plaintext, email_from, email_subject, encrypt_to_key,
                    gpgme_ctx):


    reply  = "To: " + email_from + "\n"
    reply += "Subject: " + email_subject + "\n"

    if (encrypt_to_key != None):
        plaintext_reply  = "thanks for the message!\n\n\n"
        plaintext_reply += email_quote_text(plaintext)

        # quoted printable encoding lets most ascii characters look normal
        # before the decrypted mime message is decoded.
        char_set = email.charset.Charset("utf-8")
        char_set.body_encoding = email.charset.QP

        # MIMEText doesn't allow setting the text encoding
        # so we use MIMENonMultipart.
        plaintext_mime = MIMENonMultipart('text', 'plain')
        plaintext_mime.set_payload(plaintext_reply, charset=char_set)

        encrypted_text = encrypt_sign_message(plaintext_mime.as_string(),
                                              encrypt_to_key,
                                              gpgme_ctx)

        control_mime = MIMEApplication("Version: 1",
                                       _subtype='pgp-encrypted',
                                       _encoder=email.encoders.encode_7or8bit)
        control_mime['Content-Description'] = 'PGP/MIME version identification'
        control_mime.set_charset('us-ascii')

        encoded_mime = MIMEApplication(encrypted_text,
                                       _subtype='octet-stream; name="encrypted.asc"',
                                       _encoder=email.encoders.encode_7or8bit)
        encoded_mime['Content-Description'] = 'OpenPGP encrypted message'
        encoded_mime['Content-Disposition'] = 'inline; filename="encrypted.asc"'
        encoded_mime.set_charset('us-ascii')

        message_mime = MIMEMultipart(_subtype="encrypted", protocol="application/pgp-encrypted")
        message_mime.attach(control_mime)
        message_mime.attach(encoded_mime)
        message_mime['Content-Disposition'] = 'inline'

        reply += message_mime.as_string()

    else:
        reply += "\n"
        reply += "Sorry, i couldn't find your key.\n"
        reply += "I'll need that to encrypt a message to you."

    return reply


def email_quote_text (text):

    quoted_message = re.sub(r'^', r'> ', text, flags=re.MULTILINE)

    return quoted_message


def encrypt_sign_message (plaintext, encrypt_to_key, gpgme_ctx):

    plaintext_bytes = io.BytesIO(plaintext.encode('ascii'))
    encrypted_bytes = io.BytesIO()

    gpgme_ctx.encrypt_sign([encrypt_to_key], gpgme.ENCRYPT_ALWAYS_TRUST,
            plaintext_bytes, encrypted_bytes)

    encrypted_txt = encrypted_bytes.getvalue().decode('ascii')
    return encrypted_txt


def error (error_msg):

    sys.stderr.write(progname + ": " + error_msg + "\n")


def debug (debug_msg):

    if edward_config.debug == True:
        error(debug_msg)


def handle_args ():
    if __name__ == "__main__":

        global progname
        progname = sys.argv[0]

        if len(sys.argv) > 1:
            print(progname + ": error, this program doesn't " \
                    "need any arguments.", file=sys.stderr)
            exit(1)


main()