# -*- coding: utf-8 -*-
# crypto.py
# Copyright (C) 2013,2014 LEAP
#
# 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 <http://www.gnu.org/licenses/>.
"""
Cryptographic utilities for Soledad.
"""
import os
import binascii
import hmac
import hashlib
import json
import logging
import multiprocessing

from pycryptopp.cipher.aes import AES
from pycryptopp.cipher.xsalsa20 import XSalsa20

from leap.soledad.common import soledad_assert
from leap.soledad.common import soledad_assert_type


from leap.soledad.common.crypto import (
    EncryptionSchemes,
    UnknownEncryptionScheme,
    MacMethods,
    UnknownMacMethod,
    WrongMac,
    ENC_JSON_KEY,
    ENC_SCHEME_KEY,
    ENC_METHOD_KEY,
    ENC_IV_KEY,
    MAC_KEY,
    MAC_METHOD_KEY,
)

logger = logging.getLogger(__name__)


MAC_KEY_LENGTH = 64


class EncryptionMethods(object):
    """
    Representation of encryption methods that can be used.
    """

    AES_256_CTR = 'aes-256-ctr'
    XSALSA20 = 'xsalsa20'

#
# Exceptions
#


class DocumentNotEncrypted(Exception):
    """
    Raised for failures in document encryption.
    """
    pass


class UnknownEncryptionMethod(Exception):
    """
    Raised when trying to encrypt/decrypt with unknown method.
    """
    pass


class NoSymmetricSecret(Exception):
    """
    Raised when trying to get a hashed passphrase.
    """


def encrypt_sym(data, key, method):
    """
    Encrypt C{data} using a {password}.

    Currently, the only encryption methods supported are AES-256 in CTR
    mode and XSalsa20.

    :param data: The data to be encrypted.
    :type data: str
    :param key: The key used to encrypt C{data} (must be 256 bits long).
    :type key: str
    :param method: The encryption method to use.
    :type method: str

    :return: A tuple with the initial value and the encrypted data.
    :rtype: (long, str)
    """
    soledad_assert_type(key, str)

    soledad_assert(
        len(key) == 32,  # 32 x 8 = 256 bits.
        'Wrong key size: %s bits (must be 256 bits long).' %
        (len(key) * 8))
    iv = None
    # AES-256 in CTR mode
    if method == EncryptionMethods.AES_256_CTR:
        iv = os.urandom(16)
        ciphertext = AES(key=key, iv=iv).process(data)
    # XSalsa20
    elif method == EncryptionMethods.XSALSA20:
        iv = os.urandom(24)
        ciphertext = XSalsa20(key=key, iv=iv).process(data)
    else:
        # raise if method is unknown
        raise UnknownEncryptionMethod('Unkwnown method: %s' % method)
    return binascii.b2a_base64(iv), ciphertext


def decrypt_sym(data, key, method, **kwargs):
    """
    Decrypt data using symmetric secret.

    Currently, the only encryption method supported is AES-256 CTR mode.

    :param data: The data to be decrypted.
    :type data: str
    :param key: The key used to decrypt C{data} (must be 256 bits long).
    :type key: str
    :param method: The encryption method to use.
    :type method: str
    :param kwargs: Other parameters specific to each encryption method.
    :type kwargs: dict

    :return: The decrypted data.
    :rtype: str
    """
    soledad_assert_type(key, str)
    # assert params
    soledad_assert(
        len(key) == 32,  # 32 x 8 = 256 bits.
        'Wrong key size: %s (must be 256 bits long).' % len(key))
    soledad_assert(
        'iv' in kwargs,
        '%s needs an initial value.' % method)
    # AES-256 in CTR mode
    if method == EncryptionMethods.AES_256_CTR:
        return AES(
            key=key, iv=binascii.a2b_base64(kwargs['iv'])).process(data)
    elif method == EncryptionMethods.XSALSA20:
        return XSalsa20(
            key=key, iv=binascii.a2b_base64(kwargs['iv'])).process(data)

    # raise if method is unknown
    raise UnknownEncryptionMethod('Unkwnown method: %s' % method)


def doc_mac_key(doc_id, secret):
    """
    Generate a key for calculating a MAC for a document whose id is
    C{doc_id}.

    The key is derived using HMAC having sha256 as underlying hash
    function. The key used for HMAC is the first MAC_KEY_LENGTH characters
    of Soledad's storage secret. The HMAC message is C{doc_id}.

    :param doc_id: The id of the document.
    :type doc_id: str

    :param secret: soledad secret storage
    :type secret: Soledad.storage_secret

    :return: The key.
    :rtype: str

    :raise NoSymmetricSecret: if no symmetric secret was supplied.
    """
    if secret is None:
        raise NoSymmetricSecret()

    return hmac.new(
        secret[:MAC_KEY_LENGTH],
        doc_id,
        hashlib.sha256).digest()


class SoledadCrypto(object):
    """
    General cryptographic functionality encapsulated in a
    object that can be passed along.
    """
    def __init__(self, soledad):
        """
        Initialize the crypto object.

        :param soledad: A Soledad instance for key lookup.
        :type soledad: leap.soledad.Soledad
        """
        self._soledad = soledad

    def encrypt_sym(self, data, key,
                    method=EncryptionMethods.AES_256_CTR):
        return encrypt_sym(data, key, method)

    def decrypt_sym(self, data, key,
                    method=EncryptionMethods.AES_256_CTR, **kwargs):
        return decrypt_sym(data, key, method, **kwargs)

    def doc_mac_key(self, doc_id, secret):
        return doc_mac_key(doc_id, self.secret)

    def doc_passphrase(self, doc_id):
        """
        Generate a passphrase for symmetric encryption of document's contents.

        The password is derived using HMAC having sha256 as underlying hash
        function. The key used for HMAC are the first
        C{soledad.REMOTE_STORAGE_SECRET_KENGTH} bytes of Soledad's storage
        secret stripped from the first MAC_KEY_LENGTH characters. The HMAC
        message is C{doc_id}.

        :param doc_id: The id of the document that will be encrypted using
            this passphrase.
        :type doc_id: str

        :return: The passphrase.
        :rtype: str

        :raise NoSymmetricSecret: if no symmetric secret was supplied.
        """
        if self.secret is None:
            raise NoSymmetricSecret()
        return hmac.new(
            self.secret[
                MAC_KEY_LENGTH:
                self._soledad.REMOTE_STORAGE_SECRET_LENGTH],
            doc_id,
            hashlib.sha256).digest()

    #
    # secret setters/getters
    #

    def _get_secret(self):
        return self._soledad.storage_secret

    secret = property(
        _get_secret, doc='The secret used for symmetric encryption')

#
# Crypto utilities for a SoledadDocument.
#


def mac_doc(doc_id, doc_rev, ciphertext, mac_method, secret):
    """
    Calculate a MAC for C{doc} using C{ciphertext}.

    Current MAC method used is HMAC, with the following parameters:

        * key: sha256(storage_secret, doc_id)
        * msg: doc_id + doc_rev + ciphertext
        * digestmod: sha256

    :param doc_id: The id of the document.
    :type doc_id: str
    :param doc_rev: The revision of the document.
    :type doc_rev: str
    :param ciphertext: The content of the document.
    :type ciphertext: str
    :param mac_method: The MAC method to use.
    :type mac_method: str
    :param secret: soledad secret
    :type secret: Soledad.secret_storage

    :return: The calculated MAC.
    :rtype: str
    """
    if mac_method == MacMethods.HMAC:
        return hmac.new(
            doc_mac_key(doc_id, secret),
            str(doc_id) + str(doc_rev) + ciphertext,
            hashlib.sha256).digest()
    # raise if we do not know how to handle this MAC method
    raise UnknownMacMethod('Unknown MAC method: %s.' % mac_method)


def encrypt_docstr(docstr, doc_id, doc_rev, key, secret):
    """
    Encrypt C{doc}'s content.

    Encrypt doc's contents using AES-256 CTR mode and return a valid JSON
    string representing the following:

        {
            ENC_JSON_KEY: '<encrypted doc JSON string>',
            ENC_SCHEME_KEY: 'symkey',
            ENC_METHOD_KEY: EncryptionMethods.AES_256_CTR,
            ENC_IV_KEY: '<the initial value used to encrypt>',
            MAC_KEY: '<mac>'
            MAC_METHOD_KEY: 'hmac'
        }

    :param docstr: A representation of the document to be encrypted.
    :type docstr: str or unicode.

    :param doc_id: The document id.
    :type doc_id: str

    :param doc_rev: The document revision.
    :type doc_rev: str

    :param key: The key used to encrypt ``data`` (must be 256 bits long).
    :type key: str

    :param secret:
    :type secret:

    :return: The JSON serialization of the dict representing the encrypted
        content.
    :rtype: str
    """
    # encrypt content using AES-256 CTR mode
    iv, ciphertext = encrypt_sym(
        str(docstr),  # encryption/decryption routines expect str
        key, method=EncryptionMethods.AES_256_CTR)
    # Return a representation for the encrypted content. In the following, we
    # convert binary data to hexadecimal representation so the JSON
    # serialization does not complain about what it tries to serialize.
    hex_ciphertext = binascii.b2a_hex(ciphertext)
    return json.dumps({
        ENC_JSON_KEY: hex_ciphertext,
        ENC_SCHEME_KEY: EncryptionSchemes.SYMKEY,
        ENC_METHOD_KEY: EncryptionMethods.AES_256_CTR,
        ENC_IV_KEY: iv,
        MAC_KEY: binascii.b2a_hex(mac_doc(  # store the mac as hex.
            doc_id, doc_rev, ciphertext,
            MacMethods.HMAC, secret)),
        MAC_METHOD_KEY: MacMethods.HMAC,
    })


# XXX change to docstr...
def decrypt_doc(crypto, doc):
    """
    Decrypt C{doc}'s content.

    Return the JSON string representation of the document's decrypted content.

    The content of the document should have the following structure:

        {
            ENC_JSON_KEY: '<enc_blob>',
            ENC_SCHEME_KEY: '<enc_scheme>',
            ENC_METHOD_KEY: '<enc_method>',
            ENC_IV_KEY: '<initial value used to encrypt>',  # (optional)
            MAC_KEY: '<mac>'
            MAC_METHOD_KEY: 'hmac'
        }

    C{enc_blob} is the encryption of the JSON serialization of the document's
    content. For now Soledad just deals with documents whose C{enc_scheme} is
    EncryptionSchemes.SYMKEY and C{enc_method} is
    EncryptionMethods.AES_256_CTR.

    :param crypto: A SoledadCryto instance to perform the encryption.
    :type crypto: leap.soledad.crypto.SoledadCrypto
    :param doc: The document to be decrypted.
    :type doc: SoledadDocument

    :return: The JSON serialization of the decrypted content.
    :rtype: str
    """
    soledad_assert(doc.is_tombstone() is False)
    soledad_assert(ENC_JSON_KEY in doc.content)
    soledad_assert(ENC_SCHEME_KEY in doc.content)
    soledad_assert(ENC_METHOD_KEY in doc.content)
    soledad_assert(MAC_KEY in doc.content)
    soledad_assert(MAC_METHOD_KEY in doc.content)
    # verify MAC
    ciphertext = binascii.a2b_hex(  # content is stored as hex.
        doc.content[ENC_JSON_KEY])
    mac = mac_doc(
        doc.doc_id, doc.rev,
        ciphertext,
        doc.content[MAC_METHOD_KEY], crypto.secret)
    # we compare mac's hashes to avoid possible timing attacks that might
    # exploit python's builtin comparison operator behaviour, which fails
    # immediatelly when non-matching bytes are found.
    doc_mac_hash = hashlib.sha256(
        binascii.a2b_hex(  # the mac is stored as hex
            doc.content[MAC_KEY])).digest()
    calculated_mac_hash = hashlib.sha256(mac).digest()
    if doc_mac_hash != calculated_mac_hash:
        raise WrongMac('Could not authenticate document\'s contents.')
    # decrypt doc's content
    enc_scheme = doc.content[ENC_SCHEME_KEY]
    plainjson = None
    if enc_scheme == EncryptionSchemes.SYMKEY:
        enc_method = doc.content[ENC_METHOD_KEY]
        if enc_method == EncryptionMethods.AES_256_CTR:
            soledad_assert(ENC_IV_KEY in doc.content)
            plainjson = crypto.decrypt_sym(
                ciphertext,
                crypto.doc_passphrase(doc.doc_id),
                method=enc_method,
                iv=doc.content[ENC_IV_KEY])
        else:
            raise UnknownEncryptionMethod(enc_method)
    else:
        raise UnknownEncryptionScheme(enc_scheme)
    return plainjson


def is_symmetrically_encrypted(doc):
    """
    Return True if the document was symmetrically encrypted.

    :param doc: The document to check.
    :type doc: SoledadDocument

    :rtype: bool
    """
    if doc.content and ENC_SCHEME_KEY in doc.content:
        if doc.content[ENC_SCHEME_KEY] == EncryptionSchemes.SYMKEY:
            return True
    return False


#
# Encrypt/decrypt pools of workers
#

class SyncEncryptDecryptPool(object):
    """
    Base class for encrypter/decrypter pools
    """

    def __init__(self, crypto, sync_db):
        """
        Initialize the pool of encryption-workers.

        :param crypto: A SoledadCryto instance to perform the encryption.
        :type crypto: leap.soledad.crypto.SoledadCrypto

        :param sync_db: a database connection handle
        :type sync_db: handle
        """
        self._pool = multiprocessing.Pool(self.WORKERS)
        self._crypto = crypto
        self._sync_db = sync_db


def encrypt_doc_task(doc_id, doc_rev, content, key, secret):
    encrypted_content = encrypt_docstr(
        content, doc_id, doc_rev, key, secret)
    return doc_id, doc_rev, encrypted_content


class SyncEncrypterPool(SyncEncryptDecryptPool):
    """
    of documents to be synced.
    """
    # TODO implement throttling to reduce cpu usage??
    WORKERS = 10
    TABLE_NAME = "docs_tosync"
    FIELD_NAMES = "doc_id, rev, content"

    def encrypt_doc(self, doc):
        """
        Symmetrically encrypt a document.

        :param doc: The document with contents to be encrypted.
        :type doc: SoledadDocument
        """
        docstr = doc.get_json()
        key = self._crypto.doc_passphrase(doc.doc_id)
        secret = self._crypto.secret
        args = doc.doc_id, doc.rev, docstr, key, secret

        try:
            self._pool.apply_async(encrypt_doc_task, args,
                                   callback=self.encrypt_doc_cb)
        except Exception as exc:
            logger.exception(exc)

    def encrypt_doc_cb(self, result):
        doc_id, doc_rev, content = result
        self.insert_encrypted_doc(doc_id, doc_rev, content)

    def insert_encrypted_doc(self, doc_id, doc_rev, content):
        """
        Insert the contents of the encrypted doc into the local sync
        database.

        :param doc: The document with contents to be encrypted.
        :type doc: SoledadDocument
        :param content: The encrypted document.
        :type content: str
        """
        c = self._sync_db.cursor()
        sql_del = "DELETE FROM '%s' WHERE doc_id=?" % (self.TABLE_NAME,)
        c.execute(sql_del, (doc_id, ))
        sql_ins = "INSERT INTO '%s' VALUES (?, ?, ?)" % (self.TABLE_NAME,)
        c.execute(sql_ins, (doc_id, doc_rev, content))
        self._sync_db.commit()


class SyncDecrypterPool(SyncEncryptDecryptPool):
    """
    Pool of workers that spawn subprocesses to execute the symmetric decryption
    of documents that were received.
    """
    WORKERS = 10
    TABLE_NAME = "docs_received"
    FIELD_NAMES = "doc_id, rev, content, gen, trans_id"

    def decrypt_doc(self, doc_id, rev):
        """
        Symmetrically decrypt a document.

        :param doc: The document with contents to be encrypted.
        :type doc: SoledadDocument
        """