python: Import improved python bindings.

This commit imports the python bindings from:
    https://github.com/poljar/python-olm

The bindings are imported at commit c44b145818520d69eaaa350fb95afcb846125e0f

Minor modifications were made while importing:
    - Removed travis config
    - Removed Arch Linux PKGBUILD
    - Removed the html docs, they can be rebuild by running make html in
      the docs folder
    - Slightly modified the README

The new bindings feature some improvements over the old ones:
    - Python 2 and 3 support
    - Automatic memory management
    - Automatic memory clearing before it is freed
    - Type signatures via the python typing module
    - Full test coverage
    - Properties are utilized where it makes sense (e.g. account.id)

Signed-off-by: Damir Jelić <poljar@termina.org.uk>

8 files changed, 1373 insertions, 0 deletions

diff --git a/python/olm/__init__.py b/python/olm/__init__.py
new file mode 100644
index 0000000..b52c88b
--- /dev/null
+++ b/python/olm/__init__.py
@@ -0,0 +1,26 @@
+# -*- coding: utf-8 -*-
+# libolm python bindings
+# Copyright © 2015-2017 OpenMarket Ltd
+# Copyright © 2018 Damir Jelić <poljar@termina.org.uk>
+"""
+Olm Python bindings
+~~~~~~~~~~~~~~~~~~~~~
+|  This package implements python bindings for the libolm C library.
+|  © Copyright 2015-2017 by OpenMarket Ltd
+|  © Copyright 2018 by Damir Jelić
+"""
+from .utility import ed25519_verify, OlmVerifyError
+from .account import Account, OlmAccountError
+from .session import (
+    Session,
+    InboundSession,
+    OutboundSession,
+    OlmSessionError,
+    OlmMessage,
+    OlmPreKeyMessage
+)
+from .group_session import (
+    InboundGroupSession,
+    OutboundGroupSession,
+    OlmGroupSessionError
+)
diff --git a/python/olm/__version__.py b/python/olm/__version__.py
new file mode 100644
index 0000000..dccfdd0
--- /dev/null
+++ b/python/olm/__version__.py
@@ -0,0 +1,9 @@
+__title__ = "python-olm"
+__description__ = ("python CFFI bindings for the olm "
+                   "cryptographic ratchet library")
+__url__ = "https://github.com/poljar/python-olm"
+__version__ = "0.1"
+__author__ = "Damir Jelić"
+__author_email__ = "poljar@termina.org.uk"
+__license__ = "Apache 2.0"
+__copyright__ = "Copyright 2018 Damir Jelić"
diff --git a/python/olm/_compat.py b/python/olm/_compat.py
new file mode 100644
index 0000000..67d312b
--- /dev/null
+++ b/python/olm/_compat.py
@@ -0,0 +1,24 @@
+# -*- coding: utf-8 -*-
+# libolm python bindings
+# Copyright © 2015-2017 OpenMarket Ltd
+# Copyright © 2018 Damir Jelić <poljar@termina.org.uk>
+
+from builtins import bytes, str
+from typing import AnyStr
+
+try:
+    import secrets
+    URANDOM = secrets.token_bytes  # pragma: no cover
+except ImportError:  # pragma: no cover
+    from os import urandom
+    URANDOM = urandom  # type: ignore
+
+
+def to_bytes(string):
+    # type: (AnyStr) -> bytes
+    if isinstance(string, bytes):
+        return string
+    elif isinstance(string, str):
+        return bytes(string, "utf-8")
+
+    raise TypeError("Invalid type {}".format(type(string)))
diff --git a/python/olm/_finalize.py b/python/olm/_finalize.py
new file mode 100644
index 0000000..9f467bc
--- /dev/null
+++ b/python/olm/_finalize.py
@@ -0,0 +1,65 @@
+# The MIT License (MIT)
+# Copyright (c) 2010 Benjamin Peterson <benjamin@python.org>
+
+# Permission is hereby granted, free of charge, to any person obtaining a copy
+# of this software and associated documentation files (the "Software"), to deal
+# in the Software without restriction, including without limitation the rights
+# to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+# copies of the Software, and to permit persons to whom the Software is
+# furnished to do so, subject to the following conditions:
+
+# The above copyright notice and this permission notice shall be included in
+# all copies or substantial portions of the Software.
+
+# THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND,
+# EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF
+# MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT.
+# IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM,
+# DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR
+# OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE
+# OR OTHER DEALINGS IN THE SOFTWARE.
+
+"""Finalization with weakrefs
+
+This is designed for avoiding __del__.
+"""
+from __future__ import print_function
+
+import sys
+import traceback
+import weakref
+
+__author__ = "Benjamin Peterson <benjamin@python.org>"
+
+
+class OwnerRef(weakref.ref):
+    """A simple weakref.ref subclass, so attributes can be added."""
+    pass
+
+
+def _run_finalizer(ref):
+    """Internal weakref callback to run finalizers"""
+    del _finalize_refs[id(ref)]
+    finalizer = ref.finalizer
+    item = ref.item
+    try:
+        finalizer(item)
+    except Exception:  # pragma: no cover
+        print("Exception running {}:".format(finalizer), file=sys.stderr)
+        traceback.print_exc()
+
+
+_finalize_refs = {}
+
+
+def track_for_finalization(owner, item, finalizer):
+    """Register an object for finalization.
+
+    ``owner`` is the the object which is responsible for ``item``.
+    ``finalizer`` will be called with ``item`` as its only argument when
+    ``owner`` is destroyed by the garbage collector.
+    """
+    ref = OwnerRef(owner, _run_finalizer)
+    ref.item = item
+    ref.finalizer = finalizer
+    _finalize_refs[id(ref)] = ref
diff --git a/python/olm/account.py b/python/olm/account.py
new file mode 100644
index 0000000..5705fe3
--- /dev/null
+++ b/python/olm/account.py
@@ -0,0 +1,239 @@
+# -*- coding: utf-8 -*-
+# libolm python bindings
+# Copyright © 2015-2017 OpenMarket Ltd
+# Copyright © 2018 Damir Jelić <poljar@termina.org.uk>
+"""libolm Account module.
+
+This module contains the account part of the Olm library. It contains a single
+Account class which handles the creation of new accounts as well as the storing
+and restoring of them.
+
+Examples:
+    >>> acc = Account()
+    >>> account.identity_keys()
+    >>> account.generate_one_time_keys(1)
+
+"""
+
+import json
+# pylint: disable=redefined-builtin,unused-import
+from builtins import bytes, super
+from typing import AnyStr, Dict, Optional, Type
+
+from future.utils import bytes_to_native_str
+
+# pylint: disable=no-name-in-module
+from _libolm import ffi, lib  # type: ignore
+
+from ._compat import URANDOM, to_bytes
+from ._finalize import track_for_finalization
+
+# This is imported only for type checking purposes
+if False:
+    from .session import Session  # pragma: no cover
+
+
+def _clear_account(account):
+    # type: (ffi.cdata) -> None
+    lib.olm_clear_account(account)
+
+
+class OlmAccountError(Exception):
+    """libolm Account error exception."""
+
+
+class Account(object):
+    """libolm Account class."""
+
+    def __new__(cls):
+        # type: (Type[Account]) -> Account
+        obj = super().__new__(cls)
+        obj._buf = ffi.new("char[]", lib.olm_account_size())
+        obj._account = lib.olm_account(obj._buf)
+        track_for_finalization(obj, obj._account, _clear_account)
+        return obj
+
+    def __init__(self):
+        # type: () -> None
+        """Create a new Olm account.
+
+        Creates a new account and its matching identity key pair.
+
+        Raises OlmAccountError on failure. If there weren't enough random bytes
+        for the account creation the error message for the exception will be
+        NOT_ENOUGH_RANDOM.
+        """
+        # This is needed to silence mypy not knowing the type of _account.
+        # There has to be a better way for this.
+        if False:  # pragma: no cover
+            self._account = self._account  # type: ffi.cdata
+
+        random_length = lib.olm_create_account_random_length(self._account)
+        random = URANDOM(random_length)
+        random_buffer = ffi.new("char[]", random)
+
+        self._check_error(
+            lib.olm_create_account(self._account, random_buffer,
+                                   random_length))
+
+    def _check_error(self, ret):
+        # type: (int) -> None
+        if ret != lib.olm_error():
+            return
+
+        last_error = bytes_to_native_str(
+            ffi.string((lib.olm_account_last_error(self._account))))
+
+        raise OlmAccountError(last_error)
+
+    def pickle(self, passphrase=""):
+        # type: (Optional[str]) -> bytes
+        """Store an Olm account.
+
+        Stores an account as a base64 string. Encrypts the account using the
+        supplied passphrase. Returns a byte object containing the base64
+        encoded string of the pickled account. Raises OlmAccountError on
+        failure.
+
+        Args:
+            passphrase(str, optional): The passphrase to be used to encrypt
+                the account.
+        """
+        byte_key = bytes(passphrase, "utf-8") if passphrase else b""
+        key_buffer = ffi.new("char[]", byte_key)
+
+        pickle_length = lib.olm_pickle_account_length(self._account)
+        pickle_buffer = ffi.new("char[]", pickle_length)
+
+        self._check_error(
+            lib.olm_pickle_account(self._account, key_buffer, len(byte_key),
+                                   pickle_buffer, pickle_length))
+        return ffi.unpack(pickle_buffer, pickle_length)
+
+    @classmethod
+    def from_pickle(cls, pickle, passphrase=""):
+        # type: (bytes, Optional[str]) -> Account
+        """Load a previously stored olm account.
+
+        Loads an account from a pickled base64-encoded string and returns an
+        Account object. Decrypts the account using the supplied passphrase.
+        Raises OlmAccountError on failure. If the passphrase doesn't match the
+        one used to encrypt the account then the error message for the
+        exception will be "BAD_ACCOUNT_KEY". If the base64 couldn't be decoded
+        then the error message will be "INVALID_BASE64".
+
+        Args:
+            pickle(bytes): Base64 encoded byte string containing the pickled
+                account
+            passphrase(str, optional): The passphrase used to encrypt the
+                account.
+        """
+        if not pickle:
+            raise ValueError("Pickle can't be empty")
+
+        byte_key = bytes(passphrase, "utf-8") if passphrase else b""
+        key_buffer = ffi.new("char[]", byte_key)
+        pickle_buffer = ffi.new("char[]", pickle)
+
+        obj = cls.__new__(cls)
+
+        ret = lib.olm_unpickle_account(obj._account, key_buffer, len(byte_key),
+                                       pickle_buffer, len(pickle))
+        obj._check_error(ret)
+
+        return obj
+
+    @property
+    def identity_keys(self):
+        # type: () -> Dict[str, str]
+        """dict: Public part of the identity keys of the account."""
+        out_length = lib.olm_account_identity_keys_length(self._account)
+        out_buffer = ffi.new("char[]", out_length)
+
+        self._check_error(
+            lib.olm_account_identity_keys(self._account, out_buffer,
+                                          out_length))
+        return json.loads(ffi.unpack(out_buffer, out_length).decode("utf-8"))
+
+    def sign(self, message):
+        # type: (AnyStr) -> str
+        """Signs a message with this account.
+
+        Signs a message with the private ed25519 identity key of this account.
+        Returns the signature.
+        Raises OlmAccountError on failure.
+
+        Args:
+            message(str): The message to sign.
+        """
+        bytes_message = to_bytes(message)
+        out_length = lib.olm_account_signature_length(self._account)
+        message_buffer = ffi.new("char[]", bytes_message)
+        out_buffer = ffi.new("char[]", out_length)
+
+        self._check_error(
+            lib.olm_account_sign(self._account, message_buffer,
+                                 len(bytes_message), out_buffer, out_length))
+
+        return bytes_to_native_str(ffi.unpack(out_buffer, out_length))
+
+    @property
+    def max_one_time_keys(self):
+        # type: () -> int
+        """int: The maximum number of one-time keys the account can store."""
+        return lib.olm_account_max_number_of_one_time_keys(self._account)
+
+    def mark_keys_as_published(self):
+        # type: () -> None
+        """Mark the current set of one-time keys as being published."""
+        lib.olm_account_mark_keys_as_published(self._account)
+
+    def generate_one_time_keys(self, count):
+        # type: (int) -> None
+        """Generate a number of new one-time keys.
+
+        If the total number of keys stored by this account exceeds
+        max_one_time_keys() then the old keys are discarded.
+        Raises OlmAccountError on error. If the number of random bytes is
+        too small then the error message of the exception will be
+        NOT_ENOUGH_RANDOM.
+
+        Args:
+            count(int): The number of keys to generate.
+        """
+        random_length = lib.olm_account_generate_one_time_keys_random_length(
+            self._account, count)
+        random = URANDOM(random_length)
+        random_buffer = ffi.new("char[]", random)
+        self._check_error(
+            lib.olm_account_generate_one_time_keys(
+                self._account, count, random_buffer, random_length))
+
+    @property
+    def one_time_keys(self):
+        # type: () -> Dict[str, Dict[str, str]]
+        """dict: The public part of the one-time keys for this account."""
+        out_length = lib.olm_account_one_time_keys_length(self._account)
+        out_buffer = ffi.new("char[]", out_length)
+
+        self._check_error(
+            lib.olm_account_one_time_keys(self._account, out_buffer,
+                                          out_length))
+
+        return json.loads(ffi.unpack(out_buffer, out_length).decode("utf-8"))
+
+    def remove_one_time_keys(self, session):
+        # type: (Session) -> None
+        """Remove used one-time keys.
+
+        Removes the one-time keys that the session used from the account.
+        Raises OlmAccountError on failure. If the account doesn't have any
+        matching one-time keys then the error message of the exception will be
+        "BAD_MESSAGE_KEY_ID".
+
+        Args:
+            session(Session): An Olm Session object that was created with this
+            account.
+        """
+        self._check_error(lib.olm_remove_one_time_keys(self._account,
+                                                       session._session))
diff --git a/python/olm/group_session.py b/python/olm/group_session.py
new file mode 100644
index 0000000..6b13c60
--- /dev/null
+++ b/python/olm/group_session.py
@@ -0,0 +1,467 @@
+# -*- coding: utf-8 -*-
+# libolm python bindings
+# Copyright © 2015-2017 OpenMarket Ltd
+# Copyright © 2018 Damir Jelić <poljar@termina.org.uk>
+"""libolm Group session module.
+
+This module contains the group session part of the Olm library. It contains two
+classes for creating inbound and outbound group sessions.
+
+Examples:
+    >>> outbound = OutboundGroupSession()
+    >>> InboundGroupSession(outbound.session_key)
+"""
+
+# pylint: disable=redefined-builtin,unused-import
+from builtins import bytes, super
+from typing import AnyStr, Optional, Tuple, Type
+
+from future.utils import bytes_to_native_str
+
+# pylint: disable=no-name-in-module
+from _libolm import ffi, lib  # type: ignore
+
+from ._compat import URANDOM, to_bytes
+from ._finalize import track_for_finalization
+
+
+def _clear_inbound_group_session(session):
+    # type: (ffi.cdata) -> None
+    lib.olm_clear_inbound_group_session(session)
+
+
+def _clear_outbound_group_session(session):
+    # type: (ffi.cdata) -> None
+    lib.olm_clear_outbound_group_session(session)
+
+
+class OlmGroupSessionError(Exception):
+    """libolm Group session error exception."""
+
+
+class InboundGroupSession(object):
+    """Inbound group session for encrypted multiuser communication."""
+
+    def __new__(
+        cls,              # type: Type[InboundGroupSession]
+        session_key=None  # type: Optional[str]
+    ):
+        # type: (...) -> InboundGroupSession
+        obj = super().__new__(cls)
+        obj._buf = ffi.new("char[]", lib.olm_inbound_group_session_size())
+        obj._session = lib.olm_inbound_group_session(obj._buf)
+        track_for_finalization(obj, obj._session, _clear_inbound_group_session)
+        return obj
+
+    def __init__(self, session_key):
+        # type: (AnyStr) -> None
+        """Create a new inbound group session.
+        Start a new inbound group session, from a key exported from
+        an outbound group session.
+
+        Raises OlmGroupSessionError on failure. The error message of the
+        exception will be "OLM_INVALID_BASE64" if the session key is not valid
+        base64 and "OLM_BAD_SESSION_KEY" if the session key is invalid.
+        """
+        if False:  # pragma: no cover
+            self._session = self._session  # type: ffi.cdata
+
+        byte_session_key = to_bytes(session_key)
+
+        key_buffer = ffi.new("char[]", byte_session_key)
+        ret = lib.olm_init_inbound_group_session(
+            self._session, key_buffer, len(byte_session_key)
+        )
+        self._check_error(ret)
+
+    def pickle(self, passphrase=""):
+        # type: (Optional[str]) -> bytes
+        """Store an inbound group session.
+
+        Stores a group session as a base64 string. Encrypts the session using
+        the supplied passphrase. Returns a byte object containing the base64
+        encoded string of the pickled session.
+
+        Args:
+            passphrase(str, optional): The passphrase to be used to encrypt
+                the session.
+        """
+        byte_passphrase = bytes(passphrase, "utf-8") if passphrase else b""
+
+        passphrase_buffer = ffi.new("char[]", byte_passphrase)
+        pickle_length = lib.olm_pickle_inbound_group_session_length(
+            self._session)
+        pickle_buffer = ffi.new("char[]", pickle_length)
+
+        ret = lib.olm_pickle_inbound_group_session(
+            self._session, passphrase_buffer, len(byte_passphrase),
+            pickle_buffer, pickle_length
+        )
+
+        self._check_error(ret)
+
+        return ffi.unpack(pickle_buffer, pickle_length)
+
+    @classmethod
+    def from_pickle(cls, pickle, passphrase=""):
+        # type: (bytes, Optional[str]) -> InboundGroupSession
+        """Load a previously stored inbound group session.
+
+        Loads an inbound group session from a pickled base64 string and returns
+        an InboundGroupSession object. Decrypts the session using the supplied
+        passphrase. Raises OlmSessionError on failure. If the passphrase
+        doesn't match the one used to encrypt the session then the error
+        message for the exception will be "BAD_ACCOUNT_KEY". If the base64
+        couldn't be decoded then the error message will be "INVALID_BASE64".
+
+        Args:
+            pickle(bytes): Base64 encoded byte string containing the pickled
+                session
+            passphrase(str, optional): The passphrase used to encrypt the
+                session
+        """
+        if not pickle:
+            raise ValueError("Pickle can't be empty")
+
+        byte_passphrase = bytes(passphrase, "utf-8") if passphrase else b""
+        passphrase_buffer = ffi.new("char[]", byte_passphrase)
+        pickle_buffer = ffi.new("char[]", pickle)
+
+        obj = cls.__new__(cls)
+
+        ret = lib.olm_unpickle_inbound_group_session(
+            obj._session,
+            passphrase_buffer,
+            len(byte_passphrase),
+            pickle_buffer,
+            len(pickle)
+        )
+        obj._check_error(ret)
+
+        return obj
+
+    def _check_error(self, ret):
+        # type: (int) -> None
+        if ret != lib.olm_error():
+            return
+
+        last_error = bytes_to_native_str(ffi.string(
+            lib.olm_inbound_group_session_last_error(self._session)))
+
+        raise OlmGroupSessionError(last_error)
+
+    def decrypt(self, ciphertext):
+        # type: (AnyStr) -> Tuple[str, int]
+        """Decrypt a message
+
+        Returns a tuple of the decrypted plain-text and the message index of
+        the decrypted message or raises OlmGroupSessionError on failure.
+        On failure the error message of the exception  will be:
+
+        * OLM_INVALID_BASE64         if the message is not valid base64
+        * OLM_BAD_MESSAGE_VERSION    if the message was encrypted with an
+            unsupported version of the protocol
+        * OLM_BAD_MESSAGE_FORMAT     if the message headers could not be
+            decoded
+        * OLM_BAD_MESSAGE_MAC        if the message could not be verified
+        * OLM_UNKNOWN_MESSAGE_INDEX  if we do not have a session key
+            corresponding to the message's index (i.e., it was sent before
+            the session key was shared with us)
+
+        Args:
+            ciphertext(str): Base64 encoded ciphertext containing the encrypted
+                message
+        """
+        if not ciphertext:
+            raise ValueError("Ciphertext can't be empty.")
+
+        byte_ciphertext = to_bytes(ciphertext)
+
+        ciphertext_buffer = ffi.new("char[]", byte_ciphertext)
+
+        max_plaintext_length = lib.olm_group_decrypt_max_plaintext_length(
+            self._session, ciphertext_buffer, len(byte_ciphertext)
+        )
+        plaintext_buffer = ffi.new("char[]", max_plaintext_length)
+        ciphertext_buffer = ffi.new("char[]", byte_ciphertext)
+
+        message_index = ffi.new("uint32_t*")
+        plaintext_length = lib.olm_group_decrypt(
+            self._session, ciphertext_buffer, len(byte_ciphertext),
+            plaintext_buffer, max_plaintext_length,
+            message_index
+        )
+
+        self._check_error(plaintext_length)
+
+        return bytes_to_native_str(ffi.unpack(
+            plaintext_buffer,
+            plaintext_length
+        )), message_index[0]
+
+    @property
+    def id(self):
+        # type: () -> str
+        """str: A base64 encoded identifier for this session."""
+        id_length = lib.olm_inbound_group_session_id_length(self._session)
+        id_buffer = ffi.new("char[]", id_length)
+        ret = lib.olm_inbound_group_session_id(
+            self._session,
+            id_buffer,
+            id_length
+        )
+        self._check_error(ret)
+        return bytes_to_native_str(ffi.unpack(id_buffer, id_length))
+
+    @property
+    def first_known_index(self):
+        # type: () -> int
+        """int: The first message index we know how to decrypt."""
+        return lib.olm_inbound_group_session_first_known_index(self._session)
+
+    def export_session(self, message_index):
+        # type: (int) -> str
+        """Export an inbound group session
+
+        Export the base64-encoded ratchet key for this session, at the given
+        index, in a format which can be used by import_session().
+
+        Raises OlmGroupSessionError on failure. The error message for the
+        exception will be:
+
+        * OLM_UNKNOWN_MESSAGE_INDEX if we do not have a session key
+            corresponding to the given index (ie, it was sent before the
+            session key was shared with us)
+
+        Args:
+            message_index(int): The message index at which the session should
+                be exported.
+        """
+
+        export_length = lib.olm_export_inbound_group_session_length(
+            self._session)
+
+        export_buffer = ffi.new("char[]", export_length)
+        ret = lib.olm_export_inbound_group_session(
+            self._session,
+            export_buffer,
+            export_length,
+            message_index
+        )
+        self._check_error(ret)
+        return bytes_to_native_str(ffi.unpack(export_buffer, export_length))
+
+    @classmethod
+    def import_session(cls, session_key):
+        # type: (AnyStr) -> InboundGroupSession
+        """Create an InboundGroupSession from an exported session key.
+
+        Creates an InboundGroupSession with an previously exported session key,
+        raises OlmGroupSessionError on failure. The error message for the
+        exception will be:
+
+        * OLM_INVALID_BASE64  if the session_key is not valid base64
+        * OLM_BAD_SESSION_KEY if the session_key is invalid
+
+        Args:
+            session_key(str): The exported session key with which the inbound
+                group session will be created
+        """
+        obj = cls.__new__(cls)
+
+        byte_session_key = to_bytes(session_key)
+
+        key_buffer = ffi.new("char[]", byte_session_key)
+        ret = lib.olm_import_inbound_group_session(
+            obj._session,
+            key_buffer,
+            len(byte_session_key)
+        )
+        obj._check_error(ret)
+
+        return obj
+
+
+class OutboundGroupSession(object):
+    """Outbound group session for encrypted multiuser communication."""
+
+    def __new__(cls):
+        # type: (Type[OutboundGroupSession]) -> OutboundGroupSession
+        obj = super().__new__(cls)
+        obj._buf = ffi.new("char[]", lib.olm_outbound_group_session_size())
+        obj._session = lib.olm_outbound_group_session(obj._buf)
+        track_for_finalization(
+            obj,
+            obj._session,
+            _clear_outbound_group_session
+        )
+        return obj
+
+    def __init__(self):
+        # type: () -> None
+        """Create a new outbound group session.
+
+        Start a new outbound group session. Raises OlmGroupSessionError on
+        failure. If there weren't enough random bytes for the session creation
+        the error message for the exception will be NOT_ENOUGH_RANDOM.
+        """
+        if False:  # pragma: no cover
+            self._session = self._session  # type: ffi.cdata
+
+        random_length = lib.olm_init_outbound_group_session_random_length(
+            self._session
+        )
+        random = URANDOM(random_length)
+        random_buffer = ffi.new("char[]", random)
+
+        ret = lib.olm_init_outbound_group_session(
+            self._session, random_buffer, random_length
+        )
+        self._check_error(ret)
+
+    def _check_error(self, ret):
+        # type: (int) -> None
+        if ret != lib.olm_error():
+            return
+
+        last_error = bytes_to_native_str(ffi.string(
+            lib.olm_outbound_group_session_last_error(self._session)
+        ))
+
+        raise OlmGroupSessionError(last_error)
+
+    def pickle(self, passphrase=""):
+        # type: (Optional[str]) -> bytes
+        """Store an outbound group session.
+
+        Stores a group session as a base64 string. Encrypts the session using
+        the supplied passphrase. Returns a byte object containing the base64
+        encoded string of the pickled session.
+
+        Args:
+            passphrase(str, optional): The passphrase to be used to encrypt
+                the session.
+        """
+        byte_passphrase = bytes(passphrase, "utf-8") if passphrase else b""
+        passphrase_buffer = ffi.new("char[]", byte_passphrase)
+        pickle_length = lib.olm_pickle_outbound_group_session_length(
+            self._session)
+        pickle_buffer = ffi.new("char[]", pickle_length)
+
+        ret = lib.olm_pickle_outbound_group_session(
+            self._session, passphrase_buffer, len(byte_passphrase),
+            pickle_buffer, pickle_length
+        )
+        self._check_error(ret)
+        return ffi.unpack(pickle_buffer, pickle_length)
+
+    @classmethod
+    def from_pickle(cls, pickle, passphrase=""):
+        # type: (bytes, Optional[str]) -> OutboundGroupSession
+        """Load a previously stored outbound group session.
+
+        Loads an outbound group session from a pickled base64 string and
+        returns an OutboundGroupSession object. Decrypts the session using the
+        supplied passphrase. Raises OlmSessionError on failure. If the
+        passphrase doesn't match the one used to encrypt the session then the
+        error message for the exception will be "BAD_ACCOUNT_KEY". If the
+        base64 couldn't be decoded then the error message will be
+        "INVALID_BASE64".
+
+        Args:
+            pickle(bytes): Base64 encoded byte string containing the pickled
+                session
+            passphrase(str, optional): The passphrase used to encrypt the
+        """
+        if not pickle:
+            raise ValueError("Pickle can't be empty")
+
+        byte_passphrase = bytes(passphrase, "utf-8") if passphrase else b""
+        passphrase_buffer = ffi.new("char[]", byte_passphrase)
+        pickle_buffer = ffi.new("char[]", pickle)
+
+        obj = cls.__new__(cls)
+
+        ret = lib.olm_unpickle_outbound_group_session(
+            obj._session,
+            passphrase_buffer,
+            len(byte_passphrase),
+            pickle_buffer,
+            len(pickle)
+        )
+        obj._check_error(ret)
+
+        return obj
+
+    def encrypt(self, plaintext):
+        # type: (AnyStr) -> str
+        """Encrypt a message.
+
+        Returns the encrypted ciphertext.
+
+        Args:
+            plaintext(str): A string that will be encrypted using the group
+                session.
+        """
+        byte_plaintext = to_bytes(plaintext)
+        message_length = lib.olm_group_encrypt_message_length(
+            self._session, len(byte_plaintext)
+        )
+
+        message_buffer = ffi.new("char[]", message_length)
+
+        plaintext_buffer = ffi.new("char[]", byte_plaintext)
+
+        ret = lib.olm_group_encrypt(
+            self._session,
+            plaintext_buffer, len(byte_plaintext),
+            message_buffer, message_length,
+        )
+        self._check_error(ret)
+        return bytes_to_native_str(ffi.unpack(message_buffer, message_length))
+
+    @property
+    def id(self):
+        # type: () -> str
+        """str: A base64 encoded identifier for this session."""
+        id_length = lib.olm_outbound_group_session_id_length(self._session)
+        id_buffer = ffi.new("char[]", id_length)
+
+        ret = lib.olm_outbound_group_session_id(
+            self._session,
+            id_buffer,
+            id_length
+        )
+        self._check_error(ret)
+
+        return bytes_to_native_str(ffi.unpack(id_buffer, id_length))
+
+    @property
+    def message_index(self):
+        # type: () -> int
+        """int: The current message index of the session.
+
+        Each message is encrypted with an increasing index. This is the index
+        for the next message.
+        """
+        return lib.olm_outbound_group_session_message_index(self._session)
+
+    @property
+    def session_key(self):
+        # type: () -> str
+        """The base64-encoded current ratchet key for this session.
+
+        Each message is encrypted with a different ratchet key. This function
+        returns the ratchet key that will be used for the next message.
+        """
+        key_length = lib.olm_outbound_group_session_key_length(self._session)
+        key_buffer = ffi.new("char[]", key_length)
+
+        ret = lib.olm_outbound_group_session_key(
+            self._session,
+            key_buffer,
+            key_length
+        )
+        self._check_error(ret)
+
+        return bytes_to_native_str(ffi.unpack(key_buffer, key_length))
diff --git a/python/olm/session.py b/python/olm/session.py
new file mode 100644
index 0000000..b7b2c4b
--- /dev/null
+++ b/python/olm/session.py
@@ -0,0 +1,452 @@
+# -*- coding: utf-8 -*-
+# libolm python bindings
+# Copyright © 2015-2017 OpenMarket Ltd
+# Copyright © 2018 Damir Jelić <poljar@termina.org.uk>
+"""libolm Session module.
+
+This module contains the Olm Session part of the Olm library.
+
+It is used to establish a peer-to-peer encrypted communication channel between
+two Olm accounts.
+
+Examples:
+    >>> alice = Account()
+    >>> bob = Account()
+    >>> bob.generate_one_time_keys(1)
+    >>> id_key = bob.identity_keys['curve25519']
+    >>> one_time = list(bob.one_time_keys["curve25519"].values())[0]
+    >>> session = OutboundSession(alice, id_key, one_time)
+
+"""
+
+# pylint: disable=redefined-builtin,unused-import
+from builtins import bytes, super
+from typing import AnyStr, Optional, Type
+
+from future.utils import bytes_to_native_str
+
+# pylint: disable=no-name-in-module
+from _libolm import ffi, lib  # type: ignore
+
+from ._compat import URANDOM, to_bytes
+from ._finalize import track_for_finalization
+
+# This is imported only for type checking purposes
+if False:
+    from .account import Account  # pragma: no cover
+
+
+class OlmSessionError(Exception):
+    """libolm Session exception."""
+
+
+class _OlmMessage(object):
+    def __init__(self, ciphertext, message_type):
+        # type: (AnyStr, ffi.cdata) -> None
+        if not ciphertext:
+            raise ValueError("Ciphertext can't be empty")
+
+        # I don't know why mypy wants a type annotation here nor why AnyStr
+        # doesn't work
+        self.ciphertext = ciphertext  # type: ignore
+        self.message_type = message_type
+
+    def __str__(self):
+        # type: () -> str
+        type_to_prefix = {
+            lib.OLM_MESSAGE_TYPE_PRE_KEY: "PRE_KEY",
+            lib.OLM_MESSAGE_TYPE_MESSAGE: "MESSAGE"
+        }
+
+        prefix = type_to_prefix[self.message_type]
+        return "{} {}".format(prefix, self.ciphertext)
+
+
+class OlmPreKeyMessage(_OlmMessage):
+    """Olm prekey message class
+
+    Prekey messages are used to establish an Olm session. After the first
+    message exchange the session switches to normal messages
+    """
+
+    def __init__(self, ciphertext):
+        # type: (AnyStr) -> None
+        """Create a new Olm prekey message with the supplied ciphertext
+
+        Args:
+            ciphertext(str): The ciphertext of the prekey message.
+        """
+        _OlmMessage.__init__(self, ciphertext, lib.OLM_MESSAGE_TYPE_PRE_KEY)
+
+    def __repr__(self):
+        # type: () -> str
+        return "OlmPreKeyMessage({})".format(self.ciphertext)
+
+
+class OlmMessage(_OlmMessage):
+    """Olm message class"""
+
+    def __init__(self, ciphertext):
+        # type: (AnyStr) -> None
+        """Create a new Olm message with the supplied ciphertext
+
+        Args:
+            ciphertext(str): The ciphertext of the message.
+        """
+        _OlmMessage.__init__(self, ciphertext, lib.OLM_MESSAGE_TYPE_MESSAGE)
+
+    def __repr__(self):
+        # type: () -> str
+        return "OlmMessage({})".format(self.ciphertext)
+
+
+def _clear_session(session):
+    # type: (ffi.cdata) -> None
+    lib.olm_clear_session(session)
+
+
+class Session(object):
+    """libolm Session class.
+    This is an abstract class that can't be instantiated except when unpickling
+    a previously pickled InboundSession or OutboundSession object with
+    from_pickle.
+    """
+
+    def __new__(cls):
+        # type: (Type[Session]) -> Session
+
+        obj = super().__new__(cls)
+        obj._buf = ffi.new("char[]", lib.olm_session_size())
+        obj._session = lib.olm_session(obj._buf)
+        track_for_finalization(obj, obj._session, _clear_session)
+        return obj
+
+    def __init__(self):
+        # type: () -> None
+        if type(self) is Session:
+            raise TypeError("Session class may not be instantiated.")
+
+        if False:
+            self._session = self._session  # type: ffi.cdata
+
+    def _check_error(self, ret):
+        # type: (int) -> None
+        if ret != lib.olm_error():
+            return
+
+        last_error = bytes_to_native_str(
+            ffi.string(lib.olm_session_last_error(self._session)))
+
+        raise OlmSessionError(last_error)
+
+    def pickle(self, passphrase=""):
+        # type: (Optional[str]) -> bytes
+        """Store an Olm session.
+
+        Stores a session as a base64 string. Encrypts the session using the
+        supplied passphrase. Returns a byte object containing the base64
+        encoded string of the pickled session. Raises OlmSessionError on
+        failure.
+
+        Args:
+            passphrase(str, optional): The passphrase to be used to encrypt
+                the session.
+        """
+        byte_key = bytes(passphrase, "utf-8") if passphrase else b""
+        key_buffer = ffi.new("char[]", byte_key)
+
+        pickle_length = lib.olm_pickle_session_length(self._session)
+        pickle_buffer = ffi.new("char[]", pickle_length)
+
+        self._check_error(
+            lib.olm_pickle_session(self._session, key_buffer, len(byte_key),
+                                   pickle_buffer, pickle_length))
+        return ffi.unpack(pickle_buffer, pickle_length)
+
+    @classmethod
+    def from_pickle(cls, pickle, passphrase=""):
+        # type: (bytes, Optional[str]) -> Session
+        """Load a previously stored Olm session.
+
+        Loads a session from a pickled base64 string and returns a Session
+        object. Decrypts the session using the supplied passphrase. Raises
+        OlmSessionError on failure. If the passphrase doesn't match the one
+        used to encrypt the session then the error message for the
+        exception will be "BAD_ACCOUNT_KEY". If the base64 couldn't be decoded
+        then the error message will be "INVALID_BASE64".
+
+        Args:
+            pickle(bytes): Base64 encoded byte string containing the pickled
+                session
+            passphrase(str, optional): The passphrase used to encrypt the
+                session.
+        """
+        if not pickle:
+            raise ValueError("Pickle can't be empty")
+
+        byte_key = bytes(passphrase, "utf-8") if passphrase else b""
+        key_buffer = ffi.new("char[]", byte_key)
+        pickle_buffer = ffi.new("char[]", pickle)
+
+        session = cls.__new__(cls)
+
+        ret = lib.olm_unpickle_session(session._session, key_buffer,
+                                       len(byte_key), pickle_buffer,
+                                       len(pickle))
+        session._check_error(ret)
+
+        return session
+
+    def encrypt(self, plaintext):
+        # type: (AnyStr) -> _OlmMessage
+        """Encrypts a message using the session. Returns the ciphertext as a
+        base64 encoded string on success. Raises OlmSessionError on failure. If
+        there weren't enough random bytes to encrypt the message the error
+        message for the exception will be NOT_ENOUGH_RANDOM.
+
+        Args:
+            plaintext(str): The plaintext message that will be encrypted.
+        """
+        byte_plaintext = to_bytes(plaintext)
+
+        r_length = lib.olm_encrypt_random_length(self._session)
+        random = URANDOM(r_length)
+        random_buffer = ffi.new("char[]", random)
+
+        message_type = lib.olm_encrypt_message_type(self._session)
+
+        self._check_error(message_type)
+
+        ciphertext_length = lib.olm_encrypt_message_length(
+            self._session, len(plaintext)
+        )
+        ciphertext_buffer = ffi.new("char[]", ciphertext_length)
+
+        plaintext_buffer = ffi.new("char[]", byte_plaintext)
+
+        self._check_error(lib.olm_encrypt(
+            self._session,
+            plaintext_buffer, len(byte_plaintext),
+            random_buffer, r_length,
+            ciphertext_buffer, ciphertext_length,
+        ))
+
+        if message_type == lib.OLM_MESSAGE_TYPE_PRE_KEY:
+            return OlmPreKeyMessage(
+                bytes_to_native_str(ffi.unpack(
+                    ciphertext_buffer,
+                    ciphertext_length
+                )))
+        elif message_type == lib.OLM_MESSAGE_TYPE_MESSAGE:
+            return OlmMessage(
+                bytes_to_native_str(ffi.unpack(
+                    ciphertext_buffer,
+                    ciphertext_length
+                )))
+        else:  # pragma: no cover
+            raise ValueError("Unknown message type")
+
+    def decrypt(self, message):
+        # type: (_OlmMessage) -> str
+        """Decrypts a message using the session. Returns the plaintext string
+        on success. Raises OlmSessionError on failure. If the base64 couldn't
+        be decoded then the error message will be "INVALID_BASE64". If the
+        message is for an unsupported version of the protocol the error message
+        will be "BAD_MESSAGE_VERSION". If the message couldn't be decoded then
+        the error message will be "BAD_MESSAGE_FORMAT". If the MAC on the
+        message was invalid then the error message will be "BAD_MESSAGE_MAC".
+
+        Args:
+            message(OlmMessage): The Olm message that will be decrypted. It can
+            be either an OlmPreKeyMessage or an OlmMessage.
+        """
+        if not message.ciphertext:
+            raise ValueError("Ciphertext can't be empty")
+
+        byte_ciphertext = to_bytes(message.ciphertext)
+        ciphertext_buffer = ffi.new("char[]", byte_ciphertext)
+
+        max_plaintext_length = lib.olm_decrypt_max_plaintext_length(
+            self._session, message.message_type, ciphertext_buffer,
+            len(byte_ciphertext)
+        )
+        plaintext_buffer = ffi.new("char[]", max_plaintext_length)
+        ciphertext_buffer = ffi.new("char[]", byte_ciphertext)
+        plaintext_length = lib.olm_decrypt(
+            self._session, message.message_type, ciphertext_buffer,
+            len(byte_ciphertext), plaintext_buffer, max_plaintext_length
+        )
+        self._check_error(plaintext_length)
+        return bytes_to_native_str(
+            ffi.unpack(plaintext_buffer, plaintext_length))
+
+    @property
+    def id(self):
+        # type: () -> str
+        """str: An identifier for this session. Will be the same for both
+        ends of the conversation.
+        """
+        id_length = lib.olm_session_id_length(self._session)
+        id_buffer = ffi.new("char[]", id_length)
+
+        self._check_error(
+            lib.olm_session_id(self._session, id_buffer, id_length)
+        )
+        return bytes_to_native_str(ffi.unpack(id_buffer, id_length))
+
+    def matches(self, message, identity_key=None):
+        # type: (OlmPreKeyMessage, Optional[AnyStr]) -> bool
+        """Checks if the PRE_KEY message is for this in-bound session.
+        This can happen if multiple messages are sent to this session before
+        this session sends a message in reply. Returns True if the session
+        matches. Returns False if the session does not match. Raises
+        OlmSessionError on failure. If the base64 couldn't be decoded then the
+        error message will be "INVALID_BASE64". If the message was for an
+        unsupported protocol version then the error message will be
+        "BAD_MESSAGE_VERSION". If the message couldn't be decoded then then the
+        error message will be * "BAD_MESSAGE_FORMAT".
+
+        Args:
+            message(OlmPreKeyMessage): The Olm prekey message that will checked
+                if it is intended for this session.
+            identity_key(str, optional): The identity key of the sender. To
+                check if the message was also sent using this identity key.
+        """
+        if not isinstance(message, OlmPreKeyMessage):
+            raise TypeError("Matches can only be called with prekey messages.")
+
+        if not message.ciphertext:
+            raise ValueError("Ciphertext can't be empty")
+
+        ret = None
+
+        byte_ciphertext = to_bytes(message.ciphertext)
+
+        message_buffer = ffi.new("char[]", byte_ciphertext)
+
+        if identity_key:
+            byte_id_key = to_bytes(identity_key)
+            identity_key_buffer = ffi.new("char[]", byte_id_key)
+
+            ret = lib.olm_matches_inbound_session_from(
+                self._session,
+                identity_key_buffer, len(byte_id_key),
+                message_buffer, len(byte_ciphertext)
+            )
+
+        else:
+            ret = lib.olm_matches_inbound_session(
+                self._session,
+                message_buffer, len(byte_ciphertext))
+
+        self._check_error(ret)
+
+        return bool(ret)
+
+
+class InboundSession(Session):
+    """Inbound Olm session for p2p encrypted communication.
+    """
+
+    def __new__(cls, account, message, identity_key=None):
+        # type: (Account, OlmPreKeyMessage, Optional[AnyStr]) -> Session
+        return super().__new__(cls)
+
+    def __init__(self, account, message, identity_key=None):
+        # type: (Account, OlmPreKeyMessage, Optional[AnyStr]) -> None
+        """Create a new inbound Olm session.
+
+        Create a new in-bound session for sending/receiving messages from an
+        incoming prekey message. Raises OlmSessionError on failure. If the
+        base64 couldn't be decoded then error message will be "INVALID_BASE64".
+        If the message was for an unsupported protocol version then
+        the errror message will be "BAD_MESSAGE_VERSION". If the message
+        couldn't be decoded then then the error message will be
+        "BAD_MESSAGE_FORMAT". If the message refers to an unknown one-time
+        key then the error message will be "BAD_MESSAGE_KEY_ID".
+
+        Args:
+            account(Account): The Olm Account that will be used to create this
+                session.
+            message(OlmPreKeyMessage): The Olm prekey message that will checked
+                that will be used to create this session.
+            identity_key(str, optional): The identity key of the sender. To
+                check if the message was also sent using this identity key.
+        """
+        if not message.ciphertext:
+            raise ValueError("Ciphertext can't be empty")
+
+        super().__init__()
+        byte_ciphertext = to_bytes(message.ciphertext)
+        message_buffer = ffi.new("char[]", byte_ciphertext)
+
+        if identity_key:
+            byte_id_key = to_bytes(identity_key)
+            identity_key_buffer = ffi.new("char[]", byte_id_key)
+            self._check_error(lib.olm_create_inbound_session_from(
+                self._session,
+                account._account,
+                identity_key_buffer, len(byte_id_key),
+                message_buffer, len(byte_ciphertext)
+            ))
+        else:
+            self._check_error(lib.olm_create_inbound_session(
+                self._session,
+                account._account,
+                message_buffer, len(byte_ciphertext)
+            ))
+
+
+class OutboundSession(Session):
+    """Outbound Olm session for p2p encrypted communication."""
+
+    def __new__(cls, account, identity_key, one_time_key):
+        # type: (Account, AnyStr, AnyStr) -> Session
+        return super().__new__(cls)
+
+    def __init__(self, account, identity_key, one_time_key):
+        # type: (Account, AnyStr, AnyStr) -> None
+        """Create a new outbound Olm session.
+
+        Creates a new outbound session for sending messages to a given
+        identity key and one-time key.
+
+        Raises OlmSessionError on failure. If the keys couldn't be decoded as
+        base64 then the error message will be "INVALID_BASE64". If there
+        weren't enough random bytes for the session creation the error message
+        for the exception will be NOT_ENOUGH_RANDOM.
+
+        Args:
+            account(Account): The Olm Account that will be used to create this
+                session.
+            identity_key(str): The identity key of the person with whom we want
+                to start the session.
+            one_time_key(str): A one-time key from the person with whom we want
+                to start the session.
+        """
+        if not identity_key:
+            raise ValueError("Identity key can't be empty")
+
+        if not one_time_key:
+            raise ValueError("One-time key can't be empty")
+
+        super().__init__()
+
+        byte_id_key = to_bytes(identity_key)
+        byte_one_time = to_bytes(one_time_key)
+
+        session_random_length = lib.olm_create_outbound_session_random_length(
+            self._session)
+
+        random = URANDOM(session_random_length)
+        random_buffer = ffi.new("char[]", random)
+        identity_key_buffer = ffi.new("char[]", byte_id_key)
+        one_time_key_buffer = ffi.new("char[]", byte_one_time)
+
+        self._check_error(lib.olm_create_outbound_session(
+            self._session,
+            account._account,
+            identity_key_buffer, len(byte_id_key),
+            one_time_key_buffer, len(byte_one_time),
+            random_buffer, session_random_length
+        ))
diff --git a/python/olm/utility.py b/python/olm/utility.py
new file mode 100644
index 0000000..838cf3f
--- /dev/null
+++ b/python/olm/utility.py
@@ -0,0 +1,91 @@
+# -*- coding: utf-8 -*-
+# libolm python bindings
+# Copyright © 2015-2017 OpenMarket Ltd
+# Copyright © 2018 Damir Jelić <poljar@termina.org.uk>
+"""libolm Utility module.
+
+This module contains utilities for olm.
+It only contains the ed25519_verify function for signature verification.
+
+Examples:
+    >>> alice = Account()
+
+    >>> message = "Test"
+    >>> signature = alice.sign(message)
+    >>> signing_key = alice.identity_keys["ed25519"]
+
+    >>> ed25519_verify(signing_key, message, signature)
+
+"""
+
+# pylint: disable=redefined-builtin,unused-import
+from typing import AnyStr, Type
+
+# pylint: disable=no-name-in-module
+from _libolm import ffi, lib  # type: ignore
+
+from ._compat import to_bytes
+from ._finalize import track_for_finalization
+
+
+def _clear_utility(utility):  # pragma: no cover
+    # type: (ffi.cdata) -> None
+    lib.olm_clear_utility(utility)
+
+
+class OlmVerifyError(Exception):
+    """libolm signature verification exception."""
+
+
+class _Utility(object):
+    # pylint: disable=too-few-public-methods
+    """libolm Utility class."""
+
+    _buf = None
+    _utility = None
+
+    @classmethod
+    def _allocate(cls):
+        # type: (Type[_Utility]) -> None
+        cls._buf = ffi.new("char[]", lib.olm_utility_size())
+        cls._utility = lib.olm_utility(cls._buf)
+        track_for_finalization(cls, cls._utility, _clear_utility)
+
+    @classmethod
+    def _check_error(cls, ret):
+        # type: (int) -> None
+        if ret != lib.olm_error():
+            return
+
+        raise OlmVerifyError("{}".format(
+            ffi.string(lib.olm_utility_last_error(
+                cls._utility)).decode("utf-8")))
+
+    @classmethod
+    def _ed25519_verify(cls, key, message, signature):
+        # type: (Type[_Utility], AnyStr, AnyStr, AnyStr) -> None
+        if not cls._utility:
+            cls._allocate()
+
+        byte_key = to_bytes(key)
+        byte_message = to_bytes(message)
+        byte_signature = to_bytes(signature)
+
+        cls._check_error(
+            lib.olm_ed25519_verify(cls._utility, byte_key, len(byte_key),
+                                   byte_message, len(byte_message),
+                                   byte_signature, len(byte_signature)))
+
+
+def ed25519_verify(key, message, signature):
+    # type: (AnyStr, AnyStr, AnyStr) -> None
+    """Verify an ed25519 signature.
+
+    Raises an OlmVerifyError if verification fails.
+
+    Args:
+        key(str): The ed25519 public key used for signing.
+        message(str): The signed message.
+        signature(bytes): The message signature.
+    """
+    return _Utility._ed25519_verify(key, message, signature)