python: Add PK bindings.

This patch adds bindings to the PK part of the Olm library contained in
the pk.h header file.

Encryption, decryption as well as pickling/unpickling of the decryption
object is supported.

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

2 files changed, 353 insertions, 0 deletions

diff --git a/python/olm/__init__.py b/python/olm/__init__.py
index 015c4f8..7b7423b 100644
--- a/python/olm/__init__.py
+++ b/python/olm/__init__.py
@@ -36,3 +36,10 @@ from .group_session import (
     OutboundGroupSession,
     OlmGroupSessionError
 )
+from .pk import (
+    PkMessage,
+    PkEncryption,
+    PkDecryption,
+    PkEncryptionError,
+    PkDecryptionError
+)
diff --git a/python/olm/pk.py b/python/olm/pk.py
new file mode 100644
index 0000000..b67d5a4
--- /dev/null
+++ b/python/olm/pk.py
@@ -0,0 +1,346 @@
+# -*- coding: utf-8 -*-
+# libolm python bindings
+# Copyright © 2018 Damir Jelić <poljar@termina.org.uk>
+#
+# 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.
+"""libolm PK module.
+
+This module contains bindings to the PK part of the Olm library.
+It contains two classes PkDecryption and PkEncryption that are used to
+establish an encrypted communication channel using public key encryption.
+
+Examples:
+    >>> decryption = PkDecryption()
+    >>> encryption = PkEncryption(decryption.public_key)
+    >>> plaintext = "It's a secret to everybody."
+    >>> message = encryption.encrypt(plaintext)
+    >>> decrypted_plaintext = decryption.decrypt(message)
+
+"""
+
+from builtins import super
+from typing import AnyStr, Type
+from future.utils import bytes_to_native_str
+
+from _libolm import ffi, lib  # type: ignore
+from ._finalize import track_for_finalization
+from ._compat import URANDOM, to_bytearray
+
+
+class PkEncryptionError(Exception):
+    """libolm Pk encryption exception."""
+
+
+class PkDecryptionError(Exception):
+    """libolm Pk decryption exception."""
+
+
+def _clear_pk_encryption(pk_struct):
+    lib.olm_clear_pk_encryption(pk_struct)
+
+
+class PkMessage(object):
+    """A PK encrypted message."""
+
+    def __init__(self, ephemeral_key, mac, ciphertext):
+        # type: (str, str, str) -> None
+        """Create a new PK encrypted message.
+
+        Args:
+            ephemeral_key(str): the public part of the ephemeral key
+            used (together with the recipient's key) to generate a symmetric
+            encryption key.
+            mac(str): Message Authentication Code of the encrypted message
+            ciphertext(str): The cipher text of the encrypted message
+        """
+        self.ephemeral_key = ephemeral_key
+        self.mac = mac
+        self.ciphertext = ciphertext
+
+
+class PkEncryption(object):
+    """PkEncryption class.
+
+    Represents the decryption part of a PK encrypted channel.
+    """
+
+    def __init__(self, recipient_key):
+        # type: (AnyStr) -> None
+        """Create a new PK encryption object.
+
+        Args:
+            recipient_key(str): a public key that will be used for encryption
+        """
+        if not recipient_key:
+            raise ValueError("Recipient key can't be empty")
+
+        self._buf = ffi.new("char[]", lib.olm_pk_encryption_size())
+        self._pk_encryption = lib.olm_pk_encryption(self._buf)
+        track_for_finalization(self, self._pk_encryption, _clear_pk_encryption)
+
+        byte_key = to_bytearray(recipient_key)
+        lib.olm_pk_encryption_set_recipient_key(
+            self._pk_encryption,
+            ffi.from_buffer(byte_key),
+            len(byte_key)
+        )
+
+        # clear out copies of the key
+        if byte_key is not recipient_key:  # pragma: no cover
+            for i in range(0, len(byte_key)):
+                byte_key[i] = 0
+
+    def _check_error(self, ret):  # pragma: no cover
+        # type: (int) -> None
+        if ret != lib.olm_error():
+            return
+
+        last_error = bytes_to_native_str(
+            ffi.string(lib.olm_pk_encryption_last_error(self._pk_encryption)))
+
+        raise PkEncryptionError(last_error)
+
+    def encrypt(self, plaintext):
+        # type: (AnyStr) -> PkMessage
+        """Encrypt a message.
+
+        Returns the encrypted PkMessage.
+
+        Args:
+            plaintext(str): A string that will be encrypted using the
+            PkEncryption object.
+        """
+        byte_plaintext = to_bytearray(plaintext)
+
+        r_length = lib.olm_pk_encrypt_random_length(self._pk_encryption)
+        random = URANDOM(r_length)
+        random_buffer = ffi.new("char[]", random)
+
+        ciphertext_length = lib.olm_pk_ciphertext_length(
+            self._pk_encryption, len(byte_plaintext)
+        )
+        ciphertext = ffi.new("char[]", ciphertext_length)
+
+        mac_length = lib.olm_pk_mac_length(self._pk_encryption)
+        mac = ffi.new("char[]", mac_length)
+
+        ephemeral_key_size = lib.olm_pk_key_length()
+        ephemeral_key = ffi.new("char[]", ephemeral_key_size)
+
+        ret = lib.olm_pk_encrypt(
+            self._pk_encryption,
+            ffi.from_buffer(byte_plaintext), len(byte_plaintext),
+            ciphertext, ciphertext_length,
+            mac, mac_length,
+            ephemeral_key, ephemeral_key_size,
+            random_buffer, r_length
+        )
+
+        try:
+            self._check_error(ret)
+        finally:  # pragma: no cover
+            # clear out copies of plaintext
+            if byte_plaintext is not plaintext:
+                for i in range(0, len(byte_plaintext)):
+                    byte_plaintext[i] = 0
+
+        message = PkMessage(
+            bytes_to_native_str(
+                ffi.unpack(ephemeral_key, ephemeral_key_size)),
+            bytes_to_native_str(
+                ffi.unpack(mac, mac_length)),
+            bytes_to_native_str(
+                ffi.unpack(ciphertext, ciphertext_length))
+        )
+        return message
+
+
+def _clear_pk_decryption(pk_struct):
+    lib.olm_clear_pk_decryption(pk_struct)
+
+
+class PkDecryption(object):
+    """PkDecryption class.
+
+    Represents the decryption part of a PK encrypted channel.
+
+    Attributes:
+        public_key (str): The public key of the PkDecryption object, can be
+            shared and used to create a PkEncryption object.
+
+    """
+
+    def __new__(cls):
+        # type: (Type[PkDecryption]) -> PkDecryption
+        obj = super().__new__(cls)
+        obj._buf = ffi.new("char[]", lib.olm_pk_decryption_size())
+        obj._pk_decryption = lib.olm_pk_decryption(obj._buf)
+        obj.public_key = None
+        track_for_finalization(obj, obj._pk_decryption, _clear_pk_decryption)
+        return obj
+
+    def __init__(self):
+        if False:  # pragma: no cover
+            self._pk_decryption = self._pk_decryption  # type: ffi.cdata
+
+        random_length = lib.olm_pk_private_key_length()
+        random = URANDOM(random_length)
+        random_buffer = ffi.new("char[]", random)
+
+        key_length = lib.olm_pk_key_length()
+        key_buffer = ffi.new("char[]", key_length)
+
+        ret = lib.olm_pk_key_from_private(
+            self._pk_decryption,
+            key_buffer, key_length,
+            random_buffer, random_length
+        )
+        self._check_error(ret)
+        self.public_key = bytes_to_native_str(ffi.unpack(
+            key_buffer,
+            key_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_pk_decryption_last_error(self._pk_decryption)))
+
+        raise PkDecryptionError(last_error)
+
+    def pickle(self, passphrase=""):
+        # type: (str) -> bytes
+        """Store a PkDecryption object.
+
+        Stores a PkDecryption object as a base64 string. Encrypts the object
+        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 object.
+        """
+        byte_key = to_bytearray(passphrase)
+
+        pickle_length = lib.olm_pickle_pk_decryption_length(
+            self._pk_decryption
+        )
+        pickle_buffer = ffi.new("char[]", pickle_length)
+
+        ret = lib.olm_pickle_pk_decryption(
+            self._pk_decryption,
+            ffi.from_buffer(byte_key), len(byte_key),
+            pickle_buffer, pickle_length
+        )
+        try:
+            self._check_error(ret)
+        finally:
+            # zero out copies of the passphrase
+            for i in range(0, len(byte_key)):
+                byte_key[i] = 0
+
+        return ffi.unpack(pickle_buffer, pickle_length)
+
+    @classmethod
+    def from_pickle(cls, pickle, passphrase=""):
+        # types: (bytes, str) -> PkDecryption
+        """Restore a previously stored PkDecryption object.
+
+        Creates a PkDecryption object from a pickled base64 string. Decrypts
+        the pickled object using the supplied passphrase.
+        Raises PkDecryptionError 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
+                PkDecryption object
+            passphrase(str, optional): The passphrase used to encrypt the
+                object
+        """
+        if not pickle:
+            raise ValueError("Pickle can't be empty")
+
+        byte_key = to_bytearray(passphrase)
+        pickle_buffer = ffi.new("char[]", pickle)
+
+        pubkey_length = lib.olm_pk_key_length()
+        pubkey_buffer = ffi.new("char[]", pubkey_length)
+
+        obj = cls.__new__(cls)
+
+        ret = lib.olm_unpickle_pk_decryption(
+            obj._pk_decryption,
+            ffi.from_buffer(byte_key), len(byte_key),
+            pickle_buffer, len(pickle),
+            pubkey_buffer, pubkey_length)
+
+        try:
+            obj._check_error(ret)
+        finally:
+            for i in range(0, len(byte_key)):
+                byte_key[i] = 0
+
+        obj.public_key = bytes_to_native_str(ffi.unpack(
+            pubkey_buffer,
+            pubkey_length
+        ))
+
+        return obj
+
+    def decrypt(self, message):
+        # type (PkMessage) -> str
+        """Decrypt a previously encrypted Pk message.
+
+        Returns the decrypted plaintext.
+        Raises PkDecryptionError on failure.
+
+        Args:
+            message(PkMessage): the pk message to decrypt.
+        """
+        ephemeral_key = to_bytearray(message.ephemeral_key)
+        ephemeral_key_size = len(ephemeral_key)
+
+        mac = to_bytearray(message.mac)
+        mac_length = len(mac)
+
+        ciphertext = to_bytearray(message.ciphertext)
+        ciphertext_length = len(ciphertext)
+
+        max_plaintext_length = lib.olm_pk_max_plaintext_length(
+            self._pk_decryption,
+            ciphertext_length
+        )
+        plaintext_buffer = ffi.new("char[]", max_plaintext_length)
+
+        ret = lib.olm_pk_decrypt(
+            self._pk_decryption,
+            ffi.from_buffer(ephemeral_key), ephemeral_key_size,
+            ffi.from_buffer(mac), mac_length,
+            ffi.from_buffer(ciphertext), ciphertext_length,
+            plaintext_buffer, max_plaintext_length)
+        self._check_error(ret)
+
+        plaintext = (ffi.unpack(
+            plaintext_buffer,
+            ret
+        ))
+
+        # clear out copies of the plaintext
+        lib.memset(plaintext_buffer, 0, max_plaintext_length)
+
+        return bytes_to_native_str(plaintext)