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Filename : secret.py
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# Copyright 2013 Donald Stufft and individual contributors
#
# 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.

from __future__ import absolute_import, division, print_function

import nacl.bindings
from nacl import encoding
from nacl import exceptions as exc
from nacl.utils import EncryptedMessage, StringFixer, random


class SecretBox(encoding.Encodable, StringFixer, object):
    """
    The SecretBox class encrypts and decrypts messages using the given secret
    key.

    The ciphertexts generated by :class:`~nacl.secret.Secretbox` include a 16
    byte authenticator which is checked as part of the decryption. An invalid
    authenticator will cause the decrypt function to raise an exception. The
    authenticator is not a signature. Once you've decrypted the message you've
    demonstrated the ability to create arbitrary valid message, so messages you
    send are repudiable. For non-repudiable messages, sign them after
    encryption.

    :param key: The secret key used to encrypt and decrypt messages
    :param encoder: The encoder class used to decode the given key

    :cvar KEY_SIZE: The size that the key is required to be.
    :cvar NONCE_SIZE: The size that the nonce is required to be.
    :cvar MACBYTES: The size of the authentication MAC tag in bytes.
    :cvar MESSAGEBYTES_MAX: The maximum size of a message which can be
                            safely encrypted with a single key/nonce
                            pair.
    """

    KEY_SIZE = nacl.bindings.crypto_secretbox_KEYBYTES
    NONCE_SIZE = nacl.bindings.crypto_secretbox_NONCEBYTES
    MACBYTES = nacl.bindings.crypto_secretbox_MACBYTES
    MESSAGEBYTES_MAX = nacl.bindings.crypto_secretbox_MESSAGEBYTES_MAX

    def __init__(self, key, encoder=encoding.RawEncoder):
        key = encoder.decode(key)
        if not isinstance(key, bytes):
            raise exc.TypeError("SecretBox must be created from 32 bytes")

        if len(key) != self.KEY_SIZE:
            raise exc.ValueError(
                "The key must be exactly %s bytes long" %
                self.KEY_SIZE,
            )

        self._key = key

    def __bytes__(self):
        return self._key

    def encrypt(self, plaintext, nonce=None, encoder=encoding.RawEncoder):
        """
        Encrypts the plaintext message using the given `nonce` (or generates
        one randomly if omitted) and returns the ciphertext encoded with the
        encoder.

        .. warning:: It is **VITALLY** important that the nonce is a nonce,
            i.e. it is a number used only once for any given key. If you fail
            to do this, you compromise the privacy of the messages encrypted.
            Give your nonces a different prefix, or have one side use an odd
            counter and one an even counter. Just make sure they are different.

        :param plaintext: [:class:`bytes`] The plaintext message to encrypt
        :param nonce: [:class:`bytes`] The nonce to use in the encryption
        :param encoder: The encoder to use to encode the ciphertext
        :rtype: [:class:`nacl.utils.EncryptedMessage`]
        """
        if nonce is None:
            nonce = random(self.NONCE_SIZE)

        if len(nonce) != self.NONCE_SIZE:
            raise exc.ValueError(
                "The nonce must be exactly %s bytes long" % self.NONCE_SIZE,
            )

        ciphertext = nacl.bindings.crypto_secretbox(plaintext,
                                                    nonce, self._key)

        encoded_nonce = encoder.encode(nonce)
        encoded_ciphertext = encoder.encode(ciphertext)

        return EncryptedMessage._from_parts(
            encoded_nonce,
            encoded_ciphertext,
            encoder.encode(nonce + ciphertext),
        )

    def decrypt(self, ciphertext, nonce=None, encoder=encoding.RawEncoder):
        """
        Decrypts the ciphertext using the `nonce` (explicitly, when passed as a
        parameter or implicitly, when omitted, as part of the ciphertext) and
        returns the plaintext message.

        :param ciphertext: [:class:`bytes`] The encrypted message to decrypt
        :param nonce: [:class:`bytes`] The nonce used when encrypting the
            ciphertext
        :param encoder: The encoder used to decode the ciphertext.
        :rtype: [:class:`bytes`]
        """
        # Decode our ciphertext
        ciphertext = encoder.decode(ciphertext)

        if nonce is None:
            # If we were given the nonce and ciphertext combined, split them.
            nonce = ciphertext[:self.NONCE_SIZE]
            ciphertext = ciphertext[self.NONCE_SIZE:]

        if len(nonce) != self.NONCE_SIZE:
            raise exc.ValueError(
                "The nonce must be exactly %s bytes long" % self.NONCE_SIZE,
            )

        plaintext = nacl.bindings.crypto_secretbox_open(ciphertext,
                                                        nonce, self._key)

        return plaintext