secret.py

from __future__ import absolute_import
from __future__ import division

from . import six

from . import nacl, encoding
from .exceptions import CryptoError


class SecretBox(encoding.Encodable, six.StringFixer, object):
    """
    The SecretBox class encrypted 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.
    """

    KEY_SIZE = nacl.lib.crypto_secretbox_KEYBYTES
    NONCE_SIZE = nacl.lib.crypto_secretbox_NONCEBYTES

    def __init__(self, key, encoder=encoding.RawEncoder):
        key = encoder.decode(key)

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

        self._key = key

    def __bytes__(self):
        return self._key

    def encrypt(self, plaintext, nonce, encoder=encoding.RawEncoder):
        """
        Encrypts the plaintext message using the given nonce and returns the
        ciphertext encoded with the encoder.

        .. warning:: It is **VITALLY** important that the nonce is a nonce,
            i.e. itis 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:`bytes`]
        """
        if len(nonce) != self.NONCE_SIZE:
            raise ValueError("The nonce must be exactly %s bytes long" %
                                nacl.lib.crypto_secretbox_NONCEBYTES)

        padded = b"\x00" * nacl.lib.crypto_secretbox_ZEROBYTES + plaintext
        ciphertext = nacl.ffi.new("unsigned char[]", len(padded))

        if not nacl.lib.crypto_secretbox(
                    ciphertext, padded, len(padded), nonce, self._key,
                ):
            raise CryptoError("Encryption failed")

        box_zeros = nacl.lib.crypto_secretbox_BOXZEROBYTES
        ciphertext = nacl.ffi.buffer(ciphertext, len(padded))[box_zeros:]

        return encoder.encode(ciphertext)

    def decrypt(self, ciphertext, nonce, encoder=encoding.RawEncoder):
        """
        Decrypts the ciphertext using the given nonce 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`]
        """
        if len(nonce) != self.NONCE_SIZE:
            raise ValueError("The nonce must be exactly %s bytes long" %
                                nacl.lib.crypto_secretbox_NONCEBYTES)

        ciphertext = encoder.decode(ciphertext)

        padded = b"\x00" * nacl.lib.crypto_secretbox_BOXZEROBYTES + ciphertext
        plaintext = nacl.ffi.new("unsigned char[]", len(padded))

        if not nacl.lib.crypto_secretbox_open(
                    plaintext, padded, len(padded), nonce, self._key,
                ):
            raise CryptoError(
                        "Decryption failed. Ciphertext failed verification")

        box_zeros = nacl.lib.crypto_secretbox_ZEROBYTES
        plaintext = nacl.ffi.buffer(plaintext, len(padded))[box_zeros:]

        return plaintext