pycryptodome/lib/Crypto/Cipher/_mode_siv.py

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"""
Synthetic Initialization Vector (SIV) mode.
"""

__all__ = ['SivMode']

from binascii import unhexlify, hexlify

from Crypto.Util.py3compat import *

from Crypto.Util import Counter
from Crypto.Util.number import long_to_bytes, bytes_to_long
from Crypto.Protocol.KDF import _S2V
from Crypto.Hash import BLAKE2s
from Crypto.Random import get_random_bytes


class SivMode(object):
    """Synthetic Initialization Vector (SIV).

    This is an Authenticated Encryption with Associated Data (`AEAD`_) mode.
    It provides both confidentiality and authenticity.

    The header of the message may be left in the clear, if needed, and it will
    still be subject to authentication. The decryption step tells the receiver
    if the message comes from a source that really knowns the secret key.
    Additionally, decryption detects if any part of the message - including the
    header - has been modified or corrupted.

    If the data being encrypted is completely unpredictable to an adversary
    (e.g. a secret key, for key wrapping purposes) a nonce is not strictly
    required.

    Otherwise, a *nonce* has to be provided.

    Unlike other AEAD modes such as CCM, EAX or GCM, accidental reuse of a
    nonce is not catastrophic for the confidentiality of the message. The only
    effect is that an attacker can tell when the same plaintext (and same
    associated data) is protected with the same key.

    The length of the MAC is fixed to the block size of the underlying cipher.
    The key size is twice the length of the key of the underlying cipher.

    This mode is only available for AES ciphers.

    +--------------------+---------------+-------------------+
    |      Cipher        | SIV MAC size  |   SIV key length  |
    |                    |    (bytes)    |     (bytes)       |
    +====================+===============+===================+
    |    AES-128         |      16       |        32         |
    +--------------------+---------------+-------------------+
    |    AES-192         |      16       |        48         |
    +--------------------+---------------+-------------------+
    |    AES-256         |      16       |        64         |
    +--------------------+---------------+-------------------+

    See `RFC5297`_ and the `original paper`__.

    .. _RFC5297: https://tools.ietf.org/html/rfc5297
    .. _AEAD: http://blog.cryptographyengineering.com/2012/05/how-to-choose-authenticated-encryption.html
    .. __: http://www.cs.ucdavis.edu/~rogaway/papers/keywrap.pdf
    """

    def __init__(self, factory, **kwargs):
        """Create a new block cipher, configured in
        Synthetic Initializaton Vector (SIV) mode.

        :Parameters:
          factory : object
            A symmetric cipher module from `Crypto.Cipher`
            (like `Crypto.Cipher.AES`).
        :Keywords:
          key : byte string
            The secret key to use in the symmetric cipher.
            It must be 32, 48 or 64 bytes long.
            If AES is the chosen cipher, the variants *AES-128*,
            *AES-192* and or *AES-256* will be used internally.
          nonce : byte string
            A mandatory value that must never be reused for any other encryption.
            There are no restrictions on its length,
            but it is recommended to use at least 16 bytes.

            The nonce shall never repeat for two different messages encrypted
            with the same key, but it does not need to be random.
        """

        self.block_size = factory.block_size
        self._factory = factory

        try:
            self._key = key = kwargs.pop("key")
        except KeyError, e:
            raise TypeError("Missing parameter: " + str(e))

        self._nonce = kwargs.pop("nonce", None)

        self._cipher_params = dict(kwargs)

        subkey_size = len(key) // 2
        if len(key) & 1:
            raise ValueError("MODE_SIV requires a key twice as long as"
                             " for the underlying cipher")

        self._mac_tag = None  # Cache for MAC tag
        self._kdf = _S2V(key[:subkey_size],
                         ciphermod=factory,
                         cipher_params=self._cipher_params)
        self._subkey_cipher = key[subkey_size:]

        # Allowed transitions after initialization
        self._next = [self.update, self.encrypt, self.decrypt,
                      self.digest, self.verify]

    def _create_ctr_cipher(self, mac_tag):
        """Create a new CTR cipher from the MAC in SIV mode"""

        tag_int = bytes_to_long(mac_tag)
        init_counter = tag_int ^ (tag_int & 0x8000000080000000L)
        ctr = Counter.new(self.block_size * 8,
                          initial_value=init_counter)

        return self._factory.new(
                    self._subkey_cipher,
                    self._factory.MODE_CTR,
                    counter=ctr,
                    **self._cipher_params)

    def update(self, assoc_data):
        """Protect associated data

        If there is any associated data, the caller has to invoke
        this function one or more times, before using
        ``decrypt`` or ``encrypt``.

        By *associated data* it is meant any data (e.g. packet headers) that
        will not be encrypted and will be transmitted in the clear.
        However, the receiver is still able to detect any modification to it.

        If there is no associated data, this method must not be called.

        The caller may split associated data in segments of any size, and
        invoke this method multiple times, each time with the next segment.

        :Parameters:
          assoc_data : byte string
            A piece of associated data. There are no restrictions on its size.
        """

        if self.update not in self._next:
            raise TypeError("update() can only be called"
                                " immediately after initialization")

        self._next = [self.update, self.encrypt, self.decrypt,
                      self.digest, self.verify]

        return self._kdf.update(assoc_data)

    def encrypt(self, plaintext):
        """Encrypt data with the key and the parameters set at initialization.

        A cipher object is stateful: once you have encrypted a message
        you cannot encrypt (or decrypt) another message using the same
        object.

        This method can be called only **once**.

        You cannot reuse an object for encrypting
        or decrypting other data with the same key.

        This function does not add any padding to the plaintext.

        :Parameters:
          plaintext : byte string
            The piece of data to encrypt.
            It can be of any length.
        :Return:
            the encrypted data, as a byte string.
            It is as long as *plaintext*.
        """

        if self.encrypt not in self._next:
            raise TypeError("encrypt() can only be called after"
                            " initialization or an update()")

        self._next = [self.digest]

        if self._nonce:
            self._kdf.update(self._nonce)
        self._kdf.update(plaintext)

        self._mac_tag = self._kdf.derive()
        cipher = self._create_ctr_cipher(self._mac_tag)

        return cipher.encrypt(plaintext)

    def decrypt(self, ciphertext):
        """Decrypt data with the key and the parameters set at initialization.

        For SIV, decryption and verification must take place at the same
        point. This method shall not be used.

        Use `decrypt_and_verify` instead.
        """

        raise TypeError("decrypt() not allowed for SIV mode."
                        " Use decrypt_and_verify() instead.")

    def digest(self):
        """Compute the *binary* MAC tag.

        The caller invokes this function at the very end.

        This method returns the MAC that shall be sent to the receiver,
        together with the ciphertext.

        :Return: the MAC, as a byte string.
        """

        if self.digest not in self._next:
            raise TypeError("digest() cannot be called when decrypting"
                            " or validating a message")
        self._next = [self.digest]
        if self._mac_tag is None:
            self._mac_tag = self._kdf.derive()
        return self._mac_tag

    def hexdigest(self):
        """Compute the *printable* MAC tag.

        This method is like `digest`.

        :Return: the MAC, as a hexadecimal string.
        """
        return "".join(["%02x" % bord(x) for x in self.digest()])

    def verify(self, received_mac_tag):
        """Validate the *binary* MAC tag.

        The caller invokes this function at the very end.

        This method checks if the decrypted message is indeed valid
        (that is, if the key is correct) and it has not been
        tampered with while in transit.

        :Parameters:
          received_mac_tag : byte string
            This is the *binary* MAC, as received from the sender.
        :Raises ValueError:
            if the MAC does not match. The message has been tampered with
            or the key is incorrect.
        """

        if self.verify not in self._next:
            raise TypeError("verify() cannot be called"
                            " when encrypting a message")
        self._next = [self.verify]

        if self._mac_tag is None:
            self._mac_tag = self._kdf.derive()

        secret = get_random_bytes(16)

        mac1 = BLAKE2s.new(digest_bits=160, key=secret, data=self._mac_tag)
        mac2 = BLAKE2s.new(digest_bits=160, key=secret, data=received_mac_tag)

        if mac1.digest() != mac2.digest():
            raise ValueError("MAC check failed")

    def hexverify(self, hex_mac_tag):
        """Validate the *printable* MAC tag.

        This method is like `verify`.

        :Parameters:
          hex_mac_tag : string
            This is the *printable* MAC, as received from the sender.
        :Raises ValueError:
            if the MAC does not match. The message has been tampered with
            or the key is incorrect.
        """

        self.verify(unhexlify(hex_mac_tag))

    def encrypt_and_digest(self, plaintext):
        """Perform encrypt() and digest() in one step.

        :Parameters:
          plaintext : byte string
            The piece of data to encrypt.
        :Return:
            a tuple with two byte strings:

            - the encrypted data
            - the MAC
        """

        return self.encrypt(plaintext), self.digest()

    def decrypt_and_verify(self, ciphertext, mac_tag):
        """Perform decryption and verification in one step.

        A cipher object is stateful: once you have decrypted a message
        you cannot decrypt (or encrypt) another message with the same
        object.

        You cannot reuse an object for encrypting
        or decrypting other data with the same key.

        This function does not remove any padding from the plaintext.

        :Parameters:
          ciphertext : byte string
            The piece of data to decrypt.
            It can be of any length.
          mac_tag : byte string
            This is the *binary* MAC, as received from the sender.

        :Return: the decrypted data (byte string).
        :Raises ValueError:
            if the MAC does not match. The message has been tampered with
            or the key is incorrect.
        """

        if self.decrypt not in self._next:
            raise TypeError("decrypt() can only be called"
                            " after initialization or an update()")
        self._next = [self.verify]

        # Take the MAC and start the cipher for decryption
        self._cipher = self._create_ctr_cipher(mac_tag)

        plaintext = self._cipher.decrypt(ciphertext)

        if self._nonce:
            self._kdf.update(self._nonce)
        if plaintext:
            self._kdf.update(plaintext)

        self.verify(mac_tag)
        return plaintext


def _create_siv_cipher(factory, **kwargs):
    return SivMode(factory, **kwargs)