pycryptodome/lib/Crypto/Cipher/_mode_ctr.py

# -*- coding: utf-8 -*-
#
#  Cipher/mode_ctr.py : CTR mode
#
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# the extent that dedication to the public domain is not available,
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"""
Counter (CTR) mode.
"""

__all__ = ['CtrMode']

from Crypto.Util._raw_api import (load_pycryptodome_raw_lib, VoidPointer,
                                  create_string_buffer, get_raw_buffer,
                                  SmartPointer, c_size_t, expect_byte_string)

from Crypto.Util.py3compat import *

raw_ctr_lib = load_pycryptodome_raw_lib("Crypto.Cipher._raw_ctr", """
                    int CTR_start_operation(void *cipher,
                                            uint8_t   initialCounterBlock[],
                                            size_t    initialCounterBlock_len,
                                            size_t    prefix_len,
                                            unsigned  counter_len,
                                            unsigned  littleEndian,
                                            void **pResult);
                    int CTR_encrypt(void *ctrState,
                                    const uint8_t *in,
                                    uint8_t *out,
                                    size_t data_len);
                    int CTR_decrypt(void *ctrState,
                                    const uint8_t *in,
                                    uint8_t *out,
                                    size_t data_len);
                    int CTR_stop_operation(void *ctrState);"""
                                        )


class CtrMode(object):
    """*CounTeR (CTR)* mode.

    This mode is very similar to ECB, in that
    encryption of one block is done independently of all other blocks.

    Unlike ECB, the block *position* contributes to the encryption
    and no information leaks about symbol frequency.

    Each message block is associated to a *counter* which
    must be unique across all messages that get encrypted
    with the same key (not just within the same message).
    The counter is as big as the block size.

    Counters can be generated in several ways. The most
    straightword one is to choose an *initial counter block*
    (which can be made public, similarly to the *IV* for the
    other modes) and increment its lowest **m** bits by one
    (modulo *2^m*) for each block. In most cases, **m** is
    chosen to be half the block size.

    See `NIST SP800-38A`_, Section 6.5 (for the mode) and
    Appendix B (for how to manage the *initial counter block*).

    .. _`NIST SP800-38A` : http://csrc.nist.gov/publications/nistpubs/800-38a/sp800-38a.pdf
    """

    def __init__(self, block_cipher, initial_counter_block,
                 prefix_len, counter_len, little_endian):
        """Create a new block cipher, configured in CTR mode.

        :Parameters:
          block_cipher : C pointer
            A smart pointer to the low-level block cipher instance.

          initial_counter_block : byte string
            The initial plaintext to use to generate the key stream.

            It is as large as the cipher block, and it embeds
            the initial value of the counter.

            This value must not be reused.
            It shall contain a nonce or a random component.
            Reusing the *initial counter block* for encryptions
            performed with the same key compromises confidentiality.

          prefix_len : integer
            The amount of bytes at the beginning of the counter block
            that never change.

          counter_len : integer
            The length in bytes of the counter embedded in the counter
            block.

          little_endian : boolean
            True if the counter in the counter block is an integer encoded
            in little endian mode. If False, it is big endian.
        """

        expect_byte_string(initial_counter_block)
        self._state = VoidPointer()
        result = raw_ctr_lib.CTR_start_operation(block_cipher.get(),
                                                 initial_counter_block,
                                                 c_size_t(len(initial_counter_block)),
                                                 c_size_t(prefix_len),
                                                 counter_len,
                                                 little_endian,
                                                 self._state.address_of())
        if result:
            raise ValueError("Error %X while instatiating the CTR mode"
                             % result)

        # Ensure that object disposal of this Python object will (eventually)
        # free the memory allocated by the raw library for the cipher mode
        self._state = SmartPointer(self._state.get(),
                                   raw_ctr_lib.CTR_stop_operation)

        # Memory allocated for the underlying block cipher is now owed
        # by the cipher mode
        block_cipher.release()

        #: The block size of the underlying cipher, in bytes.
        self.block_size = len(initial_counter_block)

    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.

        The data to encrypt can be broken up in two or
        more pieces and `encrypt` can be called multiple times.

        That is, the statement:

            >>> c.encrypt(a) + c.encrypt(b)

        is equivalent to:

             >>> c.encrypt(a+b)

        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*.
        """

        expect_byte_string(plaintext)
        ciphertext = create_string_buffer(len(plaintext))
        result = raw_ctr_lib.CTR_encrypt(self._state.get(),
                                         plaintext,
                                         ciphertext,
                                         c_size_t(len(plaintext)))
        if result:
            raise ValueError("Error %X while encrypting in CTR mode" % result)
        return get_raw_buffer(ciphertext)

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

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

        The data to decrypt can be broken up in two or
        more pieces and `decrypt` can be called multiple times.

        That is, the statement:

            >>> c.decrypt(a) + c.decrypt(b)

        is equivalent to:

             >>> c.decrypt(a+b)

        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.

        :Return: the decrypted data (byte string).
        """

        expect_byte_string(ciphertext)
        plaintext = create_string_buffer(len(ciphertext))
        result = raw_ctr_lib.CTR_decrypt(self._state.get(),
                                         ciphertext,
                                         plaintext,
                                         c_size_t(len(ciphertext)))
        if result:
            raise ValueError("Error %X while decrypting in CTR mode" % result)
        return get_raw_buffer(plaintext)


def _create_ctr_cipher(factory, **kwargs):
    """Instantiate a cipher object that performs CTR encryption/decryption.

    :Parameters:
      factory : module
        The underlying block cipher, a module from ``Crypto.Cipher``.

    :Keywords:
      iv : byte string
        The IV to use for CBC.

      IV : byte string
        Alias for ``iv``.

      counter : object
        Instance of ``Crypto.Util.Counter``.

    Any other keyword will be passed to the underlying block cipher.
    See the relevant documentation for details (at least ``key`` will need
    to be present).
    """

    cipher_state = factory._create_base_cipher(kwargs)
    try:
        counter = kwargs.pop("counter")
    except KeyError:
        # Required by unit test
        raise TypeError("Missing 'counter' parameter for CTR mode")

    # 'counter' used to be a callable object, but now it is
    # just a dictionary for backward compatibility.
    counter_len = counter.pop("counter_len")
    prefix = counter.pop("prefix")
    suffix = counter.pop("suffix")
    initial_value = counter.pop("initial_value")
    little_endian = counter.pop("little_endian")

    # Compute initial counter block
    words = []
    while initial_value > 0:
        words.append(bchr(initial_value & 255))
        initial_value >>= 8
    words += [bchr(0)] * max(0, counter_len - len(words))
    if not little_endian:
        words.reverse()
    initial_counter_block = prefix + b("").join(words) + suffix

    if kwargs:
        raise ValueError("Unknown parameters for CTR mode: %s"
                         % str(kwargs))
    return CtrMode(cipher_state, initial_counter_block,
                      len(prefix), counter_len, little_endian)