Module Expr Str Assign NameStore Str ImportFrom alias ImportFrom alias ClassDef bases AttributeLoad NameLoad attr body Expr Str FunctionDef arguments args NameParam NameParam defaults vararg kwarg body Expr Str Expr Call AttributeLoad AttributeLoad NameLoad attr attr NameLoad NameLoad NameLoad NameLoad NameLoad decorator_list decorator_list FunctionDef arguments args NameParam defaults vararg kwarg body Expr Str Return Call NameLoad NameLoad NameLoad NameLoad decorator_list Assign NameStore Num Assign NameStore Num Assign NameStore Num Assign NameStore Num Assign NameStore Num Assign NameStore Num Assign NameStore Num Assign NameStore Num Assign NameStore TupleLoad Num Num
N/A N/A Triple DES symmetric cipher

`Triple DES`__ (or TDES or TDEA or 3DES) is a symmetric block cipher standardized by NIST_.
It has a fixed data block size of 8 bytes. Its keys are 128 (*Option 1*) or 192
bits (*Option 2*) long.
However, 1 out of 8 bits is used for redundancy and do not contribute to
security. The effective key length is respectively 112 or 168 bits.

TDES consists of the concatenation of 3 simple `DES` ciphers.

The plaintext is first DES encrypted with *K1*, then decrypted with *K2*,
and finally encrypted again with *K3*.  The ciphertext is decrypted in the reverse manner.

The 192 bit key is a bundle of three 64 bit independent subkeys: *K1*, *K2*, and *K3*.

The 128 bit key is split into *K1* and *K2*, whereas *K1=K3*.

It is important that all subkeys are different, otherwise TDES would degrade to
single `DES`.

TDES is cryptographically secure, even though it is neither as secure nor as fast
as `AES`.

As an example, encryption can be done as follows:

    >>> from Crypto.Cipher import DES
    >>> from Crypto import Random
    >>> from Crypto.Util import Counter
    >>>
    >>> key = b'-8B key-'
    >>> nonce = Random.new().read(DES.block_size/2)
    >>> ctr = Counter.new(DES.block_size*8/2, prefix=nonce)
    >>> cipher = DES.new(key, DES.MODE_CTR, counter=ctr)
    >>> plaintext = b'We are no longer the knights who say ni!'
    >>> msg = nonce + cipher.encrypt(plaintext)

.. __: http://en.wikipedia.org/wiki/Triple_DES
.. _NIST: http://csrc.nist.gov/publications/nistpubs/800-67/SP800-67.pdf

:undocumented: __revision__, __package__
 N/A __revision__ $Id$ Crypto.Cipher blockalgo Crypto.Cipher _DES3 DES3Cipher N/A N/A blockalgo BlockAlgo N/A N/A TDES cipher object __init__ N/A N/A self key N/A args kwargs N/A N/A Initialize a TDES cipher object
        
        See also `new()` at the module level. N/A N/A N/A N/A blockalgo BlockAlgo __init__ self _DES3 key args kwargs N/A N/A new N/A N/A key N/A args kwargs N/A N/A Create a new TDES cipher

    :Parameters:
      key : byte string
        The secret key to use in the symmetric cipher.
        It must be 16 or 24 bytes long. The parity bits will be ignored.
    :Keywords:
      mode : a *MODE_** constant
        The chaining mode to use for encryption or decryption.
        Default is `MODE_ECB`.
      IV : byte string
        The initialization vector to use for encryption or decryption.
        
        It is ignored for `MODE_ECB` and `MODE_CTR`.

        For `MODE_OPENPGP`, IV must be `block_size` bytes long for encryption
        and `block_size` +2 bytes for decryption (in the latter case, it is
        actually the *encrypted* IV which was prefixed to the ciphertext).
        It is mandatory.
       
        For all other modes, it must be `block_size` bytes longs. It is optional and
        when not present it will be given a default value of all zeroes.
      counter : callable
        (*Only* `MODE_CTR`). A stateful function that returns the next
        *counter block*, which is a byte string of `block_size` bytes.
        For better performance, use `Crypto.Util.Counter`.
      segment_size : integer
        (*Only* `MODE_CFB`).The number of bits the plaintext and ciphertext
        are segmented in.
        It must be a multiple of 8. If 0 or not specified, it will be assumed to be 8.

    :Attention: it is important that all 8 byte subkeys are different,
      otherwise TDES would degrade to single `DES`.
    :Return: an `DES3Cipher` object
     N/A N/A DES3Cipher key args kwargs N/A N/A MODE_ECB 1 N/A MODE_CBC 2 N/A MODE_CFB 3 N/A MODE_PGP 4 N/A MODE_OFB 5 N/A MODE_CTR 6 N/A MODE_OPENPGP 7 N/A block_size 8 N/A key_size N/A 16 24