des(n)                  Data Encryption Standard (DES)                  des(n)



______________________________________________________________________________

NAME
       des - Implementation of the DES and triple-DES ciphers

SYNOPSIS
       package require Tcl  8.2

       package require des  1.1

       ::DES::des ?-mode [ecb|cbc|cfb|ofb]? ?-dir [encrypt|decrypt]? -key
       keydata ?-iv vector? ?-hex? ?-weak? ?-out channel? ?-chunksize size? [
       -in channel | data ]

       ::DES::Init mode keydata iv ?weak?

       ::DES::Encrypt Key data

       ::DES::Decrypt Key data

       ::DES::Reset Key iv

       ::DES::Final Key

_________________________________________________________________

DESCRIPTION
       This is an implementation in Tcl of the Data Encryption Standard (DES)
       as published by the U.S. National Institute of Standards and Technology
       (NIST) [1]. This implementation also supports triple DES (3DES)
       extension to DES. DES is a 64-bit block cipher that uses a 56-bit key.
       3DES uses a 168-bit key. DES has now officially been superceeded by AES
       but is in common use in many protocols.

       The tcllib implementation of DES and 3DES uses an implementation by Mac
       Cody and is available as a separate download from [2]. For anyone
       concerned about the details of exporting this code please see the
       TclDES web pages. The tcllib specific code is a wrapper to the TclDES
       API that presents same API for the DES cipher as for other ciphers in
       the library.

COMMANDS
       ::DES::des ?-mode [ecb|cbc|cfb|ofb]? ?-dir [encrypt|decrypt]? -key
       keydata ?-iv vector? ?-hex? ?-weak? ?-out channel? ?-chunksize size? [
       -in channel | data ]
              Perform the DES algorithm on either the data provided by the
              argument or on the data read from the -in channel. If an -out
              channel is given then the result will be written to this
              channel.

              The -key option must be given. This parameter takes a binary
              string of 8 bytes in length and is used to generate the key
              schedule.  In DES only 56 bits of key data are used. The highest
              bit from each byte is discarded.

              The -mode and -dir options are optional and default to cbc mode
              and encrypt respectively. The initialization vector -iv takes an
              8 byte binary argument. This defaults to all zeros. See MODES OF
              OPERATION for more about -mode and the use of the initialization
              vector.

              DES is a 64-bit block cipher. This means that the data must be
              provided in units that are a multiple of 8 bytes.

PROGRAMMING INTERFACE
       Internal state is maintained in an opaque structure that is returned
       from the Init function. In ECB mode the state is not affected by the
       input but for other modes some input dependent state is maintained and
       may be reset by calling the Reset function with a new initialization
       vector value.

       ::DES::Init mode keydata iv ?weak?
              Construct a new DES key schedule using the specified key data
              and the given initialization vector. The initialization vector
              is not used with ECB mode but is important for other usage
              modes.  See MODES OF OPERATION.

              There are a small number of keys that are known to be weak when
              used with DES. By default if such a key is passed in then an
              error will be raised. If there is a need to accept such keys
              then the weak parameter can be set true to avoid the error being
              thrown.

       ::DES::Encrypt Key data
              Use a prepared key acquired by calling Init to encrypt the
              provided data. The data argument should be a binary array that
              is a multiple of the DES block size of 8 bytes. The result is a
              binary array the same size as the input of encrypted data.

       ::DES::Decrypt Key data
              Decipher data using the key. Note that the same key may be used
              to encrypt and decrypt data provided that the initialization
              vector is reset appropriately for CBC mode.

       ::DES::Reset Key iv
              Reset the initialization vector. This permits the programmer to
              re-use a key and avoid the cost of re-generating the key
              schedule where the same key data is being used multiple times.

       ::DES::Final Key
              This should be called to clean up resources associated with Key.
              Once this function has been called the key may not be used
              again.

MODES OF OPERATION
       Electronic Code Book (ECB)
              ECB is the basic mode of all block ciphers. Each block is
              encrypted independently and so identical plain text will produce
              identical output when encrypted with the same key. Any
              encryption errors will only affect a single block however this
              is vulnerable to known plaintext attacks.

       Cipher Block Chaining (CBC)
              CBC mode uses the output of the last block encryption to affect
              the current block. An initialization vector of the same size as
              the cipher block size is used to handle the first block. The
              initialization vector should be chosen randomly and transmitted
              as the first block of the output. Errors in encryption affect
              the current block and the next block after which the cipher will
              correct itself. CBC is the most commonly used mode in software
              encryption.

       Cipher Feedback (CFB)
              CFB mode can be used to convert block ciphers into stream
              ciphers. In CFB mode the initialization vector is encrypted and
              the output is then xor'd with the plaintext stream. The result
              is then used as the initialization vector for the next round.
              Errors will affect the current block and the next block.

       Output Feedback (OFB)
              OFB is similar to CFB except that the output of the cipher is
              fed back into the next round and not the xor'd plain text. This
              means that errors only affect a single block but the cipher is
              more vulnerable to attack.

EXAMPLES
              % set ciphertext [DES::des -mode cbc -dir encrypt -key $secret $plaintext]
              % set plaintext [DES::des -mode cbc -dir decrypt -key $secret $ciphertext]


              set iv [string repeat \\0 8]
              set Key [DES::Init cbc \\0\\1\\2\\3\\4\\5\\6\\7 $iv]
              set ciphertext [DES::Encrypt $Key "somedata"]
              append ciphertext [DES::Encrypt $Key "moredata"]
              DES::Reset $Key $iv
              set plaintext [DES::Decrypt $Key $ciphertext]
              DES::Final $Key


REFERENCES
       [1]    "Data Encryption Standard", Federal Information Processing
              Standards Publication 46-3, 1999,
              (http://csrc.nist.gov/publications/fips/fips46-3/fips46-3.pdf)

       [2]    "TclDES: munitions-grade Tcl scripting"
              http://tcldes.sourceforge.net/

AUTHORS
       Jochen C Loewer, Mac Cody, Pat Thoyts

BUGS, IDEAS, FEEDBACK
       This document, and the package it describes, will undoubtedly contain
       bugs and other problems.  Please report such in the category des of the
       Tcllib SF Trackers [http://sourceforge.net/tracker/?group_id=12883].
       Please also report any ideas for enhancements you may have for either
       package and/or documentation.

SEE ALSO
       aes(n), blowfish(n), md5(n), rc4(n), sha1(n)

KEYWORDS
       3DES, DES, block cipher, data integrity, encryption, security

CATEGORY
       Hashes, checksums, and encryption

COPYRIGHT
       Copyright (c) 2005, Pat Thoyts <patthoyts@users.sourceforge.net>




des                                   1.1                               des(n)
