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des(3tcl) Data Encryption Standard (DES) des(3tcl) 





NAME


des - Implementation of the DES and triple-DES ciphers






SYNOPSIS


package require Tcl 8.5 9


package require des 1.2


::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



  

  
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.

  

  
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.

  

  
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.

  

  
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 Trackers
    [http://core.tcl.tk/tcllib/reportlist]. Please also report any ideas for
    enhancements you may have for either package and/or documentation.


When proposing code changes, please provide unified diffs,
    i.e the output of diff -u.


Note further that attachments are strongly preferred over
    inlined patches. Attachments can be made by going to the Edit form of
    the ticket immediately after its creation, and then using the left-most
    button in the secondary navigation bar.






SEE ALSO


aes(3tcl), blowfish(3tcl), md5(3tcl), rc4(3tcl), sha1(3tcl)






KEYWORDS


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






CATEGORY


Hashes, checksums, and encryption






COPYRIGHT


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








  

    1.2
    tcllib