des(n) Data Encryption Standard (DES) des(n)______________________________________________________________________________NAMEdes - 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 key‐
data ?-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) exten‐
sion 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 con‐
cerned 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 key‐
data ?-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 chan‐
nel.
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 pro‐
vided 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 sched‐
ule 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 encryp‐
tion errors will only affect a single block however this is vul‐
nerable 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 Stan‐
dards Publication 46-3, 1999, (http://csrc.nist.gov/publica‐
tions/fips/fips46-3/fips46-3.pdf)
[2] "TclDES: munitions-grade Tcl scripting" http://tcldes.source‐
forge.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 ALSOaes(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)
