des - Implementation of the DES and triple-DES ciphers
package require Tcl 8.2
package require des 1.1
? -key keydata
? ?-out channel
? ?-chunksize size
? [ -in
::DES::Init mode keydata iv
::DES::Encrypt Key data
::DES::Decrypt Key data
::DES::Reset Key iv
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)
. 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
The tcllib implementation of DES and 3DES uses an implementation by Mac Cody and
is available as a separate download from . 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.
- ::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
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
DES is a 64-bit block cipher. This means that the data must be provided in
units that are a multiple of 8 bytes.
Internal state is maintained in an opaque structure that is returned from the
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
- 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
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
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
- 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
% 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]
- "Data Encryption Standard", Federal Information
Processing Standards Publication 46-3, 1999, (
- "TclDES: munitions-grade Tcl scripting"
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
report any ideas for enhancements you may have for either package and/or
aes(3tcl), blowfish(3tcl), md5(3tcl), rc4(3tcl), sha1(3tcl)
3DES, DES, block cipher, data integrity, encryption, security
Hashes, checksums, and encryption
Copyright (c) 2005, Pat Thoyts <firstname.lastname@example.org>