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