1des(n) Data Encryption Standard (DES) des(n)
2______________________________________________________________________________
6
8 des - Implementation of the DES and triple-DES ciphers
9
11 package require Tcl 8.2
12 package require des 1.0
14 ::DES::des ?-mode [ecb|cbc|cfb|ofb]? ?-dir [encrypt|decrypt]? -key key‐
16 data ?-iv vector? ?-hex? ?-weak? ?-out channel? ?-chunksize size? [ -in
17 channel | data ]
18 ::DES::Init mode keydata iv ?weak?
20 ::DES::Encrypt Key data
22 ::DES::Decrypt Key data
24 ::DES::Reset Key iv
26 ::DES::Final Key
28_________________________________________________________________
30
32 This is an implementation in Tcl of the Data Encryption Standard (DES)
33 as published by the U.S. National Institute of Standards and Technology
34 (NIST) [1]. This implementation also supports triple DES (3DES) exten‐
35 sion to DES. DES is a 64-bit block cipher that uses a 56-bit key. 3DES
36 uses a 168-bit key. DES has now officially been superceeded by AES but
37 is in common use in many protocols.
38 The tcllib implementation of DES and 3DES uses an implementation by Mac
40 Cody and is available as a separate download from [2]. For anyone con‐
41 cerned about the details of exporting this code please see the TclDES
42 web pages. The tcllib specific code is a wrapper to the TclDES API that
43 presents same API for the DES cipher as for other ciphers in the
44 library.
45
47 ::DES::des ?-mode [ecb|cbc|cfb|ofb]? ?-dir [encrypt|decrypt]? -key key‐
48 data ?-iv vector? ?-hex? ?-weak? ?-out channel? ?-chunksize size? [ -in
49 channel | data ]
50 Perform the DES algorithm on either the data provided by the
51 argument or on the data read from the -in channel. If an -out
52 channel is given then the result will be written to this chan‐
53 nel.
54 The -key option must be given. This parameter takes a binary
56 string of 8 bytes in length and is used to generate the key
57 schedule. In DES only 56 bits of key data are used. The highest
58 bit from each byte is discarded.
59 The -mode and -dir options are optional and default to cbc mode
61 and encrypt respectively. The initialization vector -iv takes an
62 8 byte binary argument. This defaults to all zeros. See MODES OF
63 OPERATION for more about -mode and the use of the initialization
64 vector.
65 DES is a 64-bit block cipher. This means that the data must be
67 provided in units that are a multiple of 8 bytes.
68
70 Internal state is maintained in an opaque structure that is returned
71 from the Init function. In ECB mode the state is not affected by the
72 input but for other modes some input dependent state is maintained and
73 may be reset by calling the Reset function with a new initialization
74 vector value.
75 ::DES::Init mode keydata iv ?weak?
77 Construct a new DES key schedule using the specified key data
78 and the given initialization vector. The initialization vector
79 is not used with ECB mode but is important for other usage
80 modes. See MODES OF OPERATION.
81 There are a small number of keys that are known to be weak when
83 used with DES. By default if such a key is passed in then an
84 error will be raised. If there is a need to accept such keys
85 then the weak parameter can be set true to avoid the error being
86 thrown.
87 ::DES::Encrypt Key data
89 Use a prepared key acquired by calling Init to encrypt the pro‐
90 vided data. The data argument should be a binary array that is a
91 multiple of the DES block size of 8 bytes. The result is a
92 binary array the same size as the input of encrypted data.
93 ::DES::Decrypt Key data
95 Decipher data using the key. Note that the same key may be used
96 to encrypt and decrypt data provided that the initialization
97 vector is reset appropriately for CBC mode.
98 ::DES::Reset Key iv
100 Reset the initialization vector. This permits the programmer to
101 re-use a key and avoid the cost of re-generating the key sched‐
102 ule where the same key data is being used multiple times.
103 ::DES::Final Key
105 This should be called to clean up resources associated with Key.
106 Once this function has been called the key may not be used
107 again.
108
110 Electronic Code Book (ECB)
111 ECB is the basic mode of all block ciphers. Each block is
112 encrypted independently and so identical plain text will produce
113 identical output when encrypted with the same key. Any encryp‐
114 tion errors will only affect a single block however this is vul‐
115 nerable to known plaintext attacks.
116 Cipher Block Chaining (CBC)
118 CBC mode uses the output of the last block encryption to affect
119 the current block. An initialization vector of the same size as
120 the cipher block size is used to handle the first block. The
121 initialization vector should be chosen randomly and transmitted
122 as the first block of the output. Errors in encryption affect
123 the current block and the next block after which the cipher will
124 correct itself. CBC is the most commonly used mode in software
125 encryption.
126 Cipher Feedback (CFB)
128 CFB mode can be used to convert block ciphers into stream
129 ciphers. In CFB mode the initialization vector is encrypted and
130 the output is then xor'd with the plaintext stream. The result
131 is then used as the initialization vector for the next round.
132 Errors will affect the current block and the next block.
133 Output Feedback (OFB)
135 OFB is similar to CFB except that the output of the cipher is
136 fed back into the next round and not the xor'd plain text. This
137 means that errors only affect a single block but the cipher is
138 more vulnerable to attack.
139
141 % set ciphertext [DES::des -mode cbc -dir encrypt -key $secret $plaintext]
142 % set plaintext [DES::des -mode cbc -dir decrypt -key $secret $ciphertext]
143 set iv [string repeat \\0 8]
146 set Key [DES::Init cbc \\0\\1\\2\\3\\4\\5\\6\\7 $iv]
147 set ciphertext [DES::Encrypt $Key "somedata"]
148 append ciphertext [DES::Encrypt $Key "moredata"]
149 DES::Reset $Key $iv
150 set plaintext [DES::Decrypt $Key $ciphertext]
151 DES::Final $Key
152
155 [1] "Data Encryption Standard", Federal Information Processing Stan‐
156 dards Publication 46-3, 1999, (http://csrc.nist.gov/publica‐
157 tions/fips/fips46-3/fips46-3.pdf)
158 [2] "TclDES: munitions-grade Tcl scripting" http://tcldes.source‐
160 forge.net/
161
163 Jochen C Loewer, Mac Cody, Pat Thoyts
164
166 aes(n), blowfish(n), md5(n), rc4(n), sha1(n)
167
169 3DES, DES, block cipher, data integrity, encryption, security
170
172 Copyright (c) 2005, Pat Thoyts <patthoyts@users.sourceforge.net>
173des 1.0 des(n)