1des(n)                  Data Encryption Standard (DES)                  des(n)
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5______________________________________________________________________________
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NAME

8       des - Implementation of the DES and triple-DES ciphers
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SYNOPSIS

11       package require Tcl  8.2
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13       package require des  1.0
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15       ::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 ]
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19       ::DES::Init mode keydata iv ?weak?
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21       ::DES::Encrypt Key data
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23       ::DES::Decrypt Key data
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25       ::DES::Reset Key iv
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27       ::DES::Final Key
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29_________________________________________________________________
30

DESCRIPTION

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
39       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

COMMANDS

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
55              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
60              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
66              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

PROGRAMMING INTERFACE

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
76       ::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
82              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
88       ::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.
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94       ::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
99       ::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.
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104       ::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

MODES OF OPERATION

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
117       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
127       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
134       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

EXAMPLES

141       % set ciphertext [DES::des -mode cbc -dir encrypt -key $secret $plaintext]
142       % set plaintext [DES::des -mode cbc -dir decrypt -key $secret $ciphertext]
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144
145       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
153

REFERENCES

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
159       [2]    "TclDES: munitions-grade  Tcl  scripting"  http://tcldes.source
160              forge.net/
161

AUTHORS

163       Jochen C Loewer, Mac Cody, Pat Thoyts
164

SEE ALSO

166       aes(n), blowfish(n), md5(n), rc4(n), sha1(n)
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KEYWORDS

169       3DES, DES, block cipher, data integrity, encryption, security
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172       Copyright (c) 2005, Pat Thoyts <patthoyts@users.sourceforge.net>
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177des                                   1.0                               des(n)
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