des.1

DES( Data Encryption Standard)算法

.ds ]T 15 Nov 90
.ds CP \s-2\(co\f3 Copyright 1990 by David G. Koontz\f1\s+2
.de}F
.ev1
.}E
.if\n()s 'sp |\n(.pu-1v-1p
.if\n()t 'sp |\n(.pu-3v
.ifn 'sp |\n(.pu-4v
.ifn .tl Page %\*(]T
.if\n()s .tl - % -
.if\n()t \{.if o .tl Page %\*(CP\*(]T
.ife .tl \*(]T\*(CPPage % \}
'bp
.ev
..
.de}C
..
.po 1.0i
.nrIN 0.5i
.nr)S 12
.TH \f3DES 1 "" "\s+1\f6/dev/ktz\f1\s-1"
.SH \f3NAME
des \- encrypt/decrypt using verified NBS DES algorithm
.SH \f3SYNOPSIS
.B des
-e \|\(bv \|-d [-k key \|\(bv \|-K hex_key] [-p pad_character], or
.br
.B des
-t [-v]
.SH \f3DESCRIPTION
.I Des\^
encrypts or decrypts according to the
.B -e 
(encrypt) and 
.B -d 
(decrypt) flags reading from standard input and writing to standard output.
The
.B -k
flag passes a string of 8 characters to be used as key.  The
.B -K
flag passes a 16 character hexidecimal key.  The DES algorithm
produces a 64 bit cipher product using a 56 bit key and 64 bit plaintext value.
.PP
If no key is passed 
.I des\^
uses the routine getpass("key: ") to 
demand an 8 character key from standard input.  All key inputs must be of
the correct length or
.I des\^ 
will  terminate.
.PP
Character string keys (
.B -k 
and
.B getpass()
) have each byte shifted left one bit, to leave bit[0] undefined.  Bit[0] is
used as a parity bit, and is otherwise uninvolved in DES keys.  Shifting
character string keys provides retention of all ascii defined bits and
results in a 56 bit key.  Hex key strings, such as from the 
.B -K
option or from the test vector, are not shifted.  It is the hex key provider's
responsibility to properly position 7 bits of key into the upper 7 bits of each
hex byte.
.PP
The 
.B -p
flag passes a hexidecimal value of the range 0x0 - 0xFF to be used in place
of the default pad character (0x20).
The pad character is used to fill out the last 8 byte block if the input
stream does not supply muliples of 8 bytes.
.SH \f3TESTMODE
The 
.B -t
and 
.B -v
flags are used to toggle 
.I test\^
and 
.I verbose\^
modes.  Testmode switches input format to test vectors used in DES algorithm
verification.  Each vector includes a 16 hexdigit key, a 16 hexdigit data 
value and a 16 hexdigit result value.  All output during testmode operation is 
made to standard error output.  A summary of tests is output irregardless of
the 
.I -v\^ 
flag.  Verbose mode outputs the test source and error locations.
.SH \f3TEST VECTOR FORMAT
 An input line must contain less than 127 characters.  A line containing
a leading space is considered a vector.  A line without a leading
space is a comment.  The 
.I des\^
mode is set for vectors following a comment line that contains either
.B encrypt
or
.B decrypt
as the first 7 characters.
A line containing a vector is comprised
of a leading space and three 16 hexdigit values.
.PP
A trailing comment line in the vector file forces output of the error
total.  
.SH \f3NORMAL OPERATION
.I Des\^
encrypts and decrypts with the same key:
.PP
.RS
des -e -k key < clear >cypher
.br
des -d -k key < cypher
.RE
.PP
will output a stream identical to that contained in clear, with the exeception
of the possibility of up to 7 pad characters being appended to the decoded
stream. 
.PP
Files encrypted by
.I des\^
are compatible with the National Bureau of Standards Digital Encryption
Standard in an electronic code book (ECB) implementation.
.PP
.I Des\^
utilizes the libcrypt library used by the passwd and makekey programs.
.I Des\^ 
is slower than the 
.B crypt
command it is intended to replace, a trade off for
higher security.
.I Des\^ 
also has the capabilty of decrypting concatenated files, if
all used the same key schedule and key.
.bp
.SH \f3EXAMPLE
.IP
des -e -k abcdefgh < foo.c > fum
.PP
will use the string "abcdefgh" as key
to encrypt the contents of "foo.c", and place the encrypted
output in file "fum".  File "fum" at this point will be unreadable.
.SM \f3NOTE:\f1 
that the original file, "foo.c", remains in readable form.
To obtain readable print-out of the file "fum", it
could be decoded as follows:
.IP
des -d < fum
.PP
After the response:
.IP
key:
.PP
the user types in "abcdefgh".
.SH \f3FILES
.ta \w'des.test\ \ \ \ 'u
des.test	test vector file, derived from \f2NBS Special Pub 500-20
.br
.DT
.SH \f3SEE ALSO
crypt(1),
makekey(1).
.SH \f3BUGS
Encryption of large files is time consuming.  Some implementations of 
libcrypt do not allow the
.I edflag\^
in the routine encrypt(block,edflag) to be set to decode (TRUE).
This is done in compliance with export restrictions on cryptographic
systems.  This may be defeated by clever programming in some cases.
.PP
.I Des\^
also uses padding of the input to assemble the last block of data.  This can
result in up to 7 characters added to the decoded data.  If having the pad
character selectable is not flexible enough, a pipeline filter can be written
to protect and stuff a pad value in the plaintext domain, and strip pad
characters from the decoded output.
.br
See 
.I crypt\^ 
for additional bugs.