cipher.1

des算法实现源码

.\" @(#) cipher.1 1.0  @(#)cipher.1	1.1  91/20/91  1   
.TH CIPHER 1 
.ds )H David A. Barrett
.ds ]W Revision 1.0: Jan 1991
.SH NAME
cipher, decipher \- Fast DES-compatible encryption
.SH SYNOPSIS
.B cipher
[
.B \-de
] [ 
.B \-i
.I ivec
] [
.B \-x
.I hexkey
] [
.B \-k
.I keystr
]
.SH DESCRIPTION
.PP
.I Cipher\^
is an encryption program compatible with the National Bureau of
Standards 
.I Data Encryption Standard,
published in 
.I FIPS Publication 46-1 
(January 22, 1988).
.PP
.ne 8
.I Cipher
reads from the standard input and writes
on the standard output.
The
.I keystr\^
is a character string which
selects a particular transformation.
The output will be unintelligible to anyone who does
not specify the same 
.I keystr\^
to the
.I decipher
program.
Output will contain the same number of characters and the input but
consists of binary characters of eight-bits each.
If no key
is given (using the,
.B \-k
or 
.B \-x
option),
.I cipher\^
demands a key from the terminal and turns
off printing while the key is typed in.
This is recommended since
users running the
.I ps(1)
can see keys specified on the command line.
To assist in preventing typographic errors during key
entry, each key must be entered twice.  This feature may
be disabled by specifying the
.B \-n 
option.

.SS Options
.PP
.TP 11
.B  \-d
Used to decipher previously ciphered text.
.TP
.B  \-e
Encipher mode.  More than one
.B \-e
or
.B \-d
option forces multiple encryption.
.TP
.B \-n
inhibits key verification
.TP
.BI \-k\ keystr
Specifies a character string to use as the key for encryption/decryption.
.TP
.BI \-x\ hexkey
Specifies the key as a hexadecimal number.  One to 16 digits may be
specified.  This key is left-justified and zero-padded.
.TP
.BI \-i\ ivec
Specifies the initialization vector for cipher-feedback mode (CFB) 
as a one to 16-digit hexadecimal number.  This will be right-justified 
and zero padded.
.PP
The
.B \-x,
.B \-i,
and
.B \-k
options are primarily useful for test scripts.
.PP
Unlike the crypt command,
.I cipher
is not its own inverse; you must use the
.B \-d
option to decipher previously encrypted text or invoke the command as
.I decipher.
.PP
The security of the Data Encryption Standard has been controversial.
In particular, the number of possible keys has
been criticized as too small; only 56-bits are used.  To assist
in minimizing this risk, multiple-encryption, and "random" 
key-generation from text strings is provided.

The key string may be any length other than zero.
This string is hashed to produce the 56-bit key used for the DES.

To help thwart dictionary attacks, the key should be as long as 
convenient and contain a mix of upper-case, lower-case and 
symbolic characters (space and control characters are permitted).
Short sentences or phrases consisting of nonsense words are recommended.

These precautions should make it more difficult for someone
to use
computer(s) (or DES hardware) to guess your key by trial and
error.  This is a very serious threat and should not be ignored.
.PP
For particularly sensitive information, double
or triple encryption with independent keys should be used.
Attempts to crack the encrypted text may be made more difficult by
removing redundancy from the input by using the
.I compress(1)
command prior to enciphering and subsequent to deciphering.
(Even further security can be obtained by specifying the
.B \-n
option to 
.I compress 
so that no header is included.)
.ne 8
.PP
The following command illustrates encryption of the file
.I clear
to produce the file
.I cypher.
.PP
.RS
compress -n <clear | cipher \|>cypher
.br
Input Key:
.br
Verify Key:
.br
rm clear
.br
.RE
.PP
.PP
This command is used to examine the previously enciphered file:
.PP
.RS
cipher -d <cypher | uncompress -n | more
.br
Input Key:
.br
.RE
.PP
The following command may be used to accomplish multiple-encipherment:
(The key text is shown only for illustration)
.PP
.RS
cipher -e -e <plain >cipherfile
.br
Input Key:firsty keyzy
.br
Input Key:seconDzy keysy
.br
.RE
.PP
A prompt for each cipher key is shown.  When multiple-encipherment is
selected, care should be taken to specify the keys in the reverse order
when deciphering. All the following commands below are equivalent ways
of deciphering the above:
.PP
.RS
cipher -d -d <cipherfile >plain
.br
Input Key:seconDzy keysy
.br
Input Key:firsty keyzy
.br
cipher -dk "seconDzy keysy" -dk "firsty keyzy" <cipherfile >plain
.br
cipher -d -k "seconDzy keysy" <cipherfile \\
.br
| decipher -k "firsty keyzy" >plain
.RE
.PP
For transmitting encrypted text though the mail system, the
.I atob(1),
and
.I btoa(1),
commands may be used.
These programs are available as part of the
.I compress
4.0 package. (also included with the 
.I cipher 
package)
.PP
For example, to send a secret message, typed from the terminal:
.PP
.RS
compress -n | cipher | btoa | mail user
.RE
.PP
The receiver could save the message in the file
.I secret
and read it using the command:
.PP
.RS
atob <secret | decipher | uncompress -n
.RE
.PP
Notice that you did not have to worry about stripping mailer headers 
from the text because the
.I atob
program does that automatically.
.PP
Since the Data Encryption Standard is, by definition, implemented 
only in hardware, this algorithm should not be considered as a DES
implementation. However, it should yield the same results as
DES hardware using 64-bit Cipher-Feedback Mode (CFB). (See
FIPS Publications 81 and 74).
.PP
The key used as input to the DES is generated as follows:
The keystring is padded with blank characters to a positive multiple of 
eight.
A secondary key is generated from the first eight characters of
.I keystr
by shifting each character left by one (discarding each high-order bit). 
This secondary key is used to produce a 64-bit authentication code by
enciphering the rest of
.I keystr
using DES in cipher-block-chaining (CBC) encryption mode with
an initialization vector of zeros.
This 64-bit authentication code is used as the encryption key.
This is the same method used by FIPS publication 112 except the 
authentication code is not converted into printable form.
This is also identical to FIPS publication 113 with the input padded with
blanks instead of zeros.

The initialization vector is generated from a string of all zeros.
.PP
.SH WARNINGS
If output is piped to
.IR nroff (1)
and the encryption key is
.I not\^
given on the command line,
.I cipher\^
can leave terminal modes with echoing disabled.
(see
.IR stty (1)).
.PP
If two or more files encrypted with the same key are concatenated
and an attempt is made to decrypt the result, only the
contents of the first of the original files will be decrypted correctly.
.PP
This code is not compatible with Sun's des program.  Unlike Sun's this 
code preserves the original file size and is also suitable for use in 
communications protocols where one byte of output occurs as each byte is 
input.  As far as I know, this is the only fast implementation of DES in 
cipher-feedback mode which has the above two properties.
.SH ACKNOWLEDGEMENTS
The author has made every attempt to produce the fastest, portable
DES-compatible program possible, but considerable speed improvements 
remain.  Comments and improvements
will be greatly appreciated and should be directed to the author:
.PP
David A. Barrett ([e-mail])
.PP
Copyright 1994 by David. A. Barrett
.PP
This program is not to be distributed for profit or included in such
software without written permission from the author (this means you, TVAnswer).
No permission is required for non-profit use.
.SH FILES
.ta 1i
/dev/tty	for typed key
.DT
.SH SEE ALSO
crypt(1),
compress(1),
btoa(1),
atob(1),
makekey(1), 
des(3),
desCFB(3),
crypt(3),
stty(1).
.\" index	\fIcrypt\fR \- encode/decode files 	 \s-2CRYPT\s+1(1)\s+1
.\" index	decode/encode files 	 \s-2CIPHER\s+1(1)\s+1
.\" index	encode/decode files 	 \s-2CIPHER\s+1(1)\s+1
.\" index	decrypt/encrypt files 	 \s-2CIPHER\s+1(1)\s+1
.\" index	encrypt/decrypt files 	 \s-2CIPHER\s+1(1)\s+1
.\" index	files: encrypt/decrypt files 	 \s-2CIPHER\s+1(1)\s+1
.\"
.\" toc	\s-2CIPHER\s+1(1)\s+1:\0\0\fIcipher\fR 	 encode/decode files