cdes.1

des-bish

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.\" (c) Copyright 1987 by Matt Bishop and the Universities Space Research
.\"			 Association.
.\" This work was funded by grant NCC 2-398 from the National Aeronautics
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.\" Author's Address:
.\"		Matt Bishop
.\"		Research Institute for Advanced Computer Science
.\"		NASA Ames Research Center
.\"		Moffett Field, CA  94035
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.TH CDES 1 "Dartmouth College"
.SH NAME
cdes \- encrypt/decrypt using the Data Encryption Standard
.SH SYNOPSIS
.BR cdes " ["
.IR options " ]  ["
.IR password " ]"
.SH DESCRIPTION
.I Cdes
reads from the standard input and writes
on the standard output.
It implements all DES modes of operation described in \*(Fp 81
including alternative cipher feedback mode and both authentication modes.
All modes but the electronic code book mode require an
initialization vector;
if none is supplied,
the zero vector is used.
To protect the key and initialization vector from being read by
.IR ps (1),
.I cdes
hides its arguments on entry.
If no
.I key
is given,
one is requested from the controlling terminal if that can be opened,
or from the standard input if not.
.PP
The key and initialization vector are taken as sequences of \*(As
characters which are then mapped into their bit representations.
If either begins with `0x' or `0X',
that one is taken as a sequence of hexadecimal digits indicating the
bit pattern;
if either begins with `0b' or `0B',
that one is taken as a sequence of binary digits indicating the bit pattern.
In either case,
only the leading 64 bits of the key or initialization vector
are used,
and if fewer than 64 bits are provided, enough 0 bits are appended
to pad the key to 64 bits.
Note that if the key is not entered on the command line,
the
.B \-a
option
.I must
be given or the program will abort.
This is due to limits of the password reading function
.IR getpass (3),
which allows at most 8 characters to be entered.
.PP
By default,
the standard input is encrypted and written to the standard output.
Using the same key for encryption and decryption preserves plaintext,
so
.ce
cdes \f2key\fP < plaintext | cdes \-i \f2key\fP
is a very expensive equivalent of
.IR cat (1).
.PP
Options are:
.TP
.B \-a
the key and initialization vector strings are to be taken as \*(As.
suppressing the special interpretation given to leading `0x', `0X',
\&`0b', and `0B' characters.
Note this flag applies to
.I both
the key and initialization vector.
.TP
.B \-b
According to the DES standard,
the low-order bit of each character in the key string is deleted.
Since most \*(As representations set the high-order bit to 0,
simply deleting the low-order bit effectively reduces the size of the
key space from 2\u\s-356\s0\d to 2\u\s-348\s0\d keys.
Setting this option copies the low-order bit into the high-order bit,
thus preserving the key space size.
.TP
.B \-c
Use cipher block chaining mode.
.TP
.B \-e
Use electronic code book mode.
This is the default.
.TP
.BI \-f b
Use
.IR b -bit
cipher feedback mode.
Currently
.I b
must be a multiple of 8
between 8 and 64 inclusive.
.TP
.BI \-F b
Use
.IR b -bit
alternative cipher feedback mode.
Currently
.I b
must be a multiple of 7
between 7 and 56 inclusive.
.TP
.B \-i
invert (decrypt) the input.
.TP
.BI \-m b
Compute a message authentication code (MAC) of
.I b
bits on the input.
.I b
must be between 1 and 64 inclusive;
if
.I b
is not a multiple of 8,
enough 0 bits will be added to pad the MAC length
to the nearest multiple of 8.
Only the MAC is output.
MACs are only available in cipher block chaining mode
or in cipher feedback mode.
.TP
.BI \-o b
Use
.IR b -bit
output feedback mode.
Currently
.I b
must be a multiple of 8
between 8 and 64 inclusive.
.TP
.BI \-v v
Set the initialization vector to
.IR v ;
the vector is interpreted in the same way as the key.
The vector is ignored in electronic codebook mode.
.PP
The DES is considered a very strong cryptosystem,
and other than table lookup attacks,
key search attacks,
and Hellman's time-memory tradeoff
(all of which are very expensive and time-consuming),
no cryptanalytic methods for breaking the
DES are known in the open literature.
No doubt the choice of keys and key security
are the most vulnerable aspect of
.IR cdes .
.SH FILES
/dev/tty	controlling terminal for typed key
.SH SEE ALSO
.br
crypt(1), crypt(3)
.br
.IR "Data Encryption Standard" ,
Federal Information Processing Standard #46,
National Bureau of Standards,
U.S. Department of Commerce
(January 1977)
.br
.IR "DES Modes of Operation"
Federal Information Processing Standard #81,
National Bureau of Standards,
U.S. Department of Commerce
(December 1980)
.br
Matt Bishop,
\&``An Application of a Fast Data Encrypotion Standard Implementation,''
.I "Computing Systems"
.BR 1 (3)
pp. 223-254
(Summer 1988)
.br
Dorothy Denning,
.IR "Cryptography and Data Security" ,
Addison-Wesley Publishing Co.,
Reading, MA
\(co1982.
.SH BUGS
.PP
There is a controversy raging over whether the DES will still be secure
in a few years. The advent of special-purpose hardware could reduce
the cost of any of the methods of attack named above
so that they are no longer computationally infeasible.
.PP
As the key or key schedule is kept in memory
throughout the run of this program,
the encryption can be compromised if memory is readable.
.PP
There is no warranty of merchantability nor any warranty
of fitness for a particular purpose nor any other warranty,
either express or implied, as to the accuracy of the
enclosed materials or as to their suitability for any
particular purpose.  Accordingly, the user assumes full
responsibility for their use. Further,
the author assumes no obligation
to furnish any assistance of any kind whatsoever, or to
furnish any additional information or documentation.
.SH AUTHOR
Matt Bishop,
Department of Mathematics and Computer Science,
Bradley Hall,
Dartmouth College,
Hanover, NH  03755
.sp
Electronic mail addresses:
.br
Internet: [e-mail]
.br
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