path: root/usr.bin/bdes/bdes.1

.\"	$OpenBSD: bdes.1,v 1.7 2004/05/31 18:42:58 otto Exp $
.\"	$NetBSD: bdes.1,v 1.11 2003/08/07 11:13:11 agc Exp $
.\"
.\" Copyright (c) 1991, 1993
.\"	The Regents of the University of California.  All rights reserved.
.\"
.\" This code is derived from software contributed to Berkeley by
.\" Matt Bishop of Dartmouth College.
.\"
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.\" modification, are permitted provided that the following conditions
.\" are met:
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.\"    may be used to endorse or promote products derived from this software
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.\"	@(#)bdes.1	8.1 (Berkeley) 6/29/93
.\"
.Dd June 29, 1993
.Dt BDES 1
.Os
.Sh NAME
.Nm bdes
.Nd encrypt/decrypt using the Data Encryption Standard
.Sh SYNOPSIS
.Nm
.Op Fl abdp
.Op Fl F Ar N
.Op Fl f Ar N
.Op Fl k Ar key
.Op Fl m Ar N
.Op Fl o Ar N
.Op Fl v Ar vector
.Sh DESCRIPTION
.Nm
implements all DES modes of operation described in FIPS PUB 81,
including alternative cipher feedback mode and both authentication
modes.
.Nm
reads from the standard input and writes to the standard output.
By default, the input is encrypted using cipher block chaining mode.
Using the same key for encryption and decryption preserves plain text.
.Pp
All modes but the electronic code book mode require an initialization
vector; if none is supplied, the zero vector is used.
If no
.Ar key
is specified on the command line, the user is prompted for one (see
.Xr getpass 3
for more details).
.Pp
The options are as follows:
.Bl -tag -width "-v vector"
.It Fl a
The key and initialization vector strings are to be taken as ASCII,
suppressing the special interpretation given to leading
.Dq 0X ,
.Dq 0x ,
.Dq 0B
and
.Dq 0b
characters.
This flag applies to
.Em both
the key and initialization vector.
.It Fl b
Use electronic code book mode.
This is not recommended for messages
longer than 8 bytes, as patterns in the input will show through to the
output.
.It Fl d
Decrypt the input.
.It Fl F Ar N
Use
.Ar N Ns -bit
alternative cipher feedback mode.
Currently
.Ar N
must be a multiple of 7 between 7 and 56 inclusive (this does not conform
to the alternative CFB mode specification).
.It Fl f Ar N
Use
.Ar N Ns -bit
cipher feedback mode.
Currently
.Ar N
must be a multiple of 8 between 8 and 64 inclusive (this does not conform
to the standard CFB mode specification).
.It Fl k Ar key
Use
.Ar key
as the cryptographic key.
.It Fl m Ar N
Compute a message authentication code (MAC) of
.Ar N
bits on the input.
The value of
.Ar N
must be between 1 and 64 inclusive; if
.Ar N
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.
.It Fl o Ar N
Use
.Ar N Ns -bit
output feedback mode.
Currently
.Ar N
must be a multiple of 8 between 8 and 64 inclusive (this does not conform
to the OFB mode specification).
.It Fl p
Disable the resetting of the parity bit.
This flag forces the parity bit of the key to be used as typed, rather than
making each character be of odd parity.
It is used only if the key is given in ASCII.
.It Fl v Ar vector
Set the initialization vector to
.Ar vector ;
the vector is interpreted in the same way as the key.
The vector is ignored in electronic codebook mode.
For best security, a different
initialization vector should be used for each file.
.El
.Pp
The key and initialization vector are taken as sequences of ASCII
characters which are then mapped into their bit representations.
If either begins with
.Dq 0X
or
.Dq 0x ,
that one is taken as a sequence of hexadecimal digits indicating the
bit pattern;
if either begins with
.Dq 0B
or
.Dq 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.
.Pp
According to the DES standard, the low-order bit of each character in the
key string is deleted.
Since most ASCII representations set the high-order bit to 0, simply
deleting the low-order bit effectively reduces the size of the key space
from
.if t 2\u\s-356\s0\d
.if n 2**56
to
.if t 2\u\s-348\s0\d
.if n 2**48
keys.
To prevent this, the high-order bit must be a function depending in part
upon the low-order bit; so, the high-order bit is set to whatever value
gives odd parity.
This preserves the key space size.
Note this resetting of the parity bit is
.Em not
done if the key is given in binary or hex, and can be disabled for ASCII
keys as well.
.Pp
The DES is considered a strong cryptosystem hobbled by a short
key, and other than table lookup attacks, key search attacks, and
Hellman's time-memory tradeoff (all of which are expensive and
time-consuming), no practical cryptanalytic methods for breaking the
DES are known in the open literature.
As of this writing, the best
known cryptanalytic method is linear cryptanalysis, which requires an
average of
.if t 2\u\s-343\s0\d
.if n 2**43
known plaintext-ciphertext pairs to succeed.
Unfortunately for the DES, key search attacks (requiring only
a single known plaintext-ciphertext pair and trying
.if t 2\u\s-355\s0\d
.if n 2**55
keys on average) are becoming practical.
.Pp
As with all cryptosystems, the choice of keys and
key security remain the most vulnerable aspect of
.Nm .
.Sh IMPLEMENTATION NOTES
For implementors wishing to write software compatible with this program,
the following notes are provided.
This software is believed to be compatible with the implementation of the
data encryption standard distributed by Sun Microsystems, Inc.
.Pp
In the ECB and CBC modes, plaintext is encrypted in units of 64 bits (8 bytes,
also called a block).
To ensure that the plaintext file is encrypted correctly,
.Nm
will (internally) append from 1 to 8 bytes, the last byte containing an
integer stating how many bytes of that final block are from the plaintext
file, and encrypt the resulting block.
Hence, when decrypting, the last block may contain from 0 to 7 characters
present in the plaintext file, and the last byte tells how many.
Note that if during decryption the last byte of the file does not contain an
integer between 0 and 7, either the file has been corrupted or an incorrect
key has been given.
A similar mechanism is used for the OFB and CFB modes, except that those
simply require the length of the input to be a multiple of the mode size,
and the final byte contains an integer between 0 and one less than the number
of bytes being used as the mode.
(This was another reason that the mode size must be a multiple of 8 for those
modes.)
.Pp
Unlike Sun's implementation, unused bytes of that last block are not filled
with random data, but instead contain what was in those byte positions in
the preceding block.
This is quicker and more portable, and does not weaken the encryption
significantly.
.Pp
If the key is entered in ASCII, the parity bits of the key characters are set
so that each key character is of odd parity.
Unlike Sun's implementation, it is possible to enter binary or hexadecimal
keys on the command line, and if this is done, the parity bits are
.Em not
reset.
This allows testing using arbitrary bit patterns as keys.
.Pp
The Sun implementation always uses an initialization vector of 0
(that is, all zeroes).
By default,
.Nm
does too, but this may be changed from the command line.
.Sh SEE ALSO
.Xr crypt 3 ,
.Xr getpass 3
.Pp
.Em "Data Encryption Standard" ,
Federal Information Processing Standard #46,
National Bureau of Standards,
U.S. Department of Commerce,
Washington DC
(Jan. 1977).
.Pp
.Em "DES Modes of Operation" ,
Federal Information Processing Standard #81,
National Bureau of Standards,
U.S. Department of Commerce
Washington DC
(Dec. 1980).
.Pp
Dorothy Denning,
.Em "Cryptography and Data Security" ,
Addison-Wesley Publishing Co.,
Reading, MA
\(co1982.
.Pp
Matt Bishop,
.Em "Implementation Notes on bdes(1)" ,
Technical Report PCS-TR-91-158,
Department of Mathematics and Computer Science,
Dartmouth College,
Hanover, NH  03755
(Apr. 1991).
.Pp
M.J. Wiener,
.Em "Efficient DES Key Search" ,
Technical Report 244,
School of Computer Science,
Carleton University
(May 1994).
.Pp
Bruce Schneier,
.Em "Applied Cryptography (2nd edition)" ,
John Wiley \*[Am] Sons, Inc.,
New York, NY
\(co1996.
.Pp
M. Matsui,
.Em "Linear Cryptanalysis Method for DES Cipher" ,
Advances in Cryptology -- Eurocrypt '93 Proceedings,
Springer-Verlag
\(co1994.
.Pp
Blaze, Diffie, Rivest, Schneier, Shimomura, Thompson, and Wiener,
.Em "Minimal Key Lengths for Symmetric Ciphers To Provide Adequate Commercial Security" ,
Business Software Alliance,
http://www.bsa.org/policy/encryption/cryptographers.html
(January 1996).
.Sh BUGS
When this document was originally written, there was a controversy
raging over whether the DES would still be secure in a few years.
There is now near-universal consensus in the cryptographic community
that the key length of the DES is far too short.
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; in addition, the explosive growth in the number and speed
of modern microprocessors as well as advances in programmable logic
devices has brought an attack using only commodity hardware into the
realm of possibility.
Schneier and others currently recommend using
cryptosystems with keys of at least 90 bits when long-term security is
needed.
.Pp
As the key or key schedule is stored in memory, the encryption can be
compromised if memory is readable.
Additionally, programs which display programs' arguments may compromise the
key and initialization vector, if they are specified on the command line.
To avoid this
.Nm
overwrites its arguments, however, the obvious race cannot currently be
avoided.
.Pp
Certain specific keys should be avoided because they introduce potential
weaknesses; these keys, called the
.Em weak
and
.Em semiweak
keys, are (in hex notation, where p is either 0 or 1, and P is either
e or f):
.Bd -literal -offset indent
0x0p0p0p0p0p0p0p0p	0x0p1P0p1P0p0P0p0P
0x0pep0pep0pfp0pfp	0x0pfP0pfP0pfP0pfP
0x1P0p1P0p0P0p0P0p	0x1P1P1P1P0P0P0P0P
0x1Pep1Pep0Pfp0Pfp	0x1PfP1PfP0PfP0PfP
0xep0pep0pfp0pfp0p	0xep1Pep1pfp0Pfp0P
0xepepepepepepepep	0xepfPepfPfpfPfpfP
0xfP0pfP0pfP0pfP0p	0xfP1PfP1PfP0PfP0P
0xfPepfPepfPepfPep	0xfPfPfPfPfPfPfPfP
.Ed
.Pp
This is inherent in the DES algorithm (see Moore and Simmons,
.Do
Cycle structure of the DES with weak and semi-weak keys
.Dc ,
.Em "Advances in Cryptology \- Crypto '86 Proceedings" ,
Springer-Verlag New York, \(co1987, pp. 9-32.)