BDES(1)BDES(1)NAMEbdes - encrypt/decrypt using the Data Encryption Standard
SYNOPSISbdes [ -abdp ] [ -F N ] [ -f N ] [ -k key ]
[ -m N ] [ -o N ] [ -v vector ]
DESCRIPTION
Bdes implements all DES modes of operation described in
FIPS PUB 81, including alternative cipher feedback mode
and both authentication modes. Bdes reads from the stan-
dard 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.
All modes but the electronic code book mode require an
initialization vector; if none is supplied, the zero vec-
tor is used. If no key is specified on the command line,
the user is prompted for one (see getpass(3) for more
details).
The options are as follows:
-a The key and initialization vector strings are to be
taken as ASCII, suppressing the special interpreta-
tion given to leading ``0X'', ``0x'', ``0B'', and
``0b'' characters. This flag applies to both the
key and initialization vector.
-b Use electronic code book mode.
-d Decrypt the input.
-F Use N-bit alternative cipher feedback mode. Cur-
rently N must be a multiple of 7 between 7 and 56
inclusive (this does not conform to the alternative
CFB mode specification).
-f Use N-bit cipher feedback mode. Currently N must
be a multiple of 8 between 8 and 64 inclusive (this
does not conform to the standard CFB mode specifi-
cation).
-k Use key as the cryptographic key.
-m Compute a message authentication code (MAC) of N
bits on the input. The value of N must be between
1 and 64 inclusive; if 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 out-
put. MACs are only available in cipher block
chaining mode or in cipher feedback mode.
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BDES(1)BDES(1)-o Use N-bit output feedback mode. Currently N must
be a multiple of 8 between 8 and 64 inclusive (this
does not conform to the OFB mode specification).
-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.
-v Set the initialization vector to vector; the vector
is interpreted in the same way as the key. The
vector is ignored in electronic codebook mode.
The key and initialization vector are taken as sequences
of ASCII 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 dig-
its 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.
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 delet-
ing the low-order bit effectively reduces the size of the
key space from 256 to 248 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 what-
ever value gives odd parity. This preserves the key space
size. Note this resetting of the parity bit is not done
if the key is given in binary or hex, and can be disabled
for ASCII keys as well.
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 bdes.
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 implementa-
tion of the data encryption standard distributed by Sun
Microsystems, Inc.
In the ECB and CBC modes, plaintext is encrypted in units
of 64 bits (8 bytes, also called a block). To ensure that
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BDES(1)BDES(1)
the plaintext file is encrypted correctly, bdes will
(internally) append from 1 to 8 bytes, the last byte con-
taining an integer stating how many bytes of that final
block are from the plaintext file, and encrypt the result-
ing 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 dur-
ing decryption the last byte of the file does not contain
an integer between 0 and 7, either the file has been cor-
rupted 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 mul-
tiple 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.)
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.
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 not reset. This
allows testing using arbitrary bit patterns as keys.
The Sun implementation always uses an initialization vec-
tor of 0 (that is, all zeroes). By default, bdes does
too, but this may be changed from the command line.
SEE ALSOcrypt(3), getpass(3)
Data Encryption Standard, Federal Information Processing
Standard #46, National Bureau of Standards, U.S. Depart-
ment of Commerce, Washington DC (Jan. 1977)
DES Modes of Operation, Federal Information Processing
Standard #81, National Bureau of Standards, U.S. Depart-
ment of Commerce Washington DC (Dec. 1980)
Dorothy Denning, Cryptography and Data Security, Addison-
Wesley Publishing Co., Reading, MA (C)1982.
Matt Bishop, Implementation Notes on bdes(1), Technical
Report PCS-TR-91-158, Department of Mathematics and Com-
puter Science, Dartmouth College, Hanover, NH 03755 (Apr.
1991).
June 29, 1993 3
BDES(1)BDES(1)DISCLAIMER
THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS ``AS IS'' AND
ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
ARE DISCLAIMED. IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE
FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
SUCH DAMAGE.
BUGS
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 meth-
ods of attack named above so that they are no longer com-
putationally infeasible.
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 bdes
overwrites its arguments, however, the obvious race cannot
currently be avoided.
Certain specific keys should be avoided because they
introduce potential weaknesses; these keys, called the
weak and semiweak keys, are (in hex notation, where p is
either 0 or 1, and P is either e or f):
0x0p0p0p0p0p0p0p0p 0x0p1P0p1P0p0P0p0P
0x0pep0pep0pfp0pfp 0x0pfP0pfP0pfP0pfP
0x1P0p1P0p0P0p0P0p 0x1P1P1P1P0P0P0P0P
0x1Pep1Pep0Pfp0Pfp 0x1PfP1PfP0PfP0PfP
0xep0pep0pfp0pfp0p 0xep1Pep1pfp0Pfp0P
0xepepepepepepepep 0xepfPepfPfpfPfpfP
0xfP0pfP0pfP0pfP0p 0xfP1PfP1PfP0PfP0P
0xfPepfPepfPepfPep 0xfPfPfPfPfPfPfPfP
This is inherent in the DES algorithm (see Moore and Sim-
mons, Cycle structure of the DES with weak and semi-weak
keys, Advances in Cryptology - Crypto '86 Proceedings ,
Springer-Verlag New York, (C)1987, pp. 9-32.)
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