DES(1) General Commands Manual DES(1)
des - encrypt or decrypt data using Data Encryption Standard
des -e | -d [ -bfs ] [ -k key ] [ input-file [ output-file ] ]
des encrypts and decrypts data using the NBS Data Encryption Standard
algorithm. One of -e (for encrypt) or -d (for decrypt) must be speci-
The des command is provided to promote secure exchange of data in a
Two standard encryption modes are supported by the des program, Cipher
Block Chaining (CBC -- the default) and Electronic Code Book (ECB --
specified with -b). CBC mode treats an entire file as a unit of
encryption, that is, if insertions or deletions are made to the
encrypted file then decryption will not succeed. CBC mode also ensures
that regularities in clear data do not appear in the encrypted data.
ECB mode treats each 8 bytes as units of encryptions, so if parts of
the encrypted file are modified then other parts may still be
decrypted. Identical values of clear text encrypt to identical values
of cipher text.
The key used for the DES algorithm is obtained by prompting the user
unless the `-k key' option is given. If the key is an argument to the
des command, it is potentially visible to users executing ps(1) or a
derivative. To minimize this possibility, des takes care to destroy
the key argument immediately upon entry.
The des command attempts to use DES hardware for its job, but will use
a software implementation of the DES algorithm if the hardware is
unavailable. Normally, a warning message is printed if the DES hard-
ware is unavailable since the software is only about 1/50th as fast.
However, the -f option will suppress the warning. The -s option may be
used to force use of software instead of hardware DES.
The des command reads from standard input unless input-file is speci-
fied and writes to standard output unless output-file is given.
The following sections give information required to implement compati-
ble facilities in other environments.
Since the CBC and ECB modes of DES require units of 8 bytes to be
encrypted, files being encrypted by the des command have 1 to 8 bytes
appended to them to cause them to be a multiple of 8 bytes. The last
byte, when decrypted, gives the number of bytes (0 to 7) which are to
be saved of the last 8 bytes. The other bytes of those appended to the
input are randomized before encryption. If, when decrypting, the last
byte is not in the range of 0 to 7 then either the encrypted file has
been corrupted or an incorrect key was provided for decryption and an
error message is printed.
The DES algorithm requires an 8 byte key whose low order bits are
assumed to be odd-parity bits. The ASCII key supplied by the user is
zero padded to 8 bytes and the high order bits are set to be odd-parity
bits. The DES algorithm then ignores the low bit of each ASCII charac-
ter, but that bit's information has been preserved in the high bit due
to the parity.
The CBC mode of operation always uses an initial value of all zeros for
the initialization vector, so the first 8 bytes of a file are encrypted
the same whether in CBC or ECB mode.
-b Select ECB (eight bytes at a time) encryption mode.
-d Decrypt data.
-e Encrypt data.
-f Suppress warning message when software implementation is used.
-s Select software implementation for the encryption algorithm.
-k key Use the encryption key specified.
It would be better to use a real 56-bit key rather than an ASCII-based
56-bit pattern. Knowing that the key was derived from ASCII radically
reduces the time necessary for a brute-force cryptographic attack.
Software encryption is disabled for programs shipped outside of the
U.S. The program will still be able to encrypt files if one can obtain
an encryption chip, legally or otherwise.
9 September 1987 DES(1)