DES(1)

NAME
des - DES file encryption

SYNOPSIS

des -e|-d [-h] [-k key] [-b]

DESCRIPTION
Des is a filter that encrypts or decrypts standard input to standard output with the Data Encryption Standard (DES). Either -e (encrypt) or -d (decrypt) must be specified. If the key is not given on the command line with the -k option the command will prompt for it twice, suppressing echo and comparing the two responses to guard against mistyping.

The des command is provided to promote secure exchange of data in a standard fashion.

The -h flag controls how the key string is to be interpreted. Without the -h flag, the key is an ASCII string. Since DES ignores the low order bit of each key byte, the high order bit is set for odd parity, thus retaining the information contained in the low order bit. If the -h flag is set, the key string is interpreted as 16 hex/ASCII characters; the low order bit of each byte is again ignored as per the DES algorithm. This allows the use of any arbitrary 56-bit key, including bytes representing control characters that could not be typed if the -h option were not used.

By default, DES Cipher Block Chaining (CBC) mode is used, with an initial vector (IV) of all zeros; if the -b option is specified, Electronic Code Book (ECB, eight bytes at a time) mode is used instead.

The 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.

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 following sections give information required to implement compatible 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 character, 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.

Except for the -h option, this command is compatible with the des command on the Sun Microsystems workstation.

BUGS

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 crytographic attack.

AUTHOR
Phil Karn, KA9Q.
Manual page updated by Raphael Manfredi, from the SUN-OS des(1) manual page.