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NAME

des - encrypt or decrypt data using Data Encryption Standard
 

SYNOPSIS

des -e |  -d [-bfs] [-k key] [input-file [output-file]]

DESCRIPTION

des encrypts and decrypts data using the NBS Data Encryption Standard algorithm. One of -e (for encrypt) or -d (for decrypt) must be specified.

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

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 hardware 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 specified and writes to standard output unless output-file is given.

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.  

OPTIONS

-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.

-kkey

Use the encryption key specified.

FILES

/dev/des?

SEE ALSO

ps(1)  

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 cryptographic attack.

Index

NAME

SYNOPSIS

DESCRIPTION

OPTIONS

FILES

SEE ALSO

BUGS

Партнёры:

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