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bdes - "encrypt/decrypt using the Data Encryption Standard (DES)"


Implementation Notes
See Also


bdes [-abdp] [-F N] [-f N] [-k key] [-m N] [-o N] [-v vector]


The bdes utility implements all DES modes of operation described in "FIPS PUB 81", including alternative cipher feedback mode and both authentication modes. The bdes utility reads from the standard input and writes to the standard output. By default, the input is encrypted using cipher block chaining (CBC) mode. Using the same key for encryption and decryption preserves plain text.

All modes but the electronic code book (ECB) mode require an initialization vector; if none is supplied, the zero vector 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 interpretation given to leading "0X", "0x", "0B", and "0b" characters. This flag applies to both the key and initialization vector.
-b Use ECB mode.
-d Decrypt the input.
-F N Use N -bit alternative CFB mode. Currently N must be a multiple of 7 between 7 and 56 inclusive (this does not conform to the alternative CFB mode specification).
-f N Use N -bit CFB mode. Currently N must be a multiple of 8 between 8 and 64 inclusive (this does not conform to the standard CFB mode specification).
-k key Use key as the cryptographic key.
-m N 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 output. MACs are only available in CBC mode or in CFB mode.
-o N Use N -bit ouput feedback (OFB) 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 vector
Set the initialization vector to vector; the vector is interpreted in the same way as the key. The vector is ignored in ECB 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 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.

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 2^56 to 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 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.


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.

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, bdes 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.)

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 vector of 0 (that is, all zeroes). By default, bdes does too, but this may be changed from the command line.


.Rs "Data Encryption Standard"
.Rs "DES Modes of Operation"
.Rs "Implementation Notes on bdes(1)"



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