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-Modes of DES
-Quite a bit of the following information has been taken from
-	AS 2805.5.2
-	Australian Standard
-	Electronic funds transfer - Requirements for interfaces,
-	Part 5.2: Modes of operation for an n-bit block cipher algorithm
-	Appendix A
-
-There are several different modes in which DES can be used, they are
-as follows.
-
-Electronic Codebook Mode (ECB) (des_ecb_encrypt())
-- 64 bits are enciphered at a time.
-- The order of the blocks can be rearranged without detection.
-- The same plaintext block always produces the same ciphertext block
-  (for the same key) making it vulnerable to a 'dictionary attack'.
-- An error will only affect one ciphertext block.
-
-Cipher Block Chaining Mode (CBC) (des_cbc_encrypt())
-- a multiple of 64 bits are enciphered at a time.
-- The CBC mode produces the same ciphertext whenever the same
-  plaintext is encrypted using the same key and starting variable.
-- The chaining operation makes the ciphertext blocks dependent on the
-  current and all preceding plaintext blocks and therefore blocks can not
-  be rearranged.
-- The use of different starting variables prevents the same plaintext
-  enciphering to the same ciphertext.
-- An error will affect the current and the following ciphertext blocks.
-
-Cipher Feedback Mode (CFB) (des_cfb_encrypt())
-- a number of bits (j) <= 64 are enciphered at a time.
-- The CFB mode produces the same ciphertext whenever the same
-  plaintext is encrypted using the same key and starting variable.
-- The chaining operation makes the ciphertext variables dependent on the
-  current and all preceding variables and therefore j-bit variables are
-  chained together and con not be rearranged.
-- The use of different starting variables prevents the same plaintext
-  enciphering to the same ciphertext.
-- The strength of the CFB mode depends on the size of k (maximal if
-  j == k).  In my implementation this is always the case.
-- Selection of a small value for j will require more cycles through
-  the encipherment algorithm per unit of plaintext and thus cause
-  greater processing overheads.
-- Only multiples of j bits can be enciphered.
-- An error will affect the current and the following ciphertext variables.
-
-Output Feedback Mode (OFB) (des_ofb_encrypt())
-- a number of bits (j) <= 64 are enciphered at a time.
-- The OFB mode produces the same ciphertext whenever the same
-  plaintext enciphered using the same key and starting variable.  More
-  over, in the OFB mode the same key stream is produced when the same
-  key and start variable are used.  Consequently, for security reasons
-  a specific start variable should be used only once for a given key.
-- The absence of chaining makes the OFB more vulnerable to specific attacks.
-- The use of different start variables values prevents the same
-  plaintext enciphering to the same ciphertext, by producing different
-  key streams.
-- Selection of a small value for j will require more cycles through
-  the encipherment algorithm per unit of plaintext and thus cause
-  greater processing overheads.
-- Only multiples of j bits can be enciphered.
-- OFB mode of operation does not extend ciphertext errors in the
-  resultant plaintext output.  Every bit error in the ciphertext causes
-  only one bit to be in error in the deciphered plaintext.
-- OFB mode is not self-synchronising.  If the two operation of
-  encipherment and decipherment get out of synchronism, the system needs
-  to be re-initialised.
-- Each re-initialisation should use a value of the start variable
-different from the start variable values used before with the same
-key.  The reason for this is that an identical bit stream would be
-produced each time from the same parameters.  This would be
-susceptible to a 'known plaintext' attack.
-
-Triple ECB Mode (des_ecb3_encrypt())
-- Encrypt with key1, decrypt with key2 and encrypt with key1 again.
-- As for ECB encryption but increases the effective key length to 112 bits.
-- If both keys are the same it is equivalent to encrypting once with
-  just one key.
-
-Triple CBC Mode (des_3cbc_encrypt())
-- Encrypt with key1, decrypt with key2 and encrypt with key1 again.
-- As for CBC encryption but increases the effective key length to 112 bits.
-- If both keys are the same it is equivalent to encrypting once with
-  just one key.