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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_3ecb_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.