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des_modes(7)

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NAME    [Toc]    [Back]

       des_modes - Variants of DES and other crypto algorithms of
       OpenSSL

DESCRIPTION    [Toc]    [Back]

       Several crypto algorithms for OpenSSL can  be  used  in  a
       number  of  modes.  Those are used for using block ciphers
       in a way similar to stream ciphers, among other things.

   Electronic Codebook Mode (ECB)    [Toc]    [Back]
       Normally, this is  found  as  the  algorithm_ecb_encrypt()
       function.  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)    [Toc]    [Back]
       Normally,     this     is     found     as    the    algorithm_cbc_encrypt()function.
      Be      aware       that
       des_cbc_encrypt()  is  not  really  DES  CBC  (it does not
       update  the  IV);  use  the  des_ncbc_encrypt()   function
       instead.   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)    [Toc]    [Back]
       Normally, this is  found  as  the  algorithm_cfb_encrypt()
       function.   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 can 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).   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)    [Toc]    [Back]
       Normally,  this  is  found  as the algorithm_ofb_encrypt()
       function.  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-synchronizing.   If  the
       two  operation of encipherment and decipherment get out of
       synchronism, the system needs to be  reinitialized.   Each
       reinitialization  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    [Toc]    [Back]
       Normally, this is found  as  the  algorithm_ecb3_encrypt()
       function  .   Encrypt  with  key1,  decrypt  with key2 and
       encrypt with  key3  again.   As  for  ECB  encryption  but
       increases  the key length to 168 bits. There are theoretic
       attacks that can be  used  that  make  the  effective  key
       length 112 bits, but this attack also requires 2^56 blocks
       of memory, not very likely, even for  the  NSA.   If  both
       keys are the same it is equivalent to encrypting once with
       just one key.  If the first and last key are the same, the
       key  length  is  112  bits.  There  are attacks that could
       reduce the effective key strength to  only  slightly  more
       than 56 bits, but these require a lot of memory.  If all 3
       keys are the same, this is the same as normal ecb mode.

   Triple CBC Mode    [Toc]    [Back]
       Normally,    this    is     found     as     the     algorithm_ede3_cbc_encrypt()
  function  .   Encrypt with key1,
       decrypt with key2 and then encrypt with key3.  As for  CBC
       encryption  but  increases the key length to 168 bits with
       the same restrictions as for triple ecb mode.



                                                     des_modes(7)
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