des_modes

       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.

OVERVIEW
   Electronic Codebook Mode (ECB)
       Normally, this is found as the function algorithm_ecb_encrypt().

       o   64 bits are enciphered at a time.

       o   The order of the blocks can be rearranged without detection.

       o   The same plaintext block always produces the same ciphertext block
           (for the same key) making it vulnerable to a 'dictionary attack'.

       o   An error will only affect one ciphertext block.

   Cipher Block Chaining Mode (CBC)
       Normally, this is found as the function algorithm_cbc_encrypt().  Be
       aware that des_cbc_encrypt() is not really DES CBC (it does not update
       the IV); use des_ncbc_encrypt() instead.

       o   a multiple of 64 bits are enciphered at a time.

       o   The CBC mode produces the same ciphertext whenever the same
           plaintext is encrypted using the same key and starting variable.

       o   The chaining operation makes the ciphertext blocks dependent on the
           current and all preceding plaintext blocks and therefore blocks can
           not be rearranged.

       o   The use of different starting variables prevents the same plaintext
           enciphering to the same ciphertext.

       o   An error will affect the current and the following ciphertext
           blocks.

   Cipher Feedback Mode (CFB)
       Normally, this is found as the function algorithm_cfb_encrypt().

       o   a number of bits (j) <= 64 are enciphered at a time.

       o   The CFB mode produces the same ciphertext whenever the same
           plaintext is encrypted using the same key and starting variable.

       o   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.

       o   The use of different starting variables prevents the same plaintext
           enciphering to the same ciphertext.

       o   The strength of the CFB mode depends on the size of k (maximal if j
           == k).  In my implementation this is always the case.

       o   a number of bits (j) <= 64 are enciphered at a time.

       o   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.

       o   The absence of chaining makes the OFB more vulnerable to specific
           attacks.

       o   The use of different start variables values prevents the same
           plaintext enciphering to the same ciphertext, by producing
           different key streams.

       o   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.

       o   Only multiples of j bits can be enciphered.

       o   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.

       o   OFB mode is not self-synchronizing.  If the two operation of
           encipherment and decipherment get out of synchronism, the system
           needs to be re-initialized.

       o   Each re-initialization 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
       Normally, this is found as the function algorithm_ecb3_encrypt().

       o   Encrypt with key1, decrypt with key2 and encrypt with key3 again.

       o   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.

       o   If both keys are the same it is equivalent to encrypting once with
           just one key.

       o   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.

       This text was been written in large parts by Eric Young in his original
       documentation for SSLeay, the predecessor of OpenSSL.  In turn, he
       attributed it to:

               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

SEE ALSO
       BF_encrypt(3), DES_crypt(3)

COPYRIGHT
       Copyright 2000-2017 The OpenSSL Project Authors. All Rights Reserved.

       Licensed under the OpenSSL license (the "License").  You may not use
       this file except in compliance with the License.  You can obtain a copy
       in the file LICENSE in the source distribution or at
       <https://www.openssl.org/source/license.html>.

1.1.0h                            2018-05-12                   DES_MODES(7SSL)
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