DES_MODES(7)                OpenSSL                DES_MODES(7)





NAME
       Modes of DES - the variants of DES and other crypto
       algorithms of OpenSSL

DESCRIPTION
       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 algo-
       rithm_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 vulnera-
         ble 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 algo-
       rithm_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 algo-
       rithm_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 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 An error will affect the current and the following
         ciphertext variables.

       Output Feedback Mode (OFB)

       Normally, this is found as the function algo-
       rithm_ofb_encrypt().

       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 opera-
         tion of encipherment and decipherment get out of syn-
         chronism, 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 algo-
       rithm_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 encrypt-
         ing 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.

       o If all 3 keys are the same, this is effectively the
         same as normal ecb mode.

       Triple CBC Mode

       Normally, this is found as the function algo-
       rithm_ede3_cbc_encrypt().

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

       o As for CBC encryption but increases the key length to
         168 bits with the same restrictions as for triple ecb
         mode.

NOTES
       This text was been written in large parts by Eric Young
       in his original documentation for SSLeay, the predeces-
       sor 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
       blowfish(3), des(3), idea(3), rc2(3)



0.9.7c                     2002-03-05              DES_MODES(7)
