des_modes(7)

     DES_MODES(7)           0.9.6h (2002-03-05)           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
          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.

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          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 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
          algorithm_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.

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          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.

          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
          algorithm_ede3_cbc_encrypt().

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          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 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
          blowfish(3), des(3), idea(3), rc2(3)

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