Index for Section 7

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des_modes(7)
NAME
  des_modes - 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.

  Electronic Codebook Mode (ECB)

  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)

  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)

  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)

  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

  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

  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.

Index for Section 7

Alphabetical listing for D

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