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    Command:

    des_modes

    
    
    

    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().
    
           ? 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 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.
    
           ? 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 function algorithm_cfb_encrypt().
    
           ? 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.
    
    
           ? 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 re-initialized.
    
           ? 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().
    
           ? 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.
    
           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)
    
    
    

    1.0.1e 2013-02-11 DES_MODES(7)

    
    
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