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des_modes (7)
  • >> des_modes (7) ( Solaris man: Макропакеты и соглашения )
  • des_modes (7) ( Linux man: Макропакеты и соглашения )
  • 
    
    

    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.
    
    
    
         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.
    
         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 key
           space to 55 bit's but it requires 2^56 blocks 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().
    
    
         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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