MirOS Manual: des_modes(7)


DES_MODES(7)                 OpenSSL                 DES_MODES(7)

NAME

     des_modes - 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().

     + 64 bits are enciphered at a time.

     + The order of the blocks can be rearranged without detec-
       tion.

     + The same plaintext block always produces the same cipher-
       text 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 start-
       ing variable.

     + The chaining operation makes the ciphertext blocks depen-
       dent 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 cipher-
       text blocks.

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DES_MODES(7)                 OpenSSL                 DES_MODES(7)

     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 start-
       ing 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).  In my implementation this is always
       the case.

     + 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 cipher-
       text variables.

     Output Feedback Mode (OFB)

     Normally, this is found as the function
     algorithm_ofb_encrypt().

     + 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 vari-
       able 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 pro-
       ducing different key streams.

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DES_MODES(7)                 OpenSSL                 DES_MODES(7)

     + 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 deci-
       phered 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 effec-
       tive 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 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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DES_MODES(7)                 OpenSSL                 DES_MODES(7)

     + 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.

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)

MirOS BSD #10-current      2005-02-05                           4

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