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().
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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DES_MODES(7) OpenSSL DES_MODES(7)
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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DES_MODES(7) OpenSSL DES_MODES(7)
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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