des_modes  Variants of DES and other crypto algorithms of
OpenSSL
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) [Toc] [Back]
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) [Toc] [Back]
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) [Toc] [Back]
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 jbit 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) [Toc] [Back]
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 selfsynchronizing. 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 [Toc] [Back]
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 [Toc] [Back]
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.
des_modes(7)
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