des_modes(7)des_modes(7)NAMEdes_modes - 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.
Electronic Codebook Mode (ECB)
Normally, this is found as the algorithm_ecb_encrypt() function. 64
bits are enciphered at a time. The order of the blocks can be rear‐
ranged without detection. The same plaintext block always produces the
same ciphertext block (for the same key) making it vulnerable to a dic‐
tionary attack. An error will only affect one ciphertext block.
Cipher Block Chaining Mode (CBC)
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 cipher‐
text 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 there‐
fore blocks can not be rearranged. The use of different starting vari‐
ables 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 algorithm_cfb_encrypt() function. A
number of bits (j) <= 64 are enciphered at a time. The CFB mode pro‐
duces 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. 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 encipher‐
ment 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)
Normally, this is found as the algorithm_ofb_encrypt() function. A
number of bits (j) <= 64 are enciphered at a time. The OFB mode pro‐
duces 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 differ‐
ent 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 over‐
heads. Only multiples of j bits can be enciphered. OFB mode of opera‐
tion does not extend ciphertext errors in the resultant plaintext out‐
put. 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 syn‐
chronism, the system needs to be reinitialized. Each reinitialization
should use a value of the start variable different from the start vari‐
able 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 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
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)
