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().
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)
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 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)
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 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). 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
ciphertext 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 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
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 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 effectively
the same as normal ecb mode.
Triple
CBC Mode
Normally, this is found as the function algorithm_ede3_cbc_encrypt().
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 nbit block cipher algorithm Appendix A

SEE ALSO
blowfish(3), des(3), idea(3), rc2(3)