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
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 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
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 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 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 n-bit block cipher algorithm
Appendix A
SEE ALSO¶
blowfish(3),
des(3),
idea(3),
rc2(3)