diff options
Diffstat (limited to 'lib/libdes/MODES.DES')
| -rw-r--r-- | lib/libdes/MODES.DES | 84 |
1 files changed, 84 insertions, 0 deletions
diff --git a/lib/libdes/MODES.DES b/lib/libdes/MODES.DES new file mode 100644 index 00000000000..18934b56c33 --- /dev/null +++ b/lib/libdes/MODES.DES @@ -0,0 +1,84 @@ +Modes of DES +Quite a bit of the following information has been taken from + 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 + +There are several different modes in which DES can be used, they are +as follows. + +Electronic Codebook Mode (ECB) (des_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) (des_cbc_encrypt()) +- 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) (des_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 con 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) (des_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-synchronising. If the two operation of + encipherment and decipherment get out of synchronism, the system needs + to be re-initialised. +- Each re-initialisation 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 (des_3ecb_encrypt()) +- Encrypt with key1, decrypt with key2 and encrypt with key1 again. +- As for ECB encryption but increases the effective key length to 112 bits. +- If both keys are the same it is equivalent to encrypting once with + just one key. + +Triple CBC Mode (des_3cbc_encrypt()) +- Encrypt with key1, decrypt with key2 and encrypt with key1 again. +- As for CBC encryption but increases the effective key length to 112 bits. +- If both keys are the same it is equivalent to encrypting once with + just one key. |