From 7043a4fc4a66bc294f6914f4bebfd2b0ed2b8a75 Mon Sep 17 00:00:00 2001 From: Ulf Möller Date: Sat, 18 Mar 2000 18:37:43 +0000 Subject: superseded by des_modes.pod --- crypto/des/MODES.DES | 84 ---------------------------------------------------- 1 file changed, 84 deletions(-) delete mode 100644 crypto/des/MODES.DES (limited to 'crypto/des') diff --git a/crypto/des/MODES.DES b/crypto/des/MODES.DES deleted file mode 100644 index 0cbc44f51d..0000000000 --- a/crypto/des/MODES.DES +++ /dev/null @@ -1,84 +0,0 @@ -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_ecb3_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. -- cgit v1.2.3