From: Ulf MÃ¶ller
Date: Sat, 18 Mar 2000 18:37:43 +0000 (+0000)
Subject: superseded by des_modes.pod
XGitTag: OpenSSL_0_9_5abeta1~18
XGitUrl: https://git.openssl.org/gitweb/?p=openssl.git;a=commitdiff_plain;h=7043a4fc4a66bc294f6914f4bebfd2b0ed2b8a75
superseded by des_modes.pod

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 nbit 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 jbit 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 selfsynchronising. If the two operation of
 encipherment and decipherment get out of synchronism, the system needs
 to be reinitialised.
 Each reinitialisation 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.