5 des_modes - the variants of DES and other crypto algorithms of OpenSSL
9 Several crypto algorithms for OpenSSL can be used in a number of modes. Those
10 are used for using block ciphers in a way similar to stream ciphers, among
15 =head2 Electronic Codebook Mode (ECB)
17 Normally, this is found as the function I<algorithm>_ecb_encrypt().
23 64 bits are enciphered at a time.
27 The order of the blocks can be rearranged without detection.
31 The same plaintext block always produces the same ciphertext block
32 (for the same key) making it vulnerable to a 'dictionary attack'.
36 An error will only affect one ciphertext block.
40 =head2 Cipher Block Chaining Mode (CBC)
42 Normally, this is found as the function I<algorithm>_cbc_encrypt().
43 Be aware that des_cbc_encrypt() is not really DES CBC (it does
44 not update the IV); use des_ncbc_encrypt() instead.
50 a multiple of 64 bits are enciphered at a time.
54 The CBC mode produces the same ciphertext whenever the same
55 plaintext is encrypted using the same key and starting variable.
59 The chaining operation makes the ciphertext blocks dependent on the
60 current and all preceding plaintext blocks and therefore blocks can not
65 The use of different starting variables prevents the same plaintext
66 enciphering to the same ciphertext.
70 An error will affect the current and the following ciphertext blocks.
74 =head2 Cipher Feedback Mode (CFB)
76 Normally, this is found as the function I<algorithm>_cfb_encrypt().
82 a number of bits (j) <= 64 are enciphered at a time.
86 The CFB mode produces the same ciphertext whenever the same
87 plaintext is encrypted using the same key and starting variable.
91 The chaining operation makes the ciphertext variables dependent on the
92 current and all preceding variables and therefore j-bit variables are
93 chained together and can not be rearranged.
97 The use of different starting variables prevents the same plaintext
98 enciphering to the same ciphertext.
102 The strength of the CFB mode depends on the size of k (maximal if
103 j == k). In my implementation this is always the case.
107 Selection of a small value for j will require more cycles through
108 the encipherment algorithm per unit of plaintext and thus cause
109 greater processing overheads.
113 Only multiples of j bits can be enciphered.
117 An error will affect the current and the following ciphertext variables.
121 =head2 Output Feedback Mode (OFB)
123 Normally, this is found as the function I<algorithm>_ofb_encrypt().
129 a number of bits (j) <= 64 are enciphered at a time.
133 The OFB mode produces the same ciphertext whenever the same
134 plaintext enciphered using the same key and starting variable. More
135 over, in the OFB mode the same key stream is produced when the same
136 key and start variable are used. Consequently, for security reasons
137 a specific start variable should be used only once for a given key.
141 The absence of chaining makes the OFB more vulnerable to specific attacks.
145 The use of different start variables values prevents the same
146 plaintext enciphering to the same ciphertext, by producing different
151 Selection of a small value for j will require more cycles through
152 the encipherment algorithm per unit of plaintext and thus cause
153 greater processing overheads.
157 Only multiples of j bits can be enciphered.
161 OFB mode of operation does not extend ciphertext errors in the
162 resultant plaintext output. Every bit error in the ciphertext causes
163 only one bit to be in error in the deciphered plaintext.
167 OFB mode is not self-synchronizing. If the two operation of
168 encipherment and decipherment get out of synchronism, the system needs
169 to be re-initialized.
173 Each re-initialization should use a value of the start variable
174 different from the start variable values used before with the same
175 key. The reason for this is that an identical bit stream would be
176 produced each time from the same parameters. This would be
177 susceptible to a 'known plaintext' attack.
181 =head2 Triple ECB Mode
183 Normally, this is found as the function I<algorithm>_ecb3_encrypt().
189 Encrypt with key1, decrypt with key2 and encrypt with key3 again.
193 As for ECB encryption but increases the key length to 168 bits.
194 There are theoretic attacks that can be used that make the effective
195 key length 112 bits, but this attack also requires 2^56 blocks of
196 memory, not very likely, even for the NSA.
200 If both keys are the same it is equivalent to encrypting once with
205 If the first and last key are the same, the key length is 112 bits.
206 There are attacks that could reduce the effective key strength
207 to only slightly more than 56 bits, but these require a lot of memory.
211 If all 3 keys are the same, this is effectively the same as normal
216 =head2 Triple CBC Mode
218 Normally, this is found as the function I<algorithm>_ede3_cbc_encrypt().
224 Encrypt with key1, decrypt with key2 and then encrypt with key3.
228 As for CBC encryption but increases the key length to 168 bits with
229 the same restrictions as for triple ecb mode.
235 This text was been written in large parts by Eric Young in his original
236 documentation for SSLeay, the predecessor of OpenSSL. In turn, he attributed
241 Electronic funds transfer - Requirements for interfaces,
242 Part 5.2: Modes of operation for an n-bit block cipher algorithm
247 L<BF_encrypt(3)>, L<DES_crypt(3)>
251 Copyright 2000-2017 The OpenSSL Project Authors. All Rights Reserved.
253 Licensed under the OpenSSL license (the "License"). You may not use
254 this file except in compliance with the License. You can obtain a copy
255 in the file LICENSE in the source distribution or at
256 L<https://www.openssl.org/source/license.html>.