1=pod 2 3=head1 NAME 4 5des_modes - the variants of DES and other crypto algorithms of OpenSSL 6 7=head1 DESCRIPTION 8 9Several crypto algorithms for OpenSSL can be used in a number of modes. Those 10are used for using block ciphers in a way similar to stream ciphers, among 11other things. 12 13=head1 OVERVIEW 14 15=head2 Electronic Codebook Mode (ECB) 16 17Normally, this is found as the function I<algorithm>_ecb_encrypt(). 18 19=over 2 20 21=item * 22 2364 bits are enciphered at a time. 24 25=item * 26 27The order of the blocks can be rearranged without detection. 28 29=item * 30 31The same plaintext block always produces the same ciphertext block 32(for the same key) making it vulnerable to a 'dictionary attack'. 33 34=item * 35 36An error will only affect one ciphertext block. 37 38=back 39 40=head2 Cipher Block Chaining Mode (CBC) 41 42Normally, this is found as the function I<algorithm>_cbc_encrypt(). 43Be aware that des_cbc_encrypt() is not really DES CBC (it does 44not update the IV); use des_ncbc_encrypt() instead. 45 46=over 2 47 48=item * 49 50a multiple of 64 bits are enciphered at a time. 51 52=item * 53 54The CBC mode produces the same ciphertext whenever the same 55plaintext is encrypted using the same key and starting variable. 56 57=item * 58 59The chaining operation makes the ciphertext blocks dependent on the 60current and all preceding plaintext blocks and therefore blocks can not 61be rearranged. 62 63=item * 64 65The use of different starting variables prevents the same plaintext 66enciphering to the same ciphertext. 67 68=item * 69 70An error will affect the current and the following ciphertext blocks. 71 72=back 73 74=head2 Cipher Feedback Mode (CFB) 75 76Normally, this is found as the function I<algorithm>_cfb_encrypt(). 77 78=over 2 79 80=item * 81 82a number of bits (j) <= 64 are enciphered at a time. 83 84=item * 85 86The CFB mode produces the same ciphertext whenever the same 87plaintext is encrypted using the same key and starting variable. 88 89=item * 90 91The chaining operation makes the ciphertext variables dependent on the 92current and all preceding variables and therefore j-bit variables are 93chained together and can not be rearranged. 94 95=item * 96 97The use of different starting variables prevents the same plaintext 98enciphering to the same ciphertext. 99 100=item * 101 102The strength of the CFB mode depends on the size of k (maximal if 103j == k). In my implementation this is always the case. 104 105=item * 106 107Selection of a small value for j will require more cycles through 108the encipherment algorithm per unit of plaintext and thus cause 109greater processing overheads. 110 111=item * 112 113Only multiples of j bits can be enciphered. 114 115=item * 116 117An error will affect the current and the following ciphertext variables. 118 119=back 120 121=head2 Output Feedback Mode (OFB) 122 123Normally, this is found as the function I<algorithm>_ofb_encrypt(). 124 125=over 2 126 127=item * 128 129a number of bits (j) <= 64 are enciphered at a time. 130 131=item * 132 133The OFB mode produces the same ciphertext whenever the same 134plaintext enciphered using the same key and starting variable. More 135over, in the OFB mode the same key stream is produced when the same 136key and start variable are used. Consequently, for security reasons 137a specific start variable should be used only once for a given key. 138 139=item * 140 141The absence of chaining makes the OFB more vulnerable to specific attacks. 142 143=item * 144 145The use of different start variables values prevents the same 146plaintext enciphering to the same ciphertext, by producing different 147key streams. 148 149=item * 150 151Selection of a small value for j will require more cycles through 152the encipherment algorithm per unit of plaintext and thus cause 153greater processing overheads. 154 155=item * 156 157Only multiples of j bits can be enciphered. 158 159=item * 160 161OFB mode of operation does not extend ciphertext errors in the 162resultant plaintext output. Every bit error in the ciphertext causes 163only one bit to be in error in the deciphered plaintext. 164 165=item * 166 167OFB mode is not self-synchronizing. If the two operation of 168encipherment and decipherment get out of synchronism, the system needs 169to be re-initialized. 170 171=item * 172 173Each re-initialization should use a value of the start variable 174different from the start variable values used before with the same 175key. The reason for this is that an identical bit stream would be 176produced each time from the same parameters. This would be 177susceptible to a 'known plaintext' attack. 178 179=back 180 181=head2 Triple ECB Mode 182 183Normally, this is found as the function I<algorithm>_ecb3_encrypt(). 184 185=over 2 186 187=item * 188 189Encrypt with key1, decrypt with key2 and encrypt with key3 again. 190 191=item * 192 193As for ECB encryption but increases the key length to 168 bits. 194There are theoretic attacks that can be used that make the effective 195key length 112 bits, but this attack also requires 2^56 blocks of 196memory, not very likely, even for the NSA. 197 198=item * 199 200If both keys are the same it is equivalent to encrypting once with 201just one key. 202 203=item * 204 205If the first and last key are the same, the key length is 112 bits. 206There are attacks that could reduce the effective key strength 207to only slightly more than 56 bits, but these require a lot of memory. 208 209=item * 210 211If all 3 keys are the same, this is effectively the same as normal 212ecb mode. 213 214=back 215 216=head2 Triple CBC Mode 217 218Normally, this is found as the function I<algorithm>_ede3_cbc_encrypt(). 219 220=over 2 221 222=item * 223 224Encrypt with key1, decrypt with key2 and then encrypt with key3. 225 226=item * 227 228As for CBC encryption but increases the key length to 168 bits with 229the same restrictions as for triple ecb mode. 230 231=back 232 233=head1 NOTES 234 235This text was been written in large parts by Eric Young in his original 236documentation for SSLeay, the predecessor of OpenSSL. In turn, he attributed 237it to: 238 239 AS 2805.5.2 240 Australian Standard 241 Electronic funds transfer - Requirements for interfaces, 242 Part 5.2: Modes of operation for an n-bit block cipher algorithm 243 Appendix A 244 245=head1 SEE ALSO 246 247L<BF_encrypt(3)>, L<DES_crypt(3)> 248 249=head1 COPYRIGHT 250 251Copyright 2000-2017 The OpenSSL Project Authors. All Rights Reserved. 252 253Licensed under the Apache License 2.0 (the "License"). You may not use 254this file except in compliance with the License. You can obtain a copy 255in the file LICENSE in the source distribution or at 256L<https://www.openssl.org/source/license.html>. 257 258=cut 259
