des_modes.pod revision 1.1
1=pod 2 3=head1 NAME 4 5Modes of DES - 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 2 126 127 128=item * 129 130a number of bits (j) <= 64 are enciphered at a time. 131 132=item * 133 134The OFB mode produces the same ciphertext whenever the same 135plaintext enciphered using the same key and starting variable. More 136over, in the OFB mode the same key stream is produced when the same 137key and start variable are used. Consequently, for security reasons 138a specific start variable should be used only once for a given key. 139 140=item * 141 142The absence of chaining makes the OFB more vulnerable to specific attacks. 143 144=item * 145 146The use of different start variables values prevents the same 147plaintext enciphering to the same ciphertext, by producing different 148key streams. 149 150=item * 151 152Selection of a small value for j will require more cycles through 153the encipherment algorithm per unit of plaintext and thus cause 154greater processing overheads. 155 156=item * 157 158Only multiples of j bits can be enciphered. 159 160=item * 161 162OFB mode of operation does not extend ciphertext errors in the 163resultant plaintext output. Every bit error in the ciphertext causes 164only one bit to be in error in the deciphered plaintext. 165 166=item * 167 168OFB mode is not self-synchronizing. If the two operation of 169encipherment and decipherment get out of synchronism, the system needs 170to be re-initialized. 171 172=item * 173 174Each re-initialization should use a value of the start variable 175different from the start variable values used before with the same 176key. The reason for this is that an identical bit stream would be 177produced each time from the same parameters. This would be 178susceptible to a 'known plaintext' attack. 179 180=back 181 182=head2 Triple ECB Mode 183 184Normally, this is found as the function I<algorithm>_ecb3_encrypt(). 185 186=over 2 187 188=item * 189 190Encrypt with key1, decrypt with key2 and encrypt with key3 again. 191 192=item * 193 194As for ECB encryption but increases the key length to 168 bits. 195There are theoretic attacks that can be used that make the effective 196key length 112 bits, but this attack also requires 2^56 blocks of 197memory, not very likely, even for the NSA. 198 199=item * 200 201If both keys are the same it is equivalent to encrypting once with 202just one key. 203 204=item * 205 206If the first and last key are the same, the key length is 112 bits. 207There are attacks that could reduce the effective key strength 208to only slightly more than 56 bits, but these require a lot of memory. 209 210=item * 211 212If all 3 keys are the same, this is effectively the same as normal 213ecb mode. 214 215=back 216 217=head2 Triple CBC Mode 218 219Normally, this is found as the function I<algorithm>_ede3_cbc_encrypt(). 220 221=over 2 222 223 224=item * 225 226Encrypt with key1, decrypt with key2 and then encrypt with key3. 227 228=item * 229 230As for CBC encryption but increases the key length to 168 bits with 231the same restrictions as for triple ecb mode. 232 233=back 234 235=head1 NOTES 236 237This text was been written in large parts by Eric Young in his original 238documentation for SSLeay, the predecessor of OpenSSL. In turn, he attributed 239it to: 240 241 AS 2805.5.2 242 Australian Standard 243 Electronic funds transfer - Requirements for interfaces, 244 Part 5.2: Modes of operation for an n-bit block cipher algorithm 245 Appendix A 246 247=head1 SEE ALSO 248 249L<blowfish(3)|blowfish(3)>, L<des(3)|des(3)>, L<idea(3)|idea(3)>, 250L<rc2(3)|rc2(3)> 251 252=cut 253 254