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| 124 | .\" ======================================================================== |
| 125 | .\" |
| 126 | .IX Title "DES_MODES 7" |
| 127 | .TH DES_MODES 7 "2012-01-04" "1.0.0f" "OpenSSL" |
| 128 | .\" For nroff, turn off justification. Always turn off hyphenation; it makes |
| 129 | .\" way too many mistakes in technical documents. |
| 130 | .if n .ad l |
| 131 | .nh |
| 132 | .SH "NAME" |
| 133 | des_modes \- the variants of DES and other crypto algorithms of OpenSSL |
| 134 | .SH "DESCRIPTION" |
| 135 | .IX Header "DESCRIPTION" |
| 136 | Several crypto algorithms for OpenSSL can be used in a number of modes. Those |
| 137 | are used for using block ciphers in a way similar to stream ciphers, among |
| 138 | other things. |
| 139 | .SH "OVERVIEW" |
| 140 | .IX Header "OVERVIEW" |
| 141 | .SS "Electronic Codebook Mode (\s-1ECB\s0)" |
| 142 | .IX Subsection "Electronic Codebook Mode (ECB)" |
| 143 | Normally, this is found as the function \fIalgorithm\fR\fI_ecb_encrypt()\fR. |
| 144 | .IP "\(bu" 2 |
| 145 | 64 bits are enciphered at a time. |
| 146 | .IP "\(bu" 2 |
| 147 | The order of the blocks can be rearranged without detection. |
| 148 | .IP "\(bu" 2 |
| 149 | The same plaintext block always produces the same ciphertext block |
| 150 | (for the same key) making it vulnerable to a 'dictionary attack'. |
| 151 | .IP "\(bu" 2 |
| 152 | An error will only affect one ciphertext block. |
| 153 | .SS "Cipher Block Chaining Mode (\s-1CBC\s0)" |
| 154 | .IX Subsection "Cipher Block Chaining Mode (CBC)" |
| 155 | Normally, this is found as the function \fIalgorithm\fR\fI_cbc_encrypt()\fR. |
| 156 | Be aware that \fIdes_cbc_encrypt()\fR is not really \s-1DES\s0 \s-1CBC\s0 (it does |
| 157 | not update the \s-1IV\s0); use \fIdes_ncbc_encrypt()\fR instead. |
| 158 | .IP "\(bu" 2 |
| 159 | a multiple of 64 bits are enciphered at a time. |
| 160 | .IP "\(bu" 2 |
| 161 | The \s-1CBC\s0 mode produces the same ciphertext whenever the same |
| 162 | plaintext is encrypted using the same key and starting variable. |
| 163 | .IP "\(bu" 2 |
| 164 | The chaining operation makes the ciphertext blocks dependent on the |
| 165 | current and all preceding plaintext blocks and therefore blocks can not |
| 166 | be rearranged. |
| 167 | .IP "\(bu" 2 |
| 168 | The use of different starting variables prevents the same plaintext |
| 169 | enciphering to the same ciphertext. |
| 170 | .IP "\(bu" 2 |
| 171 | An error will affect the current and the following ciphertext blocks. |
| 172 | .SS "Cipher Feedback Mode (\s-1CFB\s0)" |
| 173 | .IX Subsection "Cipher Feedback Mode (CFB)" |
| 174 | Normally, this is found as the function \fIalgorithm\fR\fI_cfb_encrypt()\fR. |
| 175 | .IP "\(bu" 2 |
| 176 | a number of bits (j) <= 64 are enciphered at a time. |
| 177 | .IP "\(bu" 2 |
| 178 | The \s-1CFB\s0 mode produces the same ciphertext whenever the same |
| 179 | plaintext is encrypted using the same key and starting variable. |
| 180 | .IP "\(bu" 2 |
| 181 | The chaining operation makes the ciphertext variables dependent on the |
| 182 | current and all preceding variables and therefore j\-bit variables are |
| 183 | chained together and can not be rearranged. |
| 184 | .IP "\(bu" 2 |
| 185 | The use of different starting variables prevents the same plaintext |
| 186 | enciphering to the same ciphertext. |
| 187 | .IP "\(bu" 2 |
| 188 | The strength of the \s-1CFB\s0 mode depends on the size of k (maximal if |
| 189 | j == k). In my implementation this is always the case. |
| 190 | .IP "\(bu" 2 |
| 191 | Selection of a small value for j will require more cycles through |
| 192 | the encipherment algorithm per unit of plaintext and thus cause |
| 193 | greater processing overheads. |
| 194 | .IP "\(bu" 2 |
| 195 | Only multiples of j bits can be enciphered. |
| 196 | .IP "\(bu" 2 |
| 197 | An error will affect the current and the following ciphertext variables. |
| 198 | .SS "Output Feedback Mode (\s-1OFB\s0)" |
| 199 | .IX Subsection "Output Feedback Mode (OFB)" |
| 200 | Normally, this is found as the function \fIalgorithm\fR\fI_ofb_encrypt()\fR. |
| 201 | .IP "\(bu" 2 |
| 202 | a number of bits (j) <= 64 are enciphered at a time. |
| 203 | .IP "\(bu" 2 |
| 204 | The \s-1OFB\s0 mode produces the same ciphertext whenever the same |
| 205 | plaintext enciphered using the same key and starting variable. More |
| 206 | over, in the \s-1OFB\s0 mode the same key stream is produced when the same |
| 207 | key and start variable are used. Consequently, for security reasons |
| 208 | a specific start variable should be used only once for a given key. |
| 209 | .IP "\(bu" 2 |
| 210 | The absence of chaining makes the \s-1OFB\s0 more vulnerable to specific attacks. |
| 211 | .IP "\(bu" 2 |
| 212 | The use of different start variables values prevents the same |
| 213 | plaintext enciphering to the same ciphertext, by producing different |
| 214 | key streams. |
| 215 | .IP "\(bu" 2 |
| 216 | Selection of a small value for j will require more cycles through |
| 217 | the encipherment algorithm per unit of plaintext and thus cause |
| 218 | greater processing overheads. |
| 219 | .IP "\(bu" 2 |
| 220 | Only multiples of j bits can be enciphered. |
| 221 | .IP "\(bu" 2 |
| 222 | \&\s-1OFB\s0 mode of operation does not extend ciphertext errors in the |
| 223 | resultant plaintext output. Every bit error in the ciphertext causes |
| 224 | only one bit to be in error in the deciphered plaintext. |
| 225 | .IP "\(bu" 2 |
| 226 | \&\s-1OFB\s0 mode is not self-synchronizing. If the two operation of |
| 227 | encipherment and decipherment get out of synchronism, the system needs |
| 228 | to be re-initialized. |
| 229 | .IP "\(bu" 2 |
| 230 | Each re-initialization should use a value of the start variable |
| 231 | different from the start variable values used before with the same |
| 232 | key. The reason for this is that an identical bit stream would be |
| 233 | produced each time from the same parameters. This would be |
| 234 | susceptible to a 'known plaintext' attack. |
| 235 | .SS "Triple \s-1ECB\s0 Mode" |
| 236 | .IX Subsection "Triple ECB Mode" |
| 237 | Normally, this is found as the function \fIalgorithm\fR\fI_ecb3_encrypt()\fR. |
| 238 | .IP "\(bu" 2 |
| 239 | Encrypt with key1, decrypt with key2 and encrypt with key3 again. |
| 240 | .IP "\(bu" 2 |
| 241 | As for \s-1ECB\s0 encryption but increases the key length to 168 bits. |
| 242 | There are theoretic attacks that can be used that make the effective |
| 243 | key length 112 bits, but this attack also requires 2^56 blocks of |
| 244 | memory, not very likely, even for the \s-1NSA\s0. |
| 245 | .IP "\(bu" 2 |
| 246 | If both keys are the same it is equivalent to encrypting once with |
| 247 | just one key. |
| 248 | .IP "\(bu" 2 |
| 249 | If the first and last key are the same, the key length is 112 bits. |
| 250 | There are attacks that could reduce the effective key strength |
| 251 | to only slightly more than 56 bits, but these require a lot of memory. |
| 252 | .IP "\(bu" 2 |
| 253 | If all 3 keys are the same, this is effectively the same as normal |
| 254 | ecb mode. |
| 255 | .SS "Triple \s-1CBC\s0 Mode" |
| 256 | .IX Subsection "Triple CBC Mode" |
| 257 | Normally, this is found as the function \fIalgorithm\fR\fI_ede3_cbc_encrypt()\fR. |
| 258 | .IP "\(bu" 2 |
| 259 | Encrypt with key1, decrypt with key2 and then encrypt with key3. |
| 260 | .IP "\(bu" 2 |
| 261 | As for \s-1CBC\s0 encryption but increases the key length to 168 bits with |
| 262 | the same restrictions as for triple ecb mode. |
| 263 | .SH "NOTES" |
| 264 | .IX Header "NOTES" |
| 265 | This text was been written in large parts by Eric Young in his original |
| 266 | documentation for SSLeay, the predecessor of OpenSSL. In turn, he attributed |
| 267 | it to: |
| 268 | .PP |
| 269 | .Vb 5 |
| 270 | \& AS 2805.5.2 |
| 271 | \& Australian Standard |
| 272 | \& Electronic funds transfer \- Requirements for interfaces, |
| 273 | \& Part 5.2: Modes of operation for an n\-bit block cipher algorithm |
| 274 | \& Appendix A |
| 275 | .Ve |
| 276 | .SH "SEE ALSO" |
| 277 | .IX Header "SEE ALSO" |
| 278 | \&\fIblowfish\fR\|(3), \fIdes\fR\|(3), \fIidea\fR\|(3), |
| 279 | \&\fIrc2\fR\|(3) |