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a561f9ff | 124 | .\" ======================================================================== |

984263bc | 125 | .\" |

a561f9ff | 126 | .IX Title "DES_MODES 7" |

e3261593 | 127 | .TH DES_MODES 7 "2012-01-04" "1.0.0f" "OpenSSL" |

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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 | |

984263bc | 132 | .SH "NAME" |

2c0715f4 | 133 | des_modes \- the variants of DES and other crypto algorithms of OpenSSL |

984263bc MD |
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" | |

01185282 | 141 | .SS "Electronic Codebook Mode (\s-1ECB\s0)" |

984263bc MD |
142 | .IX Subsection "Electronic Codebook Mode (ECB)" |

143 | Normally, this is found as the function \fIalgorithm\fR\fI_ecb_encrypt()\fR. | |

a561f9ff | 144 | .IP "\(bu" 2 |

984263bc | 145 | 64 bits are enciphered at a time. |

a561f9ff | 146 | .IP "\(bu" 2 |

984263bc | 147 | The order of the blocks can be rearranged without detection. |

a561f9ff | 148 | .IP "\(bu" 2 |

984263bc MD |
149 | The same plaintext block always produces the same ciphertext block |

150 | (for the same key) making it vulnerable to a 'dictionary attack'. | |

a561f9ff | 151 | .IP "\(bu" 2 |

984263bc | 152 | An error will only affect one ciphertext block. |

01185282 | 153 | .SS "Cipher Block Chaining Mode (\s-1CBC\s0)" |

984263bc MD |
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. | |

a561f9ff | 158 | .IP "\(bu" 2 |

984263bc | 159 | a multiple of 64 bits are enciphered at a time. |

a561f9ff | 160 | .IP "\(bu" 2 |

984263bc MD |
161 | The \s-1CBC\s0 mode produces the same ciphertext whenever the same |

162 | plaintext is encrypted using the same key and starting variable. | |

a561f9ff | 163 | .IP "\(bu" 2 |

984263bc MD |
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. | |

a561f9ff | 167 | .IP "\(bu" 2 |

984263bc MD |
168 | The use of different starting variables prevents the same plaintext |

169 | enciphering to the same ciphertext. | |

a561f9ff | 170 | .IP "\(bu" 2 |

984263bc | 171 | An error will affect the current and the following ciphertext blocks. |

01185282 | 172 | .SS "Cipher Feedback Mode (\s-1CFB\s0)" |

984263bc MD |
173 | .IX Subsection "Cipher Feedback Mode (CFB)" |

174 | Normally, this is found as the function \fIalgorithm\fR\fI_cfb_encrypt()\fR. | |

a561f9ff | 175 | .IP "\(bu" 2 |

984263bc | 176 | a number of bits (j) <= 64 are enciphered at a time. |

a561f9ff | 177 | .IP "\(bu" 2 |

984263bc MD |
178 | The \s-1CFB\s0 mode produces the same ciphertext whenever the same |

179 | plaintext is encrypted using the same key and starting variable. | |

a561f9ff | 180 | .IP "\(bu" 2 |

984263bc | 181 | The chaining operation makes the ciphertext variables dependent on the |

a561f9ff | 182 | current and all preceding variables and therefore j\-bit variables are |

984263bc | 183 | chained together and can not be rearranged. |

a561f9ff | 184 | .IP "\(bu" 2 |

984263bc MD |
185 | The use of different starting variables prevents the same plaintext |

186 | enciphering to the same ciphertext. | |

a561f9ff | 187 | .IP "\(bu" 2 |

984263bc MD |
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. | |

a561f9ff | 190 | .IP "\(bu" 2 |

984263bc MD |
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. | |

a561f9ff | 194 | .IP "\(bu" 2 |

984263bc | 195 | Only multiples of j bits can be enciphered. |

a561f9ff | 196 | .IP "\(bu" 2 |

984263bc | 197 | An error will affect the current and the following ciphertext variables. |

01185282 | 198 | .SS "Output Feedback Mode (\s-1OFB\s0)" |

984263bc MD |
199 | .IX Subsection "Output Feedback Mode (OFB)" |

200 | Normally, this is found as the function \fIalgorithm\fR\fI_ofb_encrypt()\fR. | |

a561f9ff | 201 | .IP "\(bu" 2 |

984263bc | 202 | a number of bits (j) <= 64 are enciphered at a time. |

a561f9ff | 203 | .IP "\(bu" 2 |

984263bc MD |
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. | |

a561f9ff | 209 | .IP "\(bu" 2 |

984263bc | 210 | The absence of chaining makes the \s-1OFB\s0 more vulnerable to specific attacks. |

a561f9ff | 211 | .IP "\(bu" 2 |

984263bc MD |
212 | The use of different start variables values prevents the same |

213 | plaintext enciphering to the same ciphertext, by producing different | |

214 | key streams. | |

a561f9ff | 215 | .IP "\(bu" 2 |

984263bc MD |
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. | |

a561f9ff | 219 | .IP "\(bu" 2 |

984263bc | 220 | Only multiples of j bits can be enciphered. |

a561f9ff | 221 | .IP "\(bu" 2 |

984263bc MD |
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. | |

a561f9ff | 225 | .IP "\(bu" 2 |

e257b235 | 226 | \&\s-1OFB\s0 mode is not self-synchronizing. If the two operation of |

984263bc | 227 | encipherment and decipherment get out of synchronism, the system needs |

e257b235 | 228 | to be re-initialized. |

a561f9ff | 229 | .IP "\(bu" 2 |

984263bc MD |
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. | |

01185282 | 235 | .SS "Triple \s-1ECB\s0 Mode" |

984263bc MD |
236 | .IX Subsection "Triple ECB Mode" |

237 | Normally, this is found as the function \fIalgorithm\fR\fI_ecb3_encrypt()\fR. | |

a561f9ff | 238 | .IP "\(bu" 2 |

984263bc | 239 | Encrypt with key1, decrypt with key2 and encrypt with key3 again. |

a561f9ff | 240 | .IP "\(bu" 2 |

984263bc MD |
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. | |

a561f9ff | 245 | .IP "\(bu" 2 |

984263bc MD |
246 | If both keys are the same it is equivalent to encrypting once with |

247 | just one key. | |

a561f9ff | 248 | .IP "\(bu" 2 |

984263bc MD |
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. | |

a561f9ff | 252 | .IP "\(bu" 2 |

984263bc MD |
253 | If all 3 keys are the same, this is effectively the same as normal |

254 | ecb mode. | |

01185282 | 255 | .SS "Triple \s-1CBC\s0 Mode" |

984263bc MD |
256 | .IX Subsection "Triple CBC Mode" |

257 | Normally, this is found as the function \fIalgorithm\fR\fI_ede3_cbc_encrypt()\fR. | |

a561f9ff | 258 | .IP "\(bu" 2 |

984263bc | 259 | Encrypt with key1, decrypt with key2 and then encrypt with key3. |

a561f9ff | 260 | .IP "\(bu" 2 |

984263bc MD |
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 | |

e257b235 PA |
272 | \& Electronic funds transfer \- Requirements for interfaces, |

273 | \& Part 5.2: Modes of operation for an n\-bit block cipher algorithm | |

984263bc MD |
274 | \& Appendix A |

275 | .Ve | |

276 | .SH "SEE ALSO" | |

277 | .IX Header "SEE ALSO" | |

a561f9ff SS |
278 | \&\fIblowfish\fR\|(3), \fIdes\fR\|(3), \fIidea\fR\|(3), |

279 | \&\fIrc2\fR\|(3) |