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126 .IX Title "DES_MODES 7"
127 .TH DES_MODES 7 "2009-07-20" "0.9.6j" "libdes"
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 Modes of DES \- 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)