Update build for OpenSSL-0.9.8j upgrade.
[dragonfly.git] / secure / lib / libcrypto / man / des_modes.7
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a561f9ff 134.IX Title "DES_MODES 7"
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135.TH DES_MODES 7 "2009-01-11" "0.9.8j" "OpenSSL"
136.\" For nroff, turn off justification. Always turn off hyphenation; it makes
137.\" way too many mistakes in technical documents.
138.if n .ad l
139.nh
984263bc 140.SH "NAME"
2c0715f4 141des_modes \- the variants of DES and other crypto algorithms of OpenSSL
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142.SH "DESCRIPTION"
143.IX Header "DESCRIPTION"
144Several crypto algorithms for OpenSSL can be used in a number of modes. Those
145are used for using block ciphers in a way similar to stream ciphers, among
146other things.
147.SH "OVERVIEW"
148.IX Header "OVERVIEW"
149.Sh "Electronic Codebook Mode (\s-1ECB\s0)"
150.IX Subsection "Electronic Codebook Mode (ECB)"
151Normally, this is found as the function \fIalgorithm\fR\fI_ecb_encrypt()\fR.
a561f9ff 152.IP "\(bu" 2
984263bc 15364 bits are enciphered at a time.
a561f9ff 154.IP "\(bu" 2
984263bc 155The order of the blocks can be rearranged without detection.
a561f9ff 156.IP "\(bu" 2
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157The same plaintext block always produces the same ciphertext block
158(for the same key) making it vulnerable to a 'dictionary attack'.
a561f9ff 159.IP "\(bu" 2
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160An error will only affect one ciphertext block.
161.Sh "Cipher Block Chaining Mode (\s-1CBC\s0)"
162.IX Subsection "Cipher Block Chaining Mode (CBC)"
163Normally, this is found as the function \fIalgorithm\fR\fI_cbc_encrypt()\fR.
164Be aware that \fIdes_cbc_encrypt()\fR is not really \s-1DES\s0 \s-1CBC\s0 (it does
165not update the \s-1IV\s0); use \fIdes_ncbc_encrypt()\fR instead.
a561f9ff 166.IP "\(bu" 2
984263bc 167a multiple of 64 bits are enciphered at a time.
a561f9ff 168.IP "\(bu" 2
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169The \s-1CBC\s0 mode produces the same ciphertext whenever the same
170plaintext is encrypted using the same key and starting variable.
a561f9ff 171.IP "\(bu" 2
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172The chaining operation makes the ciphertext blocks dependent on the
173current and all preceding plaintext blocks and therefore blocks can not
174be rearranged.
a561f9ff 175.IP "\(bu" 2
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176The use of different starting variables prevents the same plaintext
177enciphering to the same ciphertext.
a561f9ff 178.IP "\(bu" 2
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179An error will affect the current and the following ciphertext blocks.
180.Sh "Cipher Feedback Mode (\s-1CFB\s0)"
181.IX Subsection "Cipher Feedback Mode (CFB)"
182Normally, this is found as the function \fIalgorithm\fR\fI_cfb_encrypt()\fR.
a561f9ff 183.IP "\(bu" 2
984263bc 184a number of bits (j) <= 64 are enciphered at a time.
a561f9ff 185.IP "\(bu" 2
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186The \s-1CFB\s0 mode produces the same ciphertext whenever the same
187plaintext is encrypted using the same key and starting variable.
a561f9ff 188.IP "\(bu" 2
984263bc 189The chaining operation makes the ciphertext variables dependent on the
a561f9ff 190current and all preceding variables and therefore j\-bit variables are
984263bc 191chained together and can not be rearranged.
a561f9ff 192.IP "\(bu" 2
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193The use of different starting variables prevents the same plaintext
194enciphering to the same ciphertext.
a561f9ff 195.IP "\(bu" 2
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196The strength of the \s-1CFB\s0 mode depends on the size of k (maximal if
197j == k). In my implementation this is always the case.
a561f9ff 198.IP "\(bu" 2
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199Selection of a small value for j will require more cycles through
200the encipherment algorithm per unit of plaintext and thus cause
201greater processing overheads.
a561f9ff 202.IP "\(bu" 2
984263bc 203Only multiples of j bits can be enciphered.
a561f9ff 204.IP "\(bu" 2
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205An error will affect the current and the following ciphertext variables.
206.Sh "Output Feedback Mode (\s-1OFB\s0)"
207.IX Subsection "Output Feedback Mode (OFB)"
208Normally, this is found as the function \fIalgorithm\fR\fI_ofb_encrypt()\fR.
a561f9ff 209.IP "\(bu" 2
984263bc 210a number of bits (j) <= 64 are enciphered at a time.
a561f9ff 211.IP "\(bu" 2
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212The \s-1OFB\s0 mode produces the same ciphertext whenever the same
213plaintext enciphered using the same key and starting variable. More
214over, in the \s-1OFB\s0 mode the same key stream is produced when the same
215key and start variable are used. Consequently, for security reasons
216a specific start variable should be used only once for a given key.
a561f9ff 217.IP "\(bu" 2
984263bc 218The absence of chaining makes the \s-1OFB\s0 more vulnerable to specific attacks.
a561f9ff 219.IP "\(bu" 2
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220The use of different start variables values prevents the same
221plaintext enciphering to the same ciphertext, by producing different
222key streams.
a561f9ff 223.IP "\(bu" 2
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224Selection of a small value for j will require more cycles through
225the encipherment algorithm per unit of plaintext and thus cause
226greater processing overheads.
a561f9ff 227.IP "\(bu" 2
984263bc 228Only multiples of j bits can be enciphered.
a561f9ff 229.IP "\(bu" 2
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230\&\s-1OFB\s0 mode of operation does not extend ciphertext errors in the
231resultant plaintext output. Every bit error in the ciphertext causes
232only one bit to be in error in the deciphered plaintext.
a561f9ff 233.IP "\(bu" 2
e257b235 234\&\s-1OFB\s0 mode is not self-synchronizing. If the two operation of
984263bc 235encipherment and decipherment get out of synchronism, the system needs
e257b235 236to be re-initialized.
a561f9ff 237.IP "\(bu" 2
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238Each re-initialization should use a value of the start variable
239different from the start variable values used before with the same
240key. The reason for this is that an identical bit stream would be
241produced each time from the same parameters. This would be
242susceptible to a 'known plaintext' attack.
243.Sh "Triple \s-1ECB\s0 Mode"
244.IX Subsection "Triple ECB Mode"
245Normally, this is found as the function \fIalgorithm\fR\fI_ecb3_encrypt()\fR.
a561f9ff 246.IP "\(bu" 2
984263bc 247Encrypt with key1, decrypt with key2 and encrypt with key3 again.
a561f9ff 248.IP "\(bu" 2
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249As for \s-1ECB\s0 encryption but increases the key length to 168 bits.
250There are theoretic attacks that can be used that make the effective
251key length 112 bits, but this attack also requires 2^56 blocks of
252memory, not very likely, even for the \s-1NSA\s0.
a561f9ff 253.IP "\(bu" 2
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254If both keys are the same it is equivalent to encrypting once with
255just one key.
a561f9ff 256.IP "\(bu" 2
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257If the first and last key are the same, the key length is 112 bits.
258There are attacks that could reduce the effective key strength
259to only slightly more than 56 bits, but these require a lot of memory.
a561f9ff 260.IP "\(bu" 2
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261If all 3 keys are the same, this is effectively the same as normal
262ecb mode.
263.Sh "Triple \s-1CBC\s0 Mode"
264.IX Subsection "Triple CBC Mode"
265Normally, this is found as the function \fIalgorithm\fR\fI_ede3_cbc_encrypt()\fR.
a561f9ff 266.IP "\(bu" 2
984263bc 267Encrypt with key1, decrypt with key2 and then encrypt with key3.
a561f9ff 268.IP "\(bu" 2
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269As for \s-1CBC\s0 encryption but increases the key length to 168 bits with
270the same restrictions as for triple ecb mode.
271.SH "NOTES"
272.IX Header "NOTES"
273This text was been written in large parts by Eric Young in his original
274documentation for SSLeay, the predecessor of OpenSSL. In turn, he attributed
275it to:
276.PP
277.Vb 5
278\& AS 2805.5.2
279\& Australian Standard
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280\& Electronic funds transfer \- Requirements for interfaces,
281\& Part 5.2: Modes of operation for an n\-bit block cipher algorithm
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282\& Appendix A
283.Ve
284.SH "SEE ALSO"
285.IX Header "SEE ALSO"
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286\&\fIblowfish\fR\|(3), \fIdes\fR\|(3), \fIidea\fR\|(3),
287\&\fIrc2\fR\|(3)