| blob | f359440a298bc80f1fe019db701096c7056e0566 |
1 .\" Automatically generated by Pod::Man 2.28 (Pod::Simple 3.28)
2 .\"
3 .\" Standard preamble:
4 .\" ========================================================================
5 .de Sp \" Vertical space (when we can't use .PP)
6 .if t .sp .5v
7 .if n .sp
8 ..
9 .de Vb \" Begin verbatim text
10 .ft CW
11 .nf
12 .ne \\$1
13 ..
14 .de Ve \" End verbatim text
15 .ft R
16 .fi
17 ..
18 .\" Set up some character translations and predefined strings. \*(-- will
19 .\" give an unbreakable dash, \*(PI will give pi, \*(L" will give a left
20 .\" double quote, and \*(R" will give a right double quote. \*(C+ will
21 .\" give a nicer C++. Capital omega is used to do unbreakable dashes and
22 .\" therefore won't be available. \*(C` and \*(C' expand to `' in nroff,
23 .\" nothing in troff, for use with C<>.
24 .tr \(*W-
25 .ds C+ C\v'-.1v'\h'-1p'\s-2+\h'-1p'+\s0\v'.1v'\h'-1p'
26 .ie n \{\
27 . ds -- \(*W-
28 . ds PI pi
29 . if (\n(.H=4u)&(1m=24u) .ds -- \(*W\h'-12u'\(*W\h'-12u'-\" diablo 10 pitch
30 . if (\n(.H=4u)&(1m=20u) .ds -- \(*W\h'-12u'\(*W\h'-8u'-\" diablo 12 pitch
31 . ds L" ""
32 . ds R" ""
33 . ds C` ""
34 . ds C' ""
35 'br\}
36 .el\{\
37 . ds -- \|\(em\|
38 . ds PI \(*p
39 . ds L" ``
40 . ds R" ''
41 . ds C`
42 . ds C'
43 'br\}
44 .\"
45 .\" Escape single quotes in literal strings from groff's Unicode transform.
46 .ie \n(.g .ds Aq \(aq
47 .el .ds Aq '
48 .\"
49 .\" If the F register is turned on, we'll generate index entries on stderr for
50 .\" titles (.TH), headers (.SH), subsections (.SS), items (.Ip), and index
51 .\" entries marked with X<> in POD. Of course, you'll have to process the
52 .\" output yourself in some meaningful fashion.
53 .\"
54 .\" Avoid warning from groff about undefined register 'F'.
55 .de IX
56 ..
57 .nr rF 0
58 .if \n(.g .if rF .nr rF 1
59 .if (\n(rF:(\n(.g==0)) \{
60 . if \nF \{
61 . de IX
62 . tm Index:\\$1\t\\n%\t"\\$2"
63 ..
64 . if !\nF==2 \{
65 . nr % 0
66 . nr F 2
67 . \}
68 . \}
69 .\}
70 .rr rF
71 .\"
72 .\" Accent mark definitions (@(#)ms.acc 1.5 88/02/08 SMI; from UCB 4.2).
73 .\" Fear. Run. Save yourself. No user-serviceable parts.
74 . \" fudge factors for nroff and troff
75 .if n \{\
76 . ds #H 0
77 . ds #V .8m
78 . ds #F .3m
79 . ds #[ \f1
80 . ds #] \fP
81 .\}
82 .if t \{\
83 . ds #H ((1u-(\\\\n(.fu%2u))*.13m)
84 . ds #V .6m
85 . ds #F 0
86 . ds #[ \&
87 . ds #] \&
88 .\}
89 . \" simple accents for nroff and troff
90 .if n \{\
91 . ds ' \&
92 . ds ` \&
93 . ds ^ \&
94 . ds , \&
95 . ds ~ ~
96 . ds /
97 .\}
98 .if t \{\
99 . ds ' \\k:\h'-(\\n(.wu*8/10-\*(#H)'\'\h"|\\n:u"
100 . ds ` \\k:\h'-(\\n(.wu*8/10-\*(#H)'\`\h'|\\n:u'
101 . ds ^ \\k:\h'-(\\n(.wu*10/11-\*(#H)'^\h'|\\n:u'
102 . ds , \\k:\h'-(\\n(.wu*8/10)',\h'|\\n:u'
103 . ds ~ \\k:\h'-(\\n(.wu-\*(#H-.1m)'~\h'|\\n:u'
104 . ds / \\k:\h'-(\\n(.wu*8/10-\*(#H)'\z\(sl\h'|\\n:u'
105 .\}
106 . \" troff and (daisy-wheel) nroff accents
107 .ds : \\k:\h'-(\\n(.wu*8/10-\*(#H+.1m+\*(#F)'\v'-\*(#V'\z.\h'.2m+\*(#F'.\h'|\\n:u'\v'\*(#V'
108 .ds 8 \h'\*(#H'\(*b\h'-\*(#H'
109 .ds o \\k:\h'-(\\n(.wu+\w'\(de'u-\*(#H)/2u'\v'-.3n'\*(#[\z\(de\v'.3n'\h'|\\n:u'\*(#]
110 .ds d- \h'\*(#H'\(pd\h'-\w'~'u'\v'-.25m'\f2\(hy\fP\v'.25m'\h'-\*(#H'
111 .ds D- D\\k:\h'-\w'D'u'\v'-.11m'\z\(hy\v'.11m'\h'|\\n:u'
112 .ds th \*(#[\v'.3m'\s+1I\s-1\v'-.3m'\h'-(\w'I'u*2/3)'\s-1o\s+1\*(#]
113 .ds Th \*(#[\s+2I\s-2\h'-\w'I'u*3/5'\v'-.3m'o\v'.3m'\*(#]
114 .ds ae a\h'-(\w'a'u*4/10)'e
115 .ds Ae A\h'-(\w'A'u*4/10)'E
116 . \" corrections for vroff
117 .if v .ds ~ \\k:\h'-(\\n(.wu*9/10-\*(#H)'\s-2\u~\d\s+2\h'|\\n:u'
118 .if v .ds ^ \\k:\h'-(\\n(.wu*10/11-\*(#H)'\v'-.4m'^\v'.4m'\h'|\\n:u'
119 . \" for low resolution devices (crt and lpr)
120 .if \n(.H>23 .if \n(.V>19 \
121 \{\
122 . ds : e
123 . ds 8 ss
124 . ds o a
125 . ds d- d\h'-1'\(ga
126 . ds D- D\h'-1'\(hy
127 . ds th \o'bp'
128 . ds Th \o'LP'
129 . ds ae ae
130 . ds Ae AE
131 .\}
132 .rm #[ #] #H #V #F C
133 .\" ========================================================================
134 .\"
135 .IX Title "DES_MODES 7"
136 .TH DES_MODES 7 "2009-07-19" "1.0.1n" "OpenSSL"
137 .\" For nroff, turn off justification. Always turn off hyphenation; it makes
138 .\" way too many mistakes in technical documents.
139 .if n .ad l
140 .nh
141 .SH "NAME"
142 des_modes \- the variants of DES and other crypto algorithms of OpenSSL
143 .SH "DESCRIPTION"
144 .IX Header "DESCRIPTION"
145 Several crypto algorithms for OpenSSL can be used in a number of modes. Those
146 are used for using block ciphers in a way similar to stream ciphers, among
147 other things.
148 .SH "OVERVIEW"
149 .IX Header "OVERVIEW"
150 .SS "Electronic Codebook Mode (\s-1ECB\s0)"
151 .IX Subsection "Electronic Codebook Mode (ECB)"
152 Normally, this is found as the function \fIalgorithm\fR\fI_ecb_encrypt()\fR.
153 .IP "\(bu" 2
154 64 bits are enciphered at a time.
155 .IP "\(bu" 2
156 The order of the blocks can be rearranged without detection.
157 .IP "\(bu" 2
158 The same plaintext block always produces the same ciphertext block
159 (for the same key) making it vulnerable to a 'dictionary attack'.
160 .IP "\(bu" 2
161 An error will only affect one ciphertext block.
162 .SS "Cipher Block Chaining Mode (\s-1CBC\s0)"
163 .IX Subsection "Cipher Block Chaining Mode (CBC)"
164 Normally, this is found as the function \fIalgorithm\fR\fI_cbc_encrypt()\fR.
165 Be aware that \fIdes_cbc_encrypt()\fR is not really \s-1DES CBC \s0(it does
166 not update the \s-1IV\s0); use \fIdes_ncbc_encrypt()\fR instead.
167 .IP "\(bu" 2
168 a multiple of 64 bits are enciphered at a time.
169 .IP "\(bu" 2
170 The \s-1CBC\s0 mode produces the same ciphertext whenever the same
171 plaintext is encrypted using the same key and starting variable.
172 .IP "\(bu" 2
173 The chaining operation makes the ciphertext blocks dependent on the
174 current and all preceding plaintext blocks and therefore blocks can not
175 be rearranged.
176 .IP "\(bu" 2
177 The use of different starting variables prevents the same plaintext
178 enciphering to the same ciphertext.
179 .IP "\(bu" 2
180 An error will affect the current and the following ciphertext blocks.
181 .SS "Cipher Feedback Mode (\s-1CFB\s0)"
182 .IX Subsection "Cipher Feedback Mode (CFB)"
183 Normally, this is found as the function \fIalgorithm\fR\fI_cfb_encrypt()\fR.
184 .IP "\(bu" 2
185 a number of bits (j) <= 64 are enciphered at a time.
186 .IP "\(bu" 2
187 The \s-1CFB\s0 mode produces the same ciphertext whenever the same
188 plaintext is encrypted using the same key and starting variable.
189 .IP "\(bu" 2
190 The chaining operation makes the ciphertext variables dependent on the
191 current and all preceding variables and therefore j\-bit variables are
192 chained together and can not be rearranged.
193 .IP "\(bu" 2
194 The use of different starting variables prevents the same plaintext
195 enciphering to the same ciphertext.
196 .IP "\(bu" 2
197 The strength of the \s-1CFB\s0 mode depends on the size of k (maximal if
198 j == k). In my implementation this is always the case.
199 .IP "\(bu" 2
200 Selection of a small value for j will require more cycles through
201 the encipherment algorithm per unit of plaintext and thus cause
202 greater processing overheads.
203 .IP "\(bu" 2
204 Only multiples of j bits can be enciphered.
205 .IP "\(bu" 2
206 An error will affect the current and the following ciphertext variables.
207 .SS "Output Feedback Mode (\s-1OFB\s0)"
208 .IX Subsection "Output Feedback Mode (OFB)"
209 Normally, this is found as the function \fIalgorithm\fR\fI_ofb_encrypt()\fR.
210 .IP "\(bu" 2
211 a number of bits (j) <= 64 are enciphered at a time.
212 .IP "\(bu" 2
213 The \s-1OFB\s0 mode produces the same ciphertext whenever the same
214 plaintext enciphered using the same key and starting variable. More
215 over, in the \s-1OFB\s0 mode the same key stream is produced when the same
216 key and start variable are used. Consequently, for security reasons
217 a specific start variable should be used only once for a given key.
218 .IP "\(bu" 2
219 The absence of chaining makes the \s-1OFB\s0 more vulnerable to specific attacks.
220 .IP "\(bu" 2
221 The use of different start variables values prevents the same
222 plaintext enciphering to the same ciphertext, by producing different
223 key streams.
224 .IP "\(bu" 2
225 Selection of a small value for j will require more cycles through
226 the encipherment algorithm per unit of plaintext and thus cause
227 greater processing overheads.
228 .IP "\(bu" 2
229 Only multiples of j bits can be enciphered.
230 .IP "\(bu" 2
231 \&\s-1OFB\s0 mode of operation does not extend ciphertext errors in the
232 resultant plaintext output. Every bit error in the ciphertext causes
233 only one bit to be in error in the deciphered plaintext.
234 .IP "\(bu" 2
235 \&\s-1OFB\s0 mode is not self-synchronizing. If the two operation of
236 encipherment and decipherment get out of synchronism, the system needs
237 to be re-initialized.
238 .IP "\(bu" 2
239 Each re-initialization should use a value of the start variable
240 different from the start variable values used before with the same
241 key. The reason for this is that an identical bit stream would be
242 produced each time from the same parameters. This would be
243 susceptible to a 'known plaintext' attack.
244 .SS "Triple \s-1ECB\s0 Mode"
245 .IX Subsection "Triple ECB Mode"
246 Normally, this is found as the function \fIalgorithm\fR\fI_ecb3_encrypt()\fR.
247 .IP "\(bu" 2
248 Encrypt with key1, decrypt with key2 and encrypt with key3 again.
249 .IP "\(bu" 2
250 As for \s-1ECB\s0 encryption but increases the key length to 168 bits.
251 There are theoretic attacks that can be used that make the effective
252 key length 112 bits, but this attack also requires 2^56 blocks of
253 memory, not very likely, even for the \s-1NSA.\s0
254 .IP "\(bu" 2
255 If both keys are the same it is equivalent to encrypting once with
256 just one key.
257 .IP "\(bu" 2
258 If the first and last key are the same, the key length is 112 bits.
259 There are attacks that could reduce the effective key strength
260 to only slightly more than 56 bits, but these require a lot of memory.
261 .IP "\(bu" 2
262 If all 3 keys are the same, this is effectively the same as normal
263 ecb mode.
264 .SS "Triple \s-1CBC\s0 Mode"
265 .IX Subsection "Triple CBC Mode"
266 Normally, this is found as the function \fIalgorithm\fR\fI_ede3_cbc_encrypt()\fR.
267 .IP "\(bu" 2
268 Encrypt with key1, decrypt with key2 and then encrypt with key3.
269 .IP "\(bu" 2
270 As for \s-1CBC\s0 encryption but increases the key length to 168 bits with
271 the same restrictions as for triple ecb mode.
272 .SH "NOTES"
273 .IX Header "NOTES"
274 This text was been written in large parts by Eric Young in his original
275 documentation for SSLeay, the predecessor of OpenSSL. In turn, he attributed
276 it to:
277 .PP
278 .Vb 5
279 \& AS 2805.5.2
280 \& Australian Standard
281 \& Electronic funds transfer \- Requirements for interfaces,
282 \& Part 5.2: Modes of operation for an n\-bit block cipher algorithm
283 \& Appendix A
284 .Ve
285 .SH "SEE ALSO"
286 .IX Header "SEE ALSO"
287 \&\fIblowfish\fR\|(3), \fIdes\fR\|(3), \fIidea\fR\|(3),
288 \&\fIrc2\fR\|(3)