xref: /openssl/doc/man7/des_modes.pod (revision 3187791e)
1=pod
2
3=head1 NAME
4
5des_modes - the variants of DES and other crypto algorithms of OpenSSL
6
7=head1 DESCRIPTION
8
9Several crypto algorithms for OpenSSL can be used in a number of modes.  Those
10are used for using block ciphers in a way similar to stream ciphers, among
11other things.
12
13=head1 OVERVIEW
14
15=head2 Electronic Codebook Mode (ECB)
16
17Normally, this is found as the function I<algorithm>_ecb_encrypt().
18
19=over 2
20
21=item *
22
2364 bits are enciphered at a time.
24
25=item *
26
27The order of the blocks can be rearranged without detection.
28
29=item *
30
31The same plaintext block always produces the same ciphertext block
32(for the same key) making it vulnerable to a 'dictionary attack'.
33
34=item *
35
36An error will only affect one ciphertext block.
37
38=back
39
40=head2 Cipher Block Chaining Mode (CBC)
41
42Normally, this is found as the function I<algorithm>_cbc_encrypt().
43Be aware that des_cbc_encrypt() is not really DES CBC (it does
44not update the IV); use des_ncbc_encrypt() instead.
45
46=over 2
47
48=item *
49
50a multiple of 64 bits are enciphered at a time.
51
52=item *
53
54The CBC mode produces the same ciphertext whenever the same
55plaintext is encrypted using the same key and starting variable.
56
57=item *
58
59The chaining operation makes the ciphertext blocks dependent on the
60current and all preceding plaintext blocks and therefore blocks can not
61be rearranged.
62
63=item *
64
65The use of different starting variables prevents the same plaintext
66enciphering to the same ciphertext.
67
68=item *
69
70An error will affect the current and the following ciphertext blocks.
71
72=back
73
74=head2 Cipher Feedback Mode (CFB)
75
76Normally, this is found as the function I<algorithm>_cfb_encrypt().
77
78=over 2
79
80=item *
81
82a number of bits (j) <= 64 are enciphered at a time.
83
84=item *
85
86The CFB mode produces the same ciphertext whenever the same
87plaintext is encrypted using the same key and starting variable.
88
89=item *
90
91The chaining operation makes the ciphertext variables dependent on the
92current and all preceding variables and therefore j-bit variables are
93chained together and can not be rearranged.
94
95=item *
96
97The use of different starting variables prevents the same plaintext
98enciphering to the same ciphertext.
99
100=item *
101
102The strength of the CFB mode depends on the size of k (maximal if
103j == k).  In my implementation this is always the case.
104
105=item *
106
107Selection of a small value for j will require more cycles through
108the encipherment algorithm per unit of plaintext and thus cause
109greater processing overheads.
110
111=item *
112
113Only multiples of j bits can be enciphered.
114
115=item *
116
117An error will affect the current and the following ciphertext variables.
118
119=back
120
121=head2 Output Feedback Mode (OFB)
122
123Normally, this is found as the function I<algorithm>_ofb_encrypt().
124
125=over 2
126
127=item *
128
129a number of bits (j) <= 64 are enciphered at a time.
130
131=item *
132
133The OFB mode produces the same ciphertext whenever the same
134plaintext enciphered using the same key and starting variable.  More
135over, in the OFB mode the same key stream is produced when the same
136key and start variable are used.  Consequently, for security reasons
137a specific start variable should be used only once for a given key.
138
139=item *
140
141The absence of chaining makes the OFB more vulnerable to specific attacks.
142
143=item *
144
145The use of different start variables values prevents the same
146plaintext enciphering to the same ciphertext, by producing different
147key streams.
148
149=item *
150
151Selection of a small value for j will require more cycles through
152the encipherment algorithm per unit of plaintext and thus cause
153greater processing overheads.
154
155=item *
156
157Only multiples of j bits can be enciphered.
158
159=item *
160
161OFB mode of operation does not extend ciphertext errors in the
162resultant plaintext output.  Every bit error in the ciphertext causes
163only one bit to be in error in the deciphered plaintext.
164
165=item *
166
167OFB mode is not self-synchronizing.  If the two operation of
168encipherment and decipherment get out of synchronism, the system needs
169to be re-initialized.
170
171=item *
172
173Each re-initialization should use a value of the start variable
174different from the start variable values used before with the same
175key.  The reason for this is that an identical bit stream would be
176produced each time from the same parameters.  This would be
177susceptible to a 'known plaintext' attack.
178
179=back
180
181=head2 Triple ECB Mode
182
183Normally, this is found as the function I<algorithm>_ecb3_encrypt().
184
185=over 2
186
187=item *
188
189Encrypt with key1, decrypt with key2 and encrypt with key3 again.
190
191=item *
192
193As for ECB encryption but increases the key length to 168 bits.
194There are theoretic attacks that can be used that make the effective
195key length 112 bits, but this attack also requires 2^56 blocks of
196memory, not very likely, even for the NSA.
197
198=item *
199
200If both keys are the same it is equivalent to encrypting once with
201just one key.
202
203=item *
204
205If the first and last key are the same, the key length is 112 bits.
206There are attacks that could reduce the effective key strength
207to only slightly more than 56 bits, but these require a lot of memory.
208
209=item *
210
211If all 3 keys are the same, this is effectively the same as normal
212ecb mode.
213
214=back
215
216=head2 Triple CBC Mode
217
218Normally, this is found as the function I<algorithm>_ede3_cbc_encrypt().
219
220=over 2
221
222=item *
223
224Encrypt with key1, decrypt with key2 and then encrypt with key3.
225
226=item *
227
228As for CBC encryption but increases the key length to 168 bits with
229the same restrictions as for triple ecb mode.
230
231=back
232
233=head1 NOTES
234
235This text was been written in large parts by Eric Young in his original
236documentation for SSLeay, the predecessor of OpenSSL.  In turn, he attributed
237it to:
238
239        AS 2805.5.2
240        Australian Standard
241        Electronic funds transfer - Requirements for interfaces,
242        Part 5.2: Modes of operation for an n-bit block cipher algorithm
243        Appendix A
244
245=head1 SEE ALSO
246
247L<BF_encrypt(3)>, L<DES_crypt(3)>
248
249=head1 COPYRIGHT
250
251Copyright 2000-2017 The OpenSSL Project Authors. All Rights Reserved.
252
253Licensed under the Apache License 2.0 (the "License").  You may not use
254this file except in compliance with the License.  You can obtain a copy
255in the file LICENSE in the source distribution or at
256L<https://www.openssl.org/source/license.html>.
257
258=cut
259