des_modes(7ssl)

1DES_MODES(7)                        OpenSSL                       DES_MODES(7)
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NAME

6       des_modes - the variants of DES and other crypto algorithms of OpenSSL
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DESCRIPTION

9       Several crypto algorithms for OpenSSL can be used in a number of modes.
10       Those are used for using block ciphers in a way similar to stream
11       ciphers, among other things.
12

OVERVIEW

14   Electronic Codebook Mode (ECB)
15       Normally, this is found as the function algorithm_ecb_encrypt().
16
17       · 64 bits are enciphered at a time.
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19       · The order of the blocks can be rearranged without detection.
20
21       · The same plaintext block always produces the same ciphertext block
22         (for the same key) making it vulnerable to a 'dictionary attack'.
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24       · An error will only affect one ciphertext block.
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26   Cipher Block Chaining Mode (CBC)
27       Normally, this is found as the function algorithm_cbc_encrypt().  Be
28       aware that des_cbc_encrypt() is not really DES CBC (it does not update
29       the IV); use des_ncbc_encrypt() instead.
30
31       · a multiple of 64 bits are enciphered at a time.
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33       · The CBC mode produces the same ciphertext whenever the same plaintext
34         is encrypted using the same key and starting variable.
35
36       · The chaining operation makes the ciphertext blocks dependent on the
37         current and all preceding plaintext blocks and therefore blocks can
38         not be rearranged.
39
40       · The use of different starting variables prevents the same plaintext
41         enciphering to the same ciphertext.
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43       · An error will affect the current and the following ciphertext blocks.
44
45   Cipher Feedback Mode (CFB)
46       Normally, this is found as the function algorithm_cfb_encrypt().
47
48       · a number of bits (j) <= 64 are enciphered at a time.
49
50       · The CFB mode produces the same ciphertext whenever the same plaintext
51         is encrypted using the same key and starting variable.
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53       · The chaining operation makes the ciphertext variables dependent on
54         the current and all preceding variables and therefore j-bit variables
55         are chained together and can not be rearranged.
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57       · The use of different starting variables prevents the same plaintext
58         enciphering to the same ciphertext.
59
60       · The strength of the CFB mode depends on the size of k (maximal if j
61         == k).  In my implementation this is always the case.
62
63       · Selection of a small value for j will require more cycles through the
64         encipherment algorithm per unit of plaintext and thus cause greater
65         processing overheads.
66
67       · Only multiples of j bits can be enciphered.
68
69       · An error will affect the current and the following ciphertext
70         variables.
71
72   Output Feedback Mode (OFB)
73       Normally, this is found as the function algorithm_ofb_encrypt().
74
75       · a number of bits (j) <= 64 are enciphered at a time.
76
77       · The OFB mode produces the same ciphertext whenever the same plaintext
78         enciphered using the same key and starting variable.  More over, in
79         the OFB mode the same key stream is produced when the same key and
80         start variable are used.  Consequently, for security reasons a
81         specific start variable should be used only once for a given key.
82
83       · The absence of chaining makes the OFB more vulnerable to specific
84         attacks.
85
86       · The use of different start variables values prevents the same
87         plaintext enciphering to the same ciphertext, by producing different
88         key streams.
89
90       · Selection of a small value for j will require more cycles through the
91         encipherment algorithm per unit of plaintext and thus cause greater
92         processing overheads.
93
94       · Only multiples of j bits can be enciphered.
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96       · OFB mode of operation does not extend ciphertext errors in the
97         resultant plaintext output.  Every bit error in the ciphertext causes
98         only one bit to be in error in the deciphered plaintext.
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100       · OFB mode is not self-synchronizing.  If the two operation of
101         encipherment and decipherment get out of synchronism, the system
102         needs to be re-initialized.
103
104       · Each re-initialization should use a value of the start variable
105         different from the start variable values used before with the same
106         key.  The reason for this is that an identical bit stream would be
107         produced each time from the same parameters.  This would be
108         susceptible to a 'known plaintext' attack.
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110   Triple ECB Mode
111       Normally, this is found as the function algorithm_ecb3_encrypt().
112
113       · Encrypt with key1, decrypt with key2 and encrypt with key3 again.
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115       · As for ECB encryption but increases the key length to 168 bits.
116         There are theoretic attacks that can be used that make the effective
117         key length 112 bits, but this attack also requires 2^56 blocks of
118         memory, not very likely, even for the NSA.
119
120       · If both keys are the same it is equivalent to encrypting once with
121         just one key.
122
123       · If the first and last key are the same, the key length is 112 bits.
124         There are attacks that could reduce the effective key strength to
125         only slightly more than 56 bits, but these require a lot of memory.
126
127       · If all 3 keys are the same, this is effectively the same as normal
128         ecb mode.
129
130   Triple CBC Mode
131       Normally, this is found as the function algorithm_ede3_cbc_encrypt().
132
133       · Encrypt with key1, decrypt with key2 and then encrypt with key3.
134
135       · As for CBC encryption but increases the key length to 168 bits with
136         the same restrictions as for triple ecb mode.
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NOTES

139       This text was been written in large parts by Eric Young in his original
140       documentation for SSLeay, the predecessor of OpenSSL.  In turn, he
141       attributed it to:
142
143               AS 2805.5.2
144               Australian Standard
145               Electronic funds transfer - Requirements for interfaces,
146               Part 5.2: Modes of operation for an n-bit block cipher algorithm
147               Appendix A
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NAME

DESCRIPTION

OVERVIEW

NOTES

SEE ALSO