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1 | .\" $OpenBSD: BN_set_flags.3,v 1.6 2023/04/27 07:22:22 tb Exp $ | ||
2 | .\" | ||
3 | .\" Copyright (c) 2017 Ingo Schwarze <schwarze@openbsd.org> | ||
4 | .\" | ||
5 | .\" Permission to use, copy, modify, and distribute this software for any | ||
6 | .\" purpose with or without fee is hereby granted, provided that the above | ||
7 | .\" copyright notice and this permission notice appear in all copies. | ||
8 | .\" | ||
9 | .\" THE SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR DISCLAIMS ALL WARRANTIES | ||
10 | .\" WITH REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED WARRANTIES OF | ||
11 | .\" MERCHANTABILITY AND FITNESS. IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR | ||
12 | .\" ANY SPECIAL, DIRECT, INDIRECT, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES | ||
13 | .\" WHATSOEVER RESULTING FROM LOSS OF USE, DATA OR PROFITS, WHETHER IN AN | ||
14 | .\" ACTION OF CONTRACT, NEGLIGENCE OR OTHER TORTIOUS ACTION, ARISING OUT OF | ||
15 | .\" OR IN CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE. | ||
16 | .\" | ||
17 | .Dd $Mdocdate: April 27 2023 $ | ||
18 | .Dt BN_SET_FLAGS 3 | ||
19 | .Os | ||
20 | .Sh NAME | ||
21 | .Nm BN_set_flags , | ||
22 | .Nm BN_get_flags | ||
23 | .Nd enable and inspect flags on BIGNUM objects | ||
24 | .Sh SYNOPSIS | ||
25 | .In openssl/bn.h | ||
26 | .Ft void | ||
27 | .Fo BN_set_flags | ||
28 | .Fa "BIGNUM *b" | ||
29 | .Fa "int flags" | ||
30 | .Fc | ||
31 | .Ft int | ||
32 | .Fo BN_get_flags | ||
33 | .Fa "const BIGNUM *b" | ||
34 | .Fa "int flags" | ||
35 | .Fc | ||
36 | .Sh DESCRIPTION | ||
37 | .Fn BN_set_flags | ||
38 | enables the given | ||
39 | .Fa flags | ||
40 | on | ||
41 | .Fa b . | ||
42 | The | ||
43 | .Fa flags | ||
44 | argument can contain zero or more of the following constants OR'ed | ||
45 | together: | ||
46 | .Bl -tag -width Ds | ||
47 | .It Dv BN_FLG_CONSTTIME | ||
48 | If this flag is set on the divident | ||
49 | .Fa a | ||
50 | or the divisor | ||
51 | .Fa d | ||
52 | in | ||
53 | .Xr BN_div 3 , | ||
54 | on the exponent | ||
55 | .Fa p | ||
56 | in | ||
57 | .Xr BN_mod_exp 3 , | ||
58 | or on the divisor | ||
59 | .Fa a | ||
60 | or the modulus | ||
61 | .Fa n | ||
62 | in | ||
63 | .Xr BN_mod_inverse 3 , | ||
64 | these functions select algorithms with an execution time independent | ||
65 | of the respective numbers, to avoid exposing sensitive information | ||
66 | to timing side-channel attacks. | ||
67 | .Pp | ||
68 | This flag is off by default for | ||
69 | .Vt BIGNUM | ||
70 | objects created with | ||
71 | .Xr BN_new 3 . | ||
72 | .It Dv BN_FLG_MALLOCED | ||
73 | If this flag is set, | ||
74 | .Xr BN_free 3 | ||
75 | and | ||
76 | .Xr BN_clear_free 3 | ||
77 | will not only clear and free the components of | ||
78 | .Fa b , | ||
79 | but also | ||
80 | .Fa b | ||
81 | itself. | ||
82 | This flag is set internally by | ||
83 | .Xr BN_new 3 . | ||
84 | Setting it manually on an existing | ||
85 | .Vt BIGNUM | ||
86 | object is usually a bad idea and can cause calls to | ||
87 | .Xr free 3 | ||
88 | with bogus arguments. | ||
89 | .It Dv BN_FLG_STATIC_DATA | ||
90 | If this flag is set, | ||
91 | .Xr BN_clear_free 3 | ||
92 | will neither clear nor free the memory used for storing the number. | ||
93 | Consequently, setting it manually on an existing | ||
94 | .Vt BIGNUM | ||
95 | object is usually a terrible idea that can cause both disclosure | ||
96 | of secret data and memory leaks. | ||
97 | This flag is automatically set on the constant | ||
98 | .Vt BIGNUM | ||
99 | object returned by | ||
100 | .Xr BN_value_one 3 . | ||
101 | .El | ||
102 | .Pp | ||
103 | .Fn BN_get_flags | ||
104 | interprets | ||
105 | .Fa flags | ||
106 | as a bitmask and returns those of the given flags that are set in | ||
107 | .Fa b , | ||
108 | OR'ed together, or 0 if none of the given | ||
109 | .Fa flags | ||
110 | is set. | ||
111 | The | ||
112 | .Fa flags | ||
113 | argument has the same syntax as for | ||
114 | .Fn BN_set_flags . | ||
115 | .Sh RETURN VALUES | ||
116 | .Fn BN_get_flags | ||
117 | returns zero or more of the above constants, OR'ed together. | ||
118 | .Sh SEE ALSO | ||
119 | .Xr BN_mod_exp 3 , | ||
120 | .Xr BN_mod_inverse 3 , | ||
121 | .Xr BN_new 3 , | ||
122 | .Xr BN_with_flags 3 | ||
123 | .Sh HISTORY | ||
124 | .Fn BN_set_flags | ||
125 | and | ||
126 | .Fn BN_get_flags | ||
127 | first appeared in SSLeay 0.9.1 and have been available since | ||
128 | .Ox 2.6 . | ||
129 | .Sh CAVEATS | ||
130 | No public interface exists to clear a flag once it is set. | ||
131 | So think twice before using | ||
132 | .Fn BN_set_flags . | ||
133 | .Sh BUGS | ||
134 | Even if the | ||
135 | .Dv BN_FLG_CONSTTIME | ||
136 | flag is set on | ||
137 | .Fa a | ||
138 | or | ||
139 | .Fa b , | ||
140 | .Fn BN_gcd | ||
141 | neither fails nor operates in constant time, potentially allowing | ||
142 | timing side-channel attacks. | ||
143 | .Pp | ||
144 | Even if the | ||
145 | .Dv BN_FLG_CONSTTIME | ||
146 | flag is set on | ||
147 | .Fa p , | ||
148 | if the modulus | ||
149 | .Fa m | ||
150 | is even, | ||
151 | .Xr BN_mod_exp 3 | ||
152 | does not operate in constant time, potentially allowing | ||
153 | timing side-channel attacks. | ||
154 | .Pp | ||
155 | If | ||
156 | .Dv BN_FLG_CONSTTIME | ||
157 | is set on | ||
158 | .Fa p , | ||
159 | .Fn BN_exp | ||
160 | fails instead of operating in constant time. | ||