xref: /openssl/doc/man3/BN_generate_prime.pod (revision 7ed6de99)
1=pod
2
3=head1 NAME
4
5BN_generate_prime_ex2, BN_generate_prime_ex, BN_is_prime_ex, BN_check_prime,
6BN_is_prime_fasttest_ex, BN_GENCB_call, BN_GENCB_new, BN_GENCB_free,
7BN_GENCB_set_old, BN_GENCB_set, BN_GENCB_get_arg, BN_generate_prime,
8BN_is_prime, BN_is_prime_fasttest - generate primes and test for primality
9
10=head1 SYNOPSIS
11
12 #include <openssl/bn.h>
13
14 int BN_generate_prime_ex2(BIGNUM *ret, int bits, int safe,
15                           const BIGNUM *add, const BIGNUM *rem, BN_GENCB *cb,
16                           BN_CTX *ctx);
17
18 int BN_generate_prime_ex(BIGNUM *ret, int bits, int safe, const BIGNUM *add,
19                          const BIGNUM *rem, BN_GENCB *cb);
20
21 int BN_check_prime(const BIGNUM *p, BN_CTX *ctx, BN_GENCB *cb);
22
23 int BN_GENCB_call(BN_GENCB *cb, int a, int b);
24
25 BN_GENCB *BN_GENCB_new(void);
26
27 void BN_GENCB_free(BN_GENCB *cb);
28
29 void BN_GENCB_set_old(BN_GENCB *gencb,
30                       void (*callback)(int, int, void *), void *cb_arg);
31
32 void BN_GENCB_set(BN_GENCB *gencb,
33                   int (*callback)(int, int, BN_GENCB *), void *cb_arg);
34
35 void *BN_GENCB_get_arg(BN_GENCB *cb);
36
37The following functions have been deprecated since OpenSSL 0.9.8, and can be
38hidden entirely by defining B<OPENSSL_API_COMPAT> with a suitable version value,
39see L<openssl_user_macros(7)>:
40
41 BIGNUM *BN_generate_prime(BIGNUM *ret, int num, int safe, BIGNUM *add,
42                           BIGNUM *rem, void (*callback)(int, int, void *),
43                           void *cb_arg);
44
45 int BN_is_prime(const BIGNUM *p, int nchecks,
46                 void (*callback)(int, int, void *), BN_CTX *ctx, void *cb_arg);
47
48 int BN_is_prime_fasttest(const BIGNUM *p, int nchecks,
49                          void (*callback)(int, int, void *), BN_CTX *ctx,
50                          void *cb_arg, int do_trial_division);
51
52The following functions have been deprecated since OpenSSL 3.0, and can be
53hidden entirely by defining B<OPENSSL_API_COMPAT> with a suitable version value,
54see L<openssl_user_macros(7)>:
55
56 int BN_is_prime_ex(const BIGNUM *p, int nchecks, BN_CTX *ctx, BN_GENCB *cb);
57
58 int BN_is_prime_fasttest_ex(const BIGNUM *p, int nchecks, BN_CTX *ctx,
59                             int do_trial_division, BN_GENCB *cb);
60
61=head1 DESCRIPTION
62
63BN_generate_prime_ex2() generates a pseudo-random prime number of
64at least bit length B<bits> using the BN_CTX provided in B<ctx>. The value of
65B<ctx> must not be NULL.
66
67The returned number is probably prime with a negligible error.
68The maximum error rate is 2^-128.
69It's 2^-287 for a 512 bit prime, 2^-435 for a 1024 bit prime,
702^-648 for a 2048 bit prime, and lower than 2^-882 for primes larger
71than 2048 bit.
72
73If B<add> is B<NULL> the returned prime number will have exact bit
74length B<bits> with the top most two bits set.
75
76If B<ret> is not B<NULL>, it will be used to store the number.
77
78If B<cb> is not B<NULL>, it is used as follows:
79
80=over 2
81
82=item *
83
84B<BN_GENCB_call(cb, 0, i)> is called after generating the i-th
85potential prime number.
86
87=item *
88
89While the number is being tested for primality,
90B<BN_GENCB_call(cb, 1, j)> is called as described below.
91
92=item *
93
94When a prime has been found, B<BN_GENCB_call(cb, 2, i)> is called.
95
96=item *
97
98The callers of BN_generate_prime_ex() may call B<BN_GENCB_call(cb, i, j)> with
99other values as described in their respective man pages; see L</SEE ALSO>.
100
101=back
102
103The prime may have to fulfill additional requirements for use in
104Diffie-Hellman key exchange:
105
106If B<add> is not B<NULL>, the prime will fulfill the condition p % B<add>
107== B<rem> (p % B<add> == 1 if B<rem> == B<NULL>) in order to suit a given
108generator.
109
110If B<safe> is true, it will be a safe prime (i.e. a prime p so
111that (p-1)/2 is also prime). If B<safe> is true, and B<rem> == B<NULL>
112the condition will be p % B<add> == 3.
113It is recommended that B<add> is a multiple of 4.
114
115The random generator must be seeded prior to calling BN_generate_prime_ex().
116If the automatic seeding or reseeding of the OpenSSL CSPRNG fails due to
117external circumstances (see L<RAND(7)>), the operation will fail.
118The random number generator configured for the OSSL_LIB_CTX associated with
119B<ctx> will be used.
120
121BN_generate_prime_ex() is the same as BN_generate_prime_ex2() except that no
122B<ctx> parameter is passed.
123In this case the random number generator associated with the default OSSL_LIB_CTX
124will be used.
125
126BN_check_prime(), BN_is_prime_ex(), BN_is_prime_fasttest_ex(), BN_is_prime()
127and BN_is_prime_fasttest() test if the number B<p> is prime.
128The functions tests until one of the tests shows that B<p> is composite,
129or all the tests passed.
130If B<p> passes all these tests, it is considered a probable prime.
131
132The test performed on B<p> are trial division by a number of small primes
133and rounds of the of the Miller-Rabin probabilistic primality test.
134
135The functions do at least 64 rounds of the Miller-Rabin test giving a maximum
136false positive rate of 2^-128.
137If the size of B<p> is more than 2048 bits, they do at least 128 rounds
138giving a maximum false positive rate of 2^-256.
139
140If B<nchecks> is larger than the minimum above (64 or 128), B<nchecks>
141rounds of the Miller-Rabin test will be done.
142
143If B<do_trial_division> set to B<0>, the trial division will be skipped.
144BN_is_prime_ex() and BN_is_prime() always skip the trial division.
145
146BN_is_prime_ex(), BN_is_prime_fasttest_ex(), BN_is_prime()
147and BN_is_prime_fasttest() are deprecated.
148
149BN_is_prime_fasttest() and BN_is_prime() behave just like
150BN_is_prime_fasttest_ex() and BN_is_prime_ex() respectively, but with the old
151style call back.
152
153B<ctx> is a preallocated B<BN_CTX> (to save the overhead of allocating and
154freeing the structure in a loop), or B<NULL>.
155
156If the trial division is done, and no divisors are found and B<cb>
157is not B<NULL>, B<BN_GENCB_call(cb, 1, -1)> is called.
158
159After each round of the Miller-Rabin probabilistic primality test,
160if B<cb> is not B<NULL>, B<BN_GENCB_call(cb, 1, j)> is called
161with B<j> the iteration (j = 0, 1, ...).
162
163BN_GENCB_call() calls the callback function held in the B<BN_GENCB> structure
164and passes the ints B<a> and B<b> as arguments. There are two types of
165B<BN_GENCB> structure that are supported: "new" style and "old" style. New
166programs should prefer the "new" style, whilst the "old" style is provided
167for backwards compatibility purposes.
168
169A B<BN_GENCB> structure should be created through a call to BN_GENCB_new(),
170and freed through a call to BN_GENCB_free(). If the argument is NULL,
171nothing is done.
172
173For "new" style callbacks a BN_GENCB structure should be initialised with a
174call to BN_GENCB_set(), where B<gencb> is a B<BN_GENCB *>, B<callback> is of
175type B<int (*callback)(int, int, BN_GENCB *)> and B<cb_arg> is a B<void *>.
176"Old" style callbacks are the same except they are initialised with a call
177to BN_GENCB_set_old() and B<callback> is of type
178B<void (*callback)(int, int, void *)>.
179
180A callback is invoked through a call to B<BN_GENCB_call>. This will check
181the type of the callback and will invoke B<callback(a, b, gencb)> for new
182style callbacks or B<callback(a, b, cb_arg)> for old style.
183
184It is possible to obtain the argument associated with a BN_GENCB structure
185(set via a call to BN_GENCB_set or BN_GENCB_set_old) using BN_GENCB_get_arg.
186
187BN_generate_prime() (deprecated) works in the same way as
188BN_generate_prime_ex() but expects an old-style callback function
189directly in the B<callback> parameter, and an argument to pass to it in
190the B<cb_arg>. BN_is_prime() and BN_is_prime_fasttest()
191can similarly be compared to BN_is_prime_ex() and
192BN_is_prime_fasttest_ex(), respectively.
193
194=head1 RETURN VALUES
195
196BN_generate_prime_ex() return 1 on success or 0 on error.
197
198BN_is_prime_ex(), BN_is_prime_fasttest_ex(), BN_is_prime(),
199BN_is_prime_fasttest() and BN_check_prime return 0 if the number is composite,
2001 if it is prime with an error probability of less than 0.25^B<nchecks>, and
201-1 on error.
202
203BN_generate_prime() returns the prime number on success, B<NULL> otherwise.
204
205BN_GENCB_new returns a pointer to a BN_GENCB structure on success, or B<NULL>
206otherwise.
207
208BN_GENCB_get_arg returns the argument previously associated with a BN_GENCB
209structure.
210
211Callback functions should return 1 on success or 0 on error.
212
213The error codes can be obtained by L<ERR_get_error(3)>.
214
215=head1 REMOVED FUNCTIONALITY
216
217As of OpenSSL 1.1.0 it is no longer possible to create a BN_GENCB structure
218directly, as in:
219
220 BN_GENCB callback;
221
222Instead applications should create a BN_GENCB structure using BN_GENCB_new:
223
224 BN_GENCB *callback;
225 callback = BN_GENCB_new();
226 if (!callback)
227     /* error */
228 ...
229 BN_GENCB_free(callback);
230
231=head1 SEE ALSO
232
233L<DH_generate_parameters(3)>, L<DSA_generate_parameters(3)>,
234L<RSA_generate_key(3)>, L<ERR_get_error(3)>, L<RAND_bytes(3)>,
235L<RAND(7)>
236
237=head1 HISTORY
238
239The BN_is_prime_ex() and BN_is_prime_fasttest_ex() functions were
240deprecated in OpenSSL 3.0.
241
242The BN_GENCB_new(), BN_GENCB_free(),
243and BN_GENCB_get_arg() functions were added in OpenSSL 1.1.0.
244
245BN_check_prime() was added in OpenSSL 3.0.
246
247=head1 COPYRIGHT
248
249Copyright 2000-2024 The OpenSSL Project Authors. All Rights Reserved.
250
251Licensed under the Apache License 2.0 (the "License").  You may not use
252this file except in compliance with the License.  You can obtain a copy
253in the file LICENSE in the source distribution or at
254L<https://www.openssl.org/source/license.html>.
255
256=cut
257