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BN_GENCB_set_old.3
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If \fBret\fR is not \fB\s-1NULL\s0\fR, it will be used to store the number. .PP If \fBcb\fR is not \fB\s-1NULL\s0\fR, it is used as follows: .IP "\(bu" 4 \&\fBBN_GENCB_call(cb, 0, i)\fR is called after generating the i\-th potential prime number. .IP "\(bu" 4 While the number is being tested for primality, \&\fBBN_GENCB_call(cb, 1, j)\fR is called as described below. .IP "\(bu" 4 When a prime has been found, \fBBN_GENCB_call(cb, 2, i)\fR is called. .PP The prime may have to fulfill additional requirements for use in Diffie-Hellman key exchange: .PP If \fBadd\fR is not \fB\s-1NULL\s0\fR, the prime will fulfill the condition p % \fBadd\fR == \fBrem\fR (p % \fBadd\fR == 1 if \fBrem\fR == \fB\s-1NULL\s0\fR) in order to suit a given generator. .PP If \fBsafe\fR is true, it will be a safe prime (i.e. a prime p so that (p\-1)/2 is also prime). .PP The \s-1PRNG\s0 must be seeded prior to calling \fIBN_generate_prime_ex()\fR. The prime number generation has a negligible error probability. .PP \&\fIBN_is_prime_ex()\fR and \fIBN_is_prime_fasttest_ex()\fR test if the number \fBp\fR is prime. The following tests are performed until one of them shows that \&\fBp\fR is composite; if \fBp\fR passes all these tests, it is considered prime. .PP \&\fIBN_is_prime_fasttest_ex()\fR, when called with \fBdo_trial_division == 1\fR, first attempts trial division by a number of small primes; if no divisors are found by this test and \fBcb\fR is not \fB\s-1NULL\s0\fR, \&\fBBN_GENCB_call(cb, 1, \-1)\fR is called. If \fBdo_trial_division == 0\fR, this test is skipped. .PP Both \fIBN_is_prime_ex()\fR and \fIBN_is_prime_fasttest_ex()\fR perform a Miller-Rabin probabilistic primality test with \fBnchecks\fR iterations. If \&\fBnchecks == BN_prime_checks\fR, a number of iterations is used that yields a false positive rate of at most 2^\-80 for random input. .PP If \fBcb\fR is not \fB\s-1NULL\s0\fR, \fBBN_GENCB_call(cb, 1, j)\fR is called after the j\-th iteration (j = 0, 1, ...). \fBctx\fR is a pre-allocated \fB\s-1BN_CTX\s0\fR (to save the overhead of allocating and freeing the structure in a loop), or \fB\s-1NULL\s0\fR. .PP BN_GENCB_call calls the callback function held in the \fB\s-1BN_GENCB\s0\fR structure and passes the ints \fBa\fR and \fBb\fR as arguments. There are two types of \&\fB\s-1BN_GENCB\s0\fR structure that are supported: \*(L"new\*(R" style and \*(L"old\*(R" style. New programs should prefer the \*(L"new\*(R" style, whilst the \*(L"old\*(R" style is provided for backwards compatibility purposes. .PP For \*(L"new\*(R" style callbacks a \s-1BN_GENCB\s0 structure should be initialised with a call to BN_GENCB_set, where \fBgencb\fR is a \fB\s-1BN_GENCB\s0 *\fR, \fBcallback\fR is of type \fBint (*callback)(int, int, \s-1BN_GENCB\s0 *)\fR and \fBcb_arg\fR is a \fBvoid *\fR. \&\*(L"Old\*(R" style callbacks are the same except they are initialised with a call to BN_GENCB_set_old and \fBcallback\fR is of type \&\fBvoid (*callback)(int, int, void *)\fR. .PP A callback is invoked through a call to \fBBN_GENCB_call\fR. This will check the type of the callback and will invoke \fBcallback(a, b, gencb)\fR for new style callbacks or \fBcallback(a, b, cb_arg)\fR for old style. .PP BN_generate_prime (deprecated) works in the same way as BN_generate_prime_ex but expects an old style callback function directly in the \fBcallback\fR parameter, and an argument to pass to it in the \fBcb_arg\fR. Similarly BN_is_prime and BN_is_prime_fasttest are deprecated and can be compared to BN_is_prime_ex and BN_is_prime_fasttest_ex respectively. .SH "RETURN VALUES" .IX Header "RETURN VALUES" \&\fIBN_generate_prime_ex()\fR return 1 on success or 0 on error. .PP \&\fIBN_is_prime_ex()\fR, \fIBN_is_prime_fasttest_ex()\fR, \fIBN_is_prime()\fR and \&\fIBN_is_prime_fasttest()\fR return 0 if the number is composite, 1 if it is prime with an error probability of less than 0.25^\fBnchecks\fR, and \&\-1 on error. .PP \&\fIBN_generate_prime()\fR returns the prime number on success, \fB\s-1NULL\s0\fR otherwise. .PP Callback functions should return 1 on success or 0 on error. .PP The error codes can be obtained by \fIERR_get_error\fR\|(3). .SH "SEE ALSO" .IX Header "SEE ALSO" \&\fIbn\fR\|(3), \fIERR_get_error\fR\|(3), \fIrand\fR\|(3) .SH "HISTORY" .IX Header "HISTORY" The \fBcb_arg\fR arguments to \fIBN_generate_prime()\fR and to \fIBN_is_prime()\fR were added in SSLeay 0.9.0. The \fBret\fR argument to \fIBN_generate_prime()\fR was added in SSLeay 0.9.1. \&\fIBN_is_prime_fasttest()\fR was added in OpenSSL 0.9.5.