Provided by: libssl-doc_1.1.0g-2ubuntu4_all

**NAME**

BN_generate_prime_ex, BN_is_prime_ex, BN_is_prime_fasttest_ex, BN_GENCB_call, BN_GENCB_new, BN_GENCB_free, BN_GENCB_set_old, BN_GENCB_set, BN_GENCB_get_arg, BN_generate_prime, BN_is_prime, BN_is_prime_fasttest - generate primes and test for primality

**SYNOPSIS**

#include <openssl/bn.h> int BN_generate_prime_ex(BIGNUM *ret, int bits, int safe, const BIGNUM *add, const BIGNUM *rem, BN_GENCB *cb); int BN_is_prime_ex(const BIGNUM *p, int nchecks, BN_CTX *ctx, BN_GENCB *cb); int BN_is_prime_fasttest_ex(const BIGNUM *p, int nchecks, BN_CTX *ctx, int do_trial_division, BN_GENCB *cb); int BN_GENCB_call(BN_GENCB *cb, int a, int b); BN_GENCB *BN_GENCB_new(void); void BN_GENCB_free(BN_GENCB *cb); void BN_GENCB_set_old(BN_GENCB *gencb, void (*callback)(int, int, void *), void *cb_arg); void BN_GENCB_set(BN_GENCB *gencb, int (*callback)(int, int, BN_GENCB *), void *cb_arg); void *BN_GENCB_get_arg(BN_GENCB *cb); Deprecated: #if OPENSSL_API_COMPAT < 0x00908000L BIGNUM *BN_generate_prime(BIGNUM *ret, int num, int safe, BIGNUM *add, BIGNUM *rem, void (*callback)(int, int, void *), void *cb_arg); int BN_is_prime(const BIGNUM *a, int checks, void (*callback)(int, int, void *), BN_CTX *ctx, void *cb_arg); int BN_is_prime_fasttest(const BIGNUM *a, int checks, void (*callback)(int, int, void *), BN_CTX *ctx, void *cb_arg, int do_trial_division); #endif

**DESCRIPTION**

BN_generate_prime_ex()generates a pseudo-random prime number of at least bit lengthbits. Ifretis notNULL, it will be used to store the number. Ifcbis notNULL, it is used as follows: ·BN_GENCB_call(cb,0,i)is called after generating the i-th potential prime number. · While the number is being tested for primality,BN_GENCB_call(cb,1,j)is called as described below. · When a prime has been found,BN_GENCB_call(cb,2,i)is called. The prime may have to fulfill additional requirements for use in Diffie-Hellman key exchange: Ifaddis notNULL, the prime will fulfill the condition p %add==rem(p %add== 1 ifrem==NULL) in order to suit a given generator. Ifsafeis true, it will be a safe prime (i.e. a prime p so that (p-1)/2 is also prime). The PRNG must be seeded prior to callingBN_generate_prime_ex(). The prime number generation has a negligible error probability.BN_is_prime_ex()andBN_is_prime_fasttest_ex()test if the numberpis prime. The following tests are performed until one of them shows thatpis composite; ifppasses all these tests, it is considered prime.BN_is_prime_fasttest_ex(), when called withdo_trial_division==1, first attempts trial division by a number of small primes; if no divisors are found by this test andcbis notNULL,BN_GENCB_call(cb,1,-1)is called. Ifdo_trial_division==0, this test is skipped. BothBN_is_prime_ex()andBN_is_prime_fasttest_ex()perform a Miller-Rabin probabilistic primality test withnchecksiterations. Ifnchecks==BN_prime_checks, a number of iterations is used that yields a false positive rate of at most 2^-80 for random input. Ifcbis notNULL,BN_GENCB_call(cb,1,j)is called after the j-th iteration (j = 0, 1, ...).ctxis a pre-allocatedBN_CTX(to save the overhead of allocating and freeing the structure in a loop), orNULL. BN_GENCB_call calls the callback function held in theBN_GENCBstructure and passes the intsaandbas arguments. There are two types ofBN_GENCBstructure that are supported: "new" style and "old" style. New programs should prefer the "new" style, whilst the "old" style is provided for backwards compatibility purposes. A BN_GENCB structure should be created through a call toBN_GENCB_new(), and freed through a call toBN_GENCB_free(). For "new" style callbacks a BN_GENCB structure should be initialised with a call toBN_GENCB_set(), wheregencbis aBN_GENCB*,callbackis of typeint(*callback)(int,int,BN_GENCB*)andcb_argis avoid*. "Old" style callbacks are the same except they are initialised with a call toBN_GENCB_set_old()andcallbackis of typevoid(*callback)(int,int,void*). A callback is invoked through a call toBN_GENCB_call. This will check the type of the callback and will invokecallback(a,b,gencb)for new style callbacks orcallback(a,b,cb_arg)for old style. It is possible to obtained the argument associated with a BN_GENCB structure (set via a call to BN_GENCB_set or BN_GENCB_set_old) using BN_GENCB_get_arg. BN_generate_prime (deprecated) works in the same way as BN_generate_prime_ex but expects an old style callback function directly in thecallbackparameter, and an argument to pass to it in thecb_arg. 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.

**RETURN** **VALUES**

BN_generate_prime_ex()return 1 on success or 0 on error.BN_is_prime_ex(),BN_is_prime_fasttest_ex(),BN_is_prime()andBN_is_prime_fasttest()return 0 if the number is composite, 1 if it is prime with an error probability of less than 0.25^nchecks, and -1 on error.BN_generate_prime()returns the prime number on success,NULLotherwise. BN_GENCB_new returns a pointer to a BN_GENCB structure on success, orNULLotherwise. BN_GENCB_get_arg returns the argument previously associated with a BN_GENCB structure. Callback functions should return 1 on success or 0 on error. The error codes can be obtained byERR_get_error(3).

**REMOVED** **FUNCTIONALITY**

As of OpenSSL 1.1.0 it is no longer possible to create a BN_GENCB structure directly, as in: BN_GENCB callback; Instead applications should create a BN_GENCB structure using BN_GENCB_new: BN_GENCB *callback; callback = BN_GENCB_new(); if(!callback) /* handle error */ ... BN_GENCB_free(callback);

**SEE** **ALSO**

ERR_get_error(3),RAND_bytes(3)

**HISTORY**

BN_GENCB_new(),BN_GENCB_free(), andBN_GENCB_get_arg()were added in OpenSSL 1.1.0

**COPYRIGHT**

Copyright 2000-2017 The OpenSSL Project Authors. All Rights Reserved. Licensed under the OpenSSL license (the "License"). You may not use this file except in compliance with the License. You can obtain a copy in the file LICENSE in the source distribution or at <https://www.openssl.org/source/license.html>.