NAME

       BN_mod_mul_reciprocal, BN_div_recp, BN_RECP_CTX_new, BN_RECP_CTX_free, BN_RECP_CTX_set -
       modular multiplication using reciprocal

SYNOPSIS

        #include <openssl/bn.h>

        BN_RECP_CTX *BN_RECP_CTX_new(void);
        void BN_RECP_CTX_free(BN_RECP_CTX *recp);

        int BN_RECP_CTX_set(BN_RECP_CTX *recp, const BIGNUM *m, BN_CTX *ctx);

        int BN_div_recp(BIGNUM *dv, BIGNUM *rem, const BIGNUM *a, BN_RECP_CTX *recp,
                        BN_CTX *ctx);

        int BN_mod_mul_reciprocal(BIGNUM *r, const BIGNUM *a, const BIGNUM *b,
                                  BN_RECP_CTX *recp, BN_CTX *ctx);

DESCRIPTION

       BN_mod_mul_reciprocal() can be used to perform an efficient BN_mod_mul(3) operation when
       the operation will be performed repeatedly with the same modulus. It computes r=(a*b)%m
       using recp=1/m, which is set as described below.  ctx is a previously allocated BN_CTX
       used for temporary variables.

       BN_RECP_CTX_new() allocates and initializes a BN_RECP structure.

       BN_RECP_CTX_free() frees the components of the BN_RECP, and, if it was created by
       BN_RECP_CTX_new(), also the structure itself.  If recp is NULL, nothing is done.

       BN_RECP_CTX_set() stores m in recp and sets it up for computing 1/m and shifting it left
       by BN_num_bits(m)+1 to make it an integer. The result and the number of bits it was
       shifted left will later be stored in recp.

       BN_div_recp() divides a by m using recp. It places the quotient in dv and the remainder in
       rem.

       The BN_RECP_CTX structure cannot be shared between threads.

RETURN VALUES

       BN_RECP_CTX_new() returns the newly allocated BN_RECP_CTX, and NULL on error.

       BN_RECP_CTX_free() has no return value.

       For the other functions, 1 is returned for success, 0 on error.  The error codes can be
       obtained by ERR_get_error(3).

SEE ALSO

       ERR_get_error(3), BN_add(3), BN_CTX_new(3)

HISTORY

       BN_RECP_CTX_init() was removed in OpenSSL 1.1.0

COPYRIGHT

       Copyright 2000-2021 The OpenSSL Project Authors. All Rights Reserved.

       Licensed under the Apache License 2.0 (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>.