- NAME
- SYNOPSIS
- DESCRIPTION
- RETURN VALUES
- SEE ALSO
- HISTORY
- COPYRIGHT

# 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, BIGNUM *a, BN_RECP_CTX *recp,
                     BN_CTX *ctx);

     int BN_mod_mul_reciprocal(BIGNUM *r, BIGNUM *a, 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 2000-2017 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.