# /usr/man/cat.3/BN_RECP_CTX_init.3.Z

## NAME

BN_mod_mul_reciprocal, BN_div_recp, BN_RECP_CTX_new, BN_RECP_CTX_init,
BN_RECP_CTX_free, BN_RECP_CTX_set - modular multiplication using recip-
rocal

## SYNOPSIS

#include <openssl/bn.h>
BN_RECP_CTX *BN_RECP_CTX_new(void);
void BN_RECP_CTX_init(BN_RECP_CTX *recp);
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***_***init()* initializes an existing uninitialized **BN_RECP**.
*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.
*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 is defined as follows:
typedef struct bn_recp_ctx_st
{
BIGNUM N; /* the divisor */
BIGNUM Nr; /* the reciprocal */
int num_bits;
int shift;
int flags;
} BN_RECP_CTX;
It 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***_***init()* and *BN***_***RECP***_***CTX***_***free()* have no return values.
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

**bn(3)**, **ERR_get_error(3)**, **BN_add(3)**, **BN_CTX_new(3)**

## HISTORY

**BN_RECP_CTX** was added in SSLeay 0.9.0. Before that, the function
*BN***_***reciprocal()* was used instead, and the *BN***_***mod***_***mul***_***reciprocal()* argu-
ments were different.
1.0.2t 2019-09-10 **BN_mod_mul_reciprocal(3)**
See also **BN_RECP_CTX_free(3)**
See also **BN_RECP_CTX_new(3)**
See also **BN_RECP_CTX_set(3)**
See also **BN_div_recp(3)**
See also **BN_mod_mul_reciprocal(3)**

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