## 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 reciprocal

## 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

## 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() arguments were different.

2021-09-02 | eCosPro License |