BN_MOD_MUL_RECIPROCAL(3) OpenSSL BN_MOD_MUL_RECIPROCAL(3)

BN_mod_mul_reciprocal, BN_div_recp, BN_RECP_CTX_new,
BN_RECP_CTX_init, BN_RECP_CTX_free, BN_RECP_CTX_set - modu-
lar multiplication using reciprocal

#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);

*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** struc-
ture. *BN_RECP_CTX_init*() initializes an existing uninitial-
ized **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 com-
puting 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 quo-
tient in **dv** and the remainder in **rem**.
The **BN_RECP_CTX** structure is defined as follows:
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BN_MOD_MUL_RECIPROCAL(3) OpenSSL BN_MOD_MUL_RECIPROCAL(3)
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.

*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).

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

**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.
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