BN_mod_mul_reciprocal(3) — Subroutines

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

The BN_mod_mul_reciprocal() function can be used to perform an efficient BN_mod_mul() 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.

The BN_RECP_CTX_new() function allocates and initializes a BN_RECP structure. The BN_RECP_CTX_init() function initializes an existing uninitialized BN_RECP.

The BN_RECP_CTX_free() function frees the components of the BN_RECP, and, if it was created by BN_RECP_CTX_new(), also the structure itself.

The BN_RECP_CTX_set() function 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.

The BN_div_recp() function 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

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

The BN_RECP_CTX_init() and BN_RECP_CTX_free() functions 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).

HISTORY

The BN_RECP_CTX structure was added in SSLeay 0.9.0. Before that, the BN_reciprocal() function was used instead, and the BN_mod_mul_reciprocal() arguments were different.

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