BN_MOD_MUL_RECIPROCAL(3)

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_CTX structure. BN_RECP_CTX_init() initializes an existing uninitialized BN_RECP_CTX.

BN_RECP_CTX_free() frees the components of the BN_RECP_CTX, 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, or NULL on error.

BN_RECP_CTX_init() and BN_RECP_CTX_free() return no 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), BN_add(3), BN_CTX_new(3), ERR_get_error(3)

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