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.