BN_MOD_MUL_RECIPROCAL(3) BSD Library Functions Manual 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 recipro‐
cal
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);
DESCRIPTIONBN_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 vari‐
ables.
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 VALUESBN_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 ALSObn(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() argu‐
ments were different.
BSD April 19, 2024 BSD
