BN_mod_mul_reciprocal(3)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 recip‐
rocal
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 effi‐
cient 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 com‐
puting 1/m and shifting it left by BN_num_bits(m)+1 to make it an inte‐
ger. 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_recipro‐
cal() arguments were different.
SEE ALSO
Functions: bn(3), err(3), BN_add(3), BN_CTX_new(3)BN_mod_mul_reciprocal(3)