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 structure.
BN_RECP_CTX_init() initializes an existing uninitialized
BN_RECP.
BN_RECP_CTX_free() frees the components of the
BN_RECP, 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, and
NULL on error.
BN_RECP_CTX_init() and
BN_RECP_CTX_free() 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).
SEE ALSO¶
bn(3),
ERR_get_error(3),
BN_add(3),
BN_CTX_new(3)
HISTORY¶
BN_RECP_CTX was added in SSLeay 0.9.0. Before that, the function
BN_reciprocal() was used instead, and the
BN_mod_mul_reciprocal() arguments were different.