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.