BN_mod_mul_reciprocal(3) OpenSSL BN_mod_mul_reciprocal(3)
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
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,
int BN_mod_mul_reciprocal(BIGNUM *r, BIGNUM *a, BIGNUM *b,
BN_RECP_CTX *recp, BN_CTX *ctx);
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() ini-
tializes 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
The BN_RECP_CTX structure is defined as follows:
typedef struct bn_recp_ctx_st
BIGNUM N; /* the divisor */
BIGNUM Nr; /* the reciprocal */
It cannot be shared between threads.
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).
bn(3), ERR_get_error(3), BN_add(3), BN_CTX_new(3)
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.
0.9.7d 2003-11-20 BN_mod_mul_reciprocal(3)