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RedHat 9 (Linux i386) - man page for bn_recp_ctx_set (redhat section 3)

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
#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);
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.
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() argu- ments were different. 0.9.7a 2002-09-25 BN_mod_mul_reciprocal(3)