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 - modu- lar 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 efficientBN_mod_mul(3) operation when the operation will be performed repeatedly with the same modulus. It computesr=(a*b)%musingrecp=1/m, which is set as described below.ctxis a previously allocatedBN_CTXused for temporary variables.BN_RECP_CTX_new() allocates and initializes aBN_RECPstruc- ture.BN_RECP_CTX_init() initializes an existing uninitial- izedBN_RECP.BN_RECP_CTX_free() frees the components of theBN_RECP, and, if it was created byBN_RECP_CTX_new(), also the structure itself.BN_RECP_CTX_set() storesminrecpand sets it up for com- puting 1/mand 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 inrecp.BN_div_recp() dividesabymusingrecp. It places the quo- tient indvand the remainder inrem. TheBN_RECP_CTXstructure is defined as follows: MirOS BSD #10-current 2005-02-05 1 BN_MOD_MUL_RECIPROCAL(3) OpenSSL BN_MOD_MUL_RECIPROCAL(3) 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 allocatedBN_RECP_CTX, and NULL on error.BN_RECP_CTX_init() andBN_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 byERR_get_error(3).

bn(3),ERR_get_error(3),BN_add(3),BN_CTX_new(3)

BN_RECP_CTXwas added in SSLeay 0.9.0. Before that, the functionBN_reciprocal() was used instead, and theBN_mod_mul_reciprocal() arguments were different. MirOS BSD #10-current 2005-02-05 2

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