BN_mod_mul_reciprocal

Section: OpenSSL (3)
Updated: 2019-02-26
Page Index

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).

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.