NAME    [Toc]    [Back]

       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    [Toc]    [Back]

        #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    [Toc]    [Back]

       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    [Toc]    [Back]

       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    [Toc]    [Back]

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

HISTORY    [Toc]    [Back]

       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.


OpenBSD 3.6                 2000-12-14                          2