BN_mod_mul_reciprocal(3)

     BN_mod_mul_reciprocal(30.9.6h (2002-09-25BN_mod_mul_reciprocal(3)

     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:

     Page 1                                         (printed 1/19/103)

     BN_mod_mul_reciprocal(30.9.6h (2002-09-25BN_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.

     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).

     SEE ALSO
          bn(3), ERR_get_error(3), BN_add(3), BN_CTX_new(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.

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