/usr/man/cat.3/BN_div_recp.3

BN_mod_mul_reciprocal(3)     OpenSSL     BN_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:

1.0.2t               Last change: 2019-09-10                    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.

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.

1.0.2t               Last change: 2019-09-10                    2

See also BN_RECP_CTX_free(3)
See also BN_RECP_CTX_init(3)
See also BN_RECP_CTX_new(3)
See also BN_RECP_CTX_set(3)
See also BN_mod_mul_reciprocal(3)