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- =pod
- =head1 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
- =head1 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);
- =head1 DESCRIPTION
- BN_mod_mul_reciprocal() can be used to perform an efficient
- L<BN_mod_mul(3)|BN_mod_mul(3)> operation when the operation will be performed
- repeatedly with the same modulus. It computes B<r>=(B<a>*B<b>)%B<m>
- using B<recp>=1/B<m>, which is set as described below. B<ctx> is a
- previously allocated B<BN_CTX> used for temporary variables.
- BN_RECP_CTX_new() allocates and initializes a B<BN_RECP> structure.
- BN_RECP_CTX_init() initializes an existing uninitialized B<BN_RECP>.
- BN_RECP_CTX_free() frees the components of the B<BN_RECP>, and, if it
- was created by BN_RECP_CTX_new(), also the structure itself.
- BN_RECP_CTX_set() stores B<m> in B<recp> and sets it up for computing
- 1/B<m> and shifting it left by BN_num_bits(B<m>)+1 to make it an
- integer. The result and the number of bits it was shifted left will
- later be stored in B<recp>.
- BN_div_recp() divides B<a> by B<m> using B<recp>. It places the quotient
- in B<dv> and the remainder in B<rem>.
- The B<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.
- =head1 RETURN VALUES
- BN_RECP_CTX_new() returns the newly allocated B<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 L<ERR_get_error(3)|ERR_get_error(3)>.
- =head1 SEE ALSO
- L<bn(3)|bn(3)>, L<ERR_get_error(3)|ERR_get_error(3)>, L<BN_add(3)|BN_add(3)>,
- L<BN_CTX_new(3)|BN_CTX_new(3)>
- =head1 HISTORY
- B<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.
- =cut
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