1=pod
2
3=head1 NAME
4
5BN_mod_mul_reciprocal,  BN_div_recp, BN_RECP_CTX_new, BN_RECP_CTX_init,
6BN_RECP_CTX_free, BN_RECP_CTX_set - modular multiplication using
7reciprocal
8
9=head1 SYNOPSIS
10
11 #include <openssl/bn.h>
12
13 BN_RECP_CTX *BN_RECP_CTX_new(void);
14 void BN_RECP_CTX_init(BN_RECP_CTX *recp);
15 void BN_RECP_CTX_free(BN_RECP_CTX *recp);
16
17 int BN_RECP_CTX_set(BN_RECP_CTX *recp, const BIGNUM *m, BN_CTX *ctx);
18
19 int BN_div_recp(BIGNUM *dv, BIGNUM *rem, BIGNUM *a, BN_RECP_CTX *recp,
20        BN_CTX *ctx);
21
22 int BN_mod_mul_reciprocal(BIGNUM *r, BIGNUM *a, BIGNUM *b,
23        BN_RECP_CTX *recp, BN_CTX *ctx);
24
25=head1 DESCRIPTION
26
27BN_mod_mul_reciprocal() can be used to perform an efficient
28L<BN_mod_mul(3)|BN_mod_mul(3)> operation when the operation will be performed
29repeatedly with the same modulus. It computes B<r>=(B<a>*B<b>)%B<m>
30using B<recp>=1/B<m>, which is set as described below.  B<ctx> is a
31previously allocated B<BN_CTX> used for temporary variables.
32
33BN_RECP_CTX_new() allocates and initializes a B<BN_RECP> structure.
34BN_RECP_CTX_init() initializes an existing uninitialized B<BN_RECP>.
35
36BN_RECP_CTX_free() frees the components of the B<BN_RECP>, and, if it
37was created by BN_RECP_CTX_new(), also the structure itself.
38
39BN_RECP_CTX_set() stores B<m> in B<recp> and sets it up for computing
401/B<m> and shifting it left by BN_num_bits(B<m>)+1 to make it an
41integer. The result and the number of bits it was shifted left will
42later be stored in B<recp>.
43
44BN_div_recp() divides B<a> by B<m> using B<recp>. It places the quotient
45in B<dv> and the remainder in B<rem>.
46
47The B<BN_RECP_CTX> structure is defined as follows:
48
49 typedef struct bn_recp_ctx_st
50	{
51	BIGNUM N;	/* the divisor */
52	BIGNUM Nr;	/* the reciprocal */
53	int num_bits;
54	int shift;
55	int flags;
56	} BN_RECP_CTX;
57
58It cannot be shared between threads.
59
60=head1 RETURN VALUES
61
62BN_RECP_CTX_new() returns the newly allocated B<BN_RECP_CTX>, and NULL
63on error.
64
65BN_RECP_CTX_init() and BN_RECP_CTX_free() have no return values.
66
67For the other functions, 1 is returned for success, 0 on error.
68The error codes can be obtained by L<ERR_get_error(3)|ERR_get_error(3)>.
69
70=head1 SEE ALSO
71
72L<bn(3)|bn(3)>, L<ERR_get_error(3)|ERR_get_error(3)>, L<BN_add(3)|BN_add(3)>,
73L<BN_CTX_new(3)|BN_CTX_new(3)>
74
75=head1 HISTORY
76
77B<BN_RECP_CTX> was added in SSLeay 0.9.0. Before that, the function
78BN_reciprocal() was used instead, and the BN_mod_mul_reciprocal()
79arguments were different.
80
81=cut
82