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