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.IX Title "BN_mod_mul_reciprocal 3"
.TH BN_mod_mul_reciprocal 3 2019-12-20 1.0.2u OpenSSL
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.\" way too many mistakes in technical documents.
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.nh
.SH 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
.SH SYNOPSIS
.IX Header "SYNOPSIS"
.Vb 1
\& #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);
.Ve
.SH DESCRIPTION
.IX Header "DESCRIPTION"
\&\fBBN_mod_mul_reciprocal()\fR can be used to perform an efficient
\&\fBBN_mod_mul\fR\|(3) operation when the operation will be performed
repeatedly with the same modulus. It computes \fBr\fR=(\fBa\fR*\fBb\fR)%\fBm\fR
using \fBrecp\fR=1/\fBm\fR, which is set as described below.  \fBctx\fR is a
previously allocated \fBBN_CTX\fR used for temporary variables.
.PP
\&\fBBN_RECP_CTX_new()\fR allocates and initializes a \fBBN_RECP\fR structure.
\&\fBBN_RECP_CTX_init()\fR initializes an existing uninitialized \fBBN_RECP\fR.
.PP
\&\fBBN_RECP_CTX_free()\fR frees the components of the \fBBN_RECP\fR, and, if it
was created by \fBBN_RECP_CTX_new()\fR, also the structure itself.
.PP
\&\fBBN_RECP_CTX_set()\fR stores \fBm\fR in \fBrecp\fR and sets it up for computing
1/\fBm\fR and shifting it left by BN_num_bits(\fBm\fR)+1 to make it an
integer. The result and the number of bits it was shifted left will
later be stored in \fBrecp\fR.
.PP
\&\fBBN_div_recp()\fR divides \fBa\fR by \fBm\fR using \fBrecp\fR. It places the quotient
in \fBdv\fR and the remainder in \fBrem\fR.
.PP
The \fBBN_RECP_CTX\fR structure is defined as follows:
.PP
.Vb 8
\& typedef struct bn_recp_ctx_st
\&        {
\&        BIGNUM N;       /* the divisor */
\&        BIGNUM Nr;      /* the reciprocal */
\&        int num_bits;
\&        int shift;
\&        int flags;
\&        } BN_RECP_CTX;
.Ve
.PP
It cannot be shared between threads.
.SH "RETURN VALUES"
.IX Header "RETURN VALUES"
\&\fBBN_RECP_CTX_new()\fR returns the newly allocated \fBBN_RECP_CTX\fR, and NULL
on error.
.PP
\&\fBBN_RECP_CTX_init()\fR and \fBBN_RECP_CTX_free()\fR have no return values.
.PP
For the other functions, 1 is returned for success, 0 on error.
The error codes can be obtained by \fBERR_get_error\fR\|(3).
.SH "SEE ALSO"
.IX Header "SEE ALSO"
\&\fBbn\fR\|(3), \fBERR_get_error\fR\|(3), \fBBN_add\fR\|(3),
\&\fBBN_CTX_new\fR\|(3)
.SH HISTORY
.IX Header "HISTORY"
\&\fBBN_RECP_CTX\fR was added in SSLeay 0.9.0. Before that, the function
\&\fBBN_reciprocal()\fR was used instead, and the \fBBN_mod_mul_reciprocal()\fR
arguments were different.