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.\" ========================================================================
.\"
.IX Title "BN_MOD_MUL_RECIPROCAL 3"
.TH BN_MOD_MUL_RECIPROCAL 3 "2023-09-11" "1.1.1w" "OpenSSL"
.\" For nroff, turn off justification. Always turn off hyphenation; it makes
.\" way too many mistakes in technical documents.
.if n .ad l
.nh
.SH "NAME"
BN_mod_mul_reciprocal, BN_div_recp, BN_RECP_CTX_new, 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_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"
\&\fIBN_mod_mul_reciprocal()\fR can be used to perform an efficient
\&\fIBN_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 \fB\s-1BN_CTX\s0\fR used for temporary variables.
.PP
\&\fIBN_RECP_CTX_new()\fR allocates and initializes a \fB\s-1BN_RECP\s0\fR structure.
.PP
\&\fIBN_RECP_CTX_free()\fR frees the components of the \fB\s-1BN_RECP\s0\fR, and, if it
was created by \fIBN_RECP_CTX_new()\fR, also the structure itself.
If \fBrecp\fR is \s-1NULL,\s0 nothing is done.
.PP
\&\fIBN_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
\&\fIBN_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 \fB\s-1BN_RECP_CTX\s0\fR structure cannot be shared between threads.
.SH "RETURN VALUES"
.IX Header "RETURN VALUES"
\&\fIBN_RECP_CTX_new()\fR returns the newly allocated \fB\s-1BN_RECP_CTX\s0\fR, and \s-1NULL\s0
on error.
.PP
\&\fIBN_RECP_CTX_free()\fR has no return value.
.PP
For the other functions, 1 is returned for success, 0 on error.
The error codes can be obtained by \fIERR_get_error\fR\|(3).
.SH "SEE ALSO"
.IX Header "SEE ALSO"
\&\fIERR_get_error\fR\|(3), \fIBN_add\fR\|(3),
\&\fIBN_CTX_new\fR\|(3)
.SH "HISTORY"
.IX Header "HISTORY"
\&\fIBN_RECP_CTX_init()\fR was removed in OpenSSL 1.1.0
.SH "COPYRIGHT"
.IX Header "COPYRIGHT"
Copyright 2000\-2017 The OpenSSL Project Authors. All Rights Reserved.
.PP
Licensed under the OpenSSL license (the \*(L"License\*(R"). You may not use
this file except in compliance with the License. You can obtain a copy
in the file \s-1LICENSE\s0 in the source distribution or at
<https://www.openssl.org/source/license.html>.