.\" $OpenBSD: BN_mod_inverse.3,v 1.10 2018/04/29 15:58:21 schwarze Exp $
.\" OpenSSL b97fdb57 Nov 11 09:33:09 2016 +0100
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.Dd $Mdocdate: April 29 2018 $
.Dt BN_MOD_INVERSE 3
.Os
.Sh NAME
.Nm BN_mod_inverse
.Nd compute inverse modulo n
.Sh SYNOPSIS
.In openssl/bn.h
.Ft BIGNUM *
.Fo BN_mod_inverse
.Fa "BIGNUM *r"
.Fa "BIGNUM *a"
.Fa "const BIGNUM *n"
.Fa "BN_CTX *ctx"
.Fc
.Sh DESCRIPTION
.Fn BN_mod_inverse
computes the inverse of
.Fa a
modulo
.Fa n
and places the result in
.Fa r
.Pq Li (a*r)%n==1 .
If
.Fa r
is
.Dv NULL ,
a new
.Vt BIGNUM
is created.
.Pp
If the flag
.Dv BN_FLG_CONSTTIME
is set on
.Fa a
or
.Fa n ,
it operates in constant time.
.Pp
.Fa ctx
is a previously allocated
.Vt BN_CTX
used for temporary variables.
.Fa r
may be the same
.Vt BIGNUM
as
.Fa a
or
.Fa n .
.Sh RETURN VALUES
.Fn BN_mod_inverse
returns the
.Vt BIGNUM
containing the inverse, or
.Dv NULL
on error.
The error codes can be obtained by
.Xr ERR_get_error 3 .
.Sh SEE ALSO
.Xr BN_add 3 ,
.Xr BN_new 3 ,
.Xr BN_set_flags 3
.Sh HISTORY
.Fn BN_mod_inverse
first appeared in SSLeay 0.5.1 and has been available since
.Ox 2.4 .
.Pp
The
.Fa r
argument was added in SSLeay 0.9.1 and
.Ox 2.6 .