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.Dd March 24, 2020
.Dt CRYPTO_X25519_INVERSE 3MONOCYPHER
.Os
.Sh NAME
.Nm crypto_x25519_inverse
.Nd X25519 scalar multiplication with the multiplicative inverse of a scalar
.Sh SYNOPSIS
.In monocypher.h
.Ft void
.Fo crypto_x25519_inverse
.Fa "uint8_t blind_salt[32]"
.Fa "const uint8_t private_key[32]"
.Fa "const uint8_t curve_point"
.Fc
.Sh DESCRIPTION
The
.Fn crypto_x25519_inverse
function performs scalar multiplication of the multiplicative inverse
of a scalar for X25519.
.Sy This is a highly advanced, specialized feature ;
unless you are implementing a protocol that requires this specifically,
.Sy you likely have no reason to be using these functions
and are probably looking for
.Xr crypto_key_exchange 3monocypher
or
.Xr crypto_x25519 3monocypher
instead.
Expect elliptic curve jargon on this page.
.Pp
This function is used, for example, with exponential blinding in
oblivious pseudo-random functions (OPRFs).
The arguments are:
.Bl -tag -width Ds
.It Fa blind_salt
The output point.
.It Fa private_key
The private key (scalar) to use.
First, the value is clamped;
then the clamped value's multiplicative inverse (modulo the curve order)
is determined;
the clamped value's multiplicative inverse then has its cofactor
cleared,
and that final value is then used for scalar multiplication.
.It Fa curve_point
The curve point on X25519 to multiply with the multiplicative inverse
(modulo the curve order) of
.Fa private_key .
.El
.Sh SEE ALSO
.Xr crypto_x25519 3monocypher ,
.Xr intro 3monocypher
.Sh HISTORY
The
.Fn crypto_x25519_inverse
function first appeared in Monocypher 3.1.0.