.\" $OpenBSD: ECDH_compute_key.3,v 1.3 2023/08/29 10:07:42 tb Exp $
.\" Copyright (c) 2019 Ingo Schwarze <schwarze@openbsd.org>
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.Dd $Mdocdate: August 29 2023 $
.Dt ECDH_COMPUTE_KEY 3
.Os
.Sh NAME
.Nm ECDH_compute_key ,
.Nm ECDH_size
.Nd Elliptic Curve Diffie-Hellman key exchange
.Sh SYNOPSIS
.In openssl/ec.h
.Ft int
.Fo ECDH_compute_key
.Fa "void *out"
.Fa "size_t outlen"
.Fa "const EC_POINT *public_key"
.Fa "EC_KEY *ecdh"
.Fa "void *(*KDF)(const void *in, size_t inlen, void *out, size_t *outlen)"
.Fc
.Ft int
.Fo ECDH_size
.Fa "const EC_KEY *ecdh"
.Fc
.Sh DESCRIPTION
.Fn ECDH_compute_key
performs Elliptic Curve Diffie-Hellman key agreement.
It combines the private key contained in
.Fa ecdh
with the other party's
.Fa public_key ,
takes the
.Fa x
component of the affine coordinates,
and optionally applies the key derivation function
.Fa KDF .
It stores the resulting symmetric key in the buffer
.Fa out ,
which is
.Fa outlen
bytes long.
If
.Fa KDF
is
.Dv NULL ,
.Fa outlen
must be at least
.Fn ECDH_size ecdh .
.Pp
.Fn ECDH_size
returns the number of bytes needed to store an affine coordinate of a
point on the elliptic curve used by
.Fa ecdh ,
which is one eighth of the degree of the finite field underlying
that elliptic curve, rounded up to the next integer number.
.Sh RETURN VALUES
.Fn ECDH_compute_key
returns the length of the computed key in bytes or -1 if an error occurs.
.Pp
.Fn ECDH_size
returns the number of bytes needed to store an affine coordinate.
.Sh SEE ALSO
.Xr DH_generate_key 3 ,
.Xr DH_size 3 ,
.Xr EC_GROUP_new 3 ,
.Xr EC_KEY_new 3 ,
.Xr EC_POINT_new 3 ,
.Xr X25519 3
.Sh HISTORY
.Fn ECDH_compute_key
first appeared in OpenSSL 0.9.8 and has been available since
.Ox 4.5 .
.Pp
.Fn ECDH_size
first appeared in
.Ox 6.1 .