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=head1 NAME

X25519,
X448
- EVP_PKEY X25519 and X448 support

=head1 DESCRIPTION

The B<X25519> and B<X448> EVP_PKEY implementation supports key generation and
key derivation using B<X25519> and B<X448>. It has associated private and public
key formats compatible with RFC 8410.

No additional parameters can be set during key generation.

The peer public key must be set using EVP_PKEY_derive_set_peer() when
performing key derivation.

=head1 NOTES

A context for the B<X25519> algorithm can be obtained by calling:

 EVP_PKEY_CTX *pctx = EVP_PKEY_CTX_new_id(EVP_PKEY_X25519, NULL);

For the B<X448> algorithm a context can be obtained by calling:

 EVP_PKEY_CTX *pctx = EVP_PKEY_CTX_new_id(EVP_PKEY_X448, NULL);

X25519 or X448 private keys can be set directly using
L<EVP_PKEY_new_raw_private_key(3)> or loaded from a PKCS#8 private key file
using L<PEM_read_bio_PrivateKey(3)> (or similar function). Completely new keys
can also be generated (see the example below). Setting a private key also sets
the associated public key.

X25519 or X448 public keys can be set directly using
L<EVP_PKEY_new_raw_public_key(3)> or loaded from a SubjectPublicKeyInfo
structure in a PEM file using L<PEM_read_bio_PUBKEY(3)> (or similar function).

=head1 EXAMPLES

This example generates an B<X25519> private key and writes it to standard
output in PEM format:

 #include <openssl/evp.h>
 #include <openssl/pem.h>
 ...
 EVP_PKEY *pkey = NULL;
 EVP_PKEY_CTX *pctx = EVP_PKEY_CTX_new_id(EVP_PKEY_X25519, NULL);
 EVP_PKEY_keygen_init(pctx);
 EVP_PKEY_keygen(pctx, &pkey);
 EVP_PKEY_CTX_free(pctx);
 PEM_write_PrivateKey(stdout, pkey, NULL, NULL, 0, NULL, NULL);

The key derivation example in L<EVP_PKEY_derive(3)> can be used with
B<X25519> and B<X448>.

=head1 SEE ALSO

L<EVP_PKEY_CTX_new(3)>,
L<EVP_PKEY_keygen(3)>,
L<EVP_PKEY_derive(3)>,
L<EVP_PKEY_derive_set_peer(3)>

=head1 COPYRIGHT

Copyright 2017-2020 The OpenSSL Project Authors. All Rights Reserved.

Licensed under the OpenSSL license (the "License").  You may not use
this file except in compliance with the License.  You can obtain a copy
in the file LICENSE in the source distribution or at
L<https://www.openssl.org/source/license.html>.

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