Add X25519 doc

Reviewed-by: Matt Caswell <matt@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/3059)

doc/man7/X25519.pod

@@ -0,0 +1,58 @@
+=pod
+
+=head1 NAME
+
+X25519 - EVP_PKEY X25519 support
+
+=head1 DESCRIPTION
+
+The B<X25519> EVP_PKEY implementation supports key generation and key
+derivation using B<X25519>. It has associated private and public key formats
+compatible with draft-ietf-curdle-pkix-03.
+
+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(NID_X25519, NULL);
+
+=head1 EXAMPLE
+
+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(NID_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 derviation example in L<EVP_PKEY_derive(3)> can be used with
+B<X25519>.
+
+=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 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>.
+
+=cut