Generate Ed25519 keys when invoked without arguments

Ed25519 public keys are very convenient due to their small size.
OpenSSH has supported Ed25519 since version 6.5 (January 2014).

OK djm@ markus@ sthen@ deraadt@

usr.bin/ssh/ssh-keygen.1

@@ -1,4 +1,4 @@
-.\"	$OpenBSD: ssh-keygen.1,v 1.229 2023/07/23 20:04:45 naddy Exp $
+.\"	$OpenBSD: ssh-keygen.1,v 1.230 2023/09/04 10:29:58 job Exp $
 .\"
 .\" Author: Tatu Ylonen <ylo@cs.hut.fi>
 .\" Copyright (c) 1995 Tatu Ylonen <ylo@cs.hut.fi>, Espoo, Finland
@@ -35,7 +35,7 @@
 .\" (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF
 .\" THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
 .\"
-.Dd $Mdocdate: July 23 2023 $
+.Dd $Mdocdate: September 4 2023 $
 .Dt SSH-KEYGEN 1
 .Os
 .Sh NAME
@@ -185,7 +185,7 @@ The type of key to be generated is specified with the
 option.
 If invoked without any arguments,
 .Nm
-will generate an RSA key.
+will generate an Ed25519 key.
 .Pp
 .Nm
 is also used to generate groups for use in Diffie-Hellman group

usr.bin/ssh/ssh-keygen.c

@@ -1,4 +1,4 @@
-/* $OpenBSD: ssh-keygen.c,v 1.470 2023/07/17 04:01:10 djm Exp $ */
+/* $OpenBSD: ssh-keygen.c,v 1.471 2023/09/04 10:29:58 job Exp $ */
 /*
  * Author: Tatu Ylonen <ylo@cs.hut.fi>
  * Copyright (c) 1994 Tatu Ylonen <ylo@cs.hut.fi>, Espoo, Finland
@@ -61,11 +61,7 @@
 #include "ssh-pkcs11.h"
 #endif
 
-#ifdef WITH_OPENSSL
-# define DEFAULT_KEY_TYPE_NAME "rsa"
-#else
-# define DEFAULT_KEY_TYPE_NAME "ed25519"
-#endif
+#define DEFAULT_KEY_TYPE_NAME "ed25519"
 
 /*
  * Default number of bits in the RSA, DSA and ECDSA keys.  These value can be
@@ -252,7 +248,7 @@ ask_filename(struct passwd *pw, const char *prompt)
 	char *name = NULL;
 
 	if (key_type_name == NULL)
-		name = _PATH_SSH_CLIENT_ID_RSA;
+		name = _PATH_SSH_CLIENT_ID_ED25519;
 	else {
 		switch (sshkey_type_from_name(key_type_name)) {
 		case KEY_DSA_CERT: