upstream: Use ed25519 keys for kex tests

since that's supported by OpenSSH even when built without OpenSSL.
Only test diffie-hellman kex if OpenSSH is compiled with support for it.

OpenBSD-Regress-ID: a5d09ef9bbd171f9e4ec73ed0d9eeb49a8878e97

regress/dropbear-kex.sh

@@ -1,4 +1,4 @@
-#	$OpenBSD: dropbear-kex.sh,v 1.2 2024/06/18 06:14:27 anton Exp $
+#	$OpenBSD: dropbear-kex.sh,v 1.3 2024/06/19 10:10:46 dtucker Exp $
 #	Placed in the Public Domain.
 
 tid="dropbear kex"
@@ -7,21 +7,19 @@ if test "x$REGRESS_INTEROP_DROPBEAR" != "xyes" ; then
 	skip "dropbear interop tests not enabled"
 fi
 
-cat >>$OBJ/sshd_proxy <<EOD
-PubkeyAcceptedAlgorithms +ssh-rsa
-HostkeyAlgorithms +ssh-rsa
-EOD
 cp $OBJ/sshd_proxy $OBJ/sshd_proxy.bak
 
-kex="curve25519-sha256 curve25519-sha256@libssh.org
-    diffie-hellman-group14-sha256 diffie-hellman-group14-sha1"
+kex="curve25519-sha256 curve25519-sha256@libssh.org"
+if $SSH -Q kex | grep 'diffie-hellman-group14-sha1'; then
+	kex="$kex diffie-hellman-group14-sha256 diffie-hellman-group14-sha1"
+fi
 
 for k in $kex; do
 	verbose "$tid: kex $k"
 	rm -f ${COPY}
 	# dbclient doesn't have switch for kex, so force in server
 	(cat $OBJ/sshd_proxy.bak; echo "KexAlgorithms $k") >$OBJ/sshd_proxy
-	env HOME=$OBJ dbclient -y -i $OBJ/.dropbear/id_rsa 2>$OBJ/dbclient.log \
+	env HOME=$OBJ dbclient -y -i $OBJ/.dropbear/id_ed25519 2>$OBJ/dbclient.log \
 	    -J "$OBJ/ssh_proxy.sh" somehost cat ${DATA} > ${COPY}
 	if [ $? -ne 0 ]; then
 		fail "ssh cat $DATA failed"