Use key type supported by server if possible

Check the key types supported by the server. If the data is not
available, default to the previous behavior which is using the rsa key
type.

Update insecure key check to match against any key files located within
the keys directory. For now, this effectively allows matching either rsa
or ed25519 insecure private keys.

plugins/communicators/ssh/communicator.rb

@@ -185,7 +185,29 @@ module VagrantPlugins
             @machine.guest.capability?(:remove_public_key)
           raise Vagrant::Errors::SSHInsertKeyUnsupported if !cap
 
-          _pub, priv, openssh = Vagrant::Util::Keypair.create
+          # Check for supported key type
+          key_type = catch(:key_type) do
+            begin
+              Vagrant::Util::Keypair::PREFER_KEY_TYPES.each do |type_name, type|
+                throw :key_type, type if supports_key_type?(type_name)
+              end
+              nil
+            rescue => err
+              @logger.warn("Failed to check key types server supports: #{err}")
+              nil
+            end
+          end
+
+          @logger.debug("Detected key type for new private key: #{key_type}")
+
+          # If no key type was discovered, default to rsa
+          if key_type.nil?
+            @logger.debug("Failed to detect supported key type, defaulting to rsa")
+            key_type = :rsa
+          end
+
+          @logger.info("Creating new ssh keypair (type: #{key_type.inspect})")
+          _pub, priv, openssh = Vagrant::Util::Keypair.create(type: key_type)
 
           @logger.info("Inserting key to avoid password: #{openssh}")
           @machine.ui.detail("\n"+I18n.t("vagrant.inserting_random_key"))
@@ -748,8 +770,9 @@ module VagrantPlugins
       def insecure_key?(path)
         return false if !path
         return false if !File.file?(path)
-        source_path = Vagrant.source_root.join("keys", "vagrant")
-        return File.read(path).chomp == source_path.read.chomp
+        Dir.glob(Vagrant.source_root.join("keys", "vagrant.key.*")).any? do |source_path|
+          File.read(path).chomp == File.read(source_path).chomp
+        end
       end
 
       def create_remote_directory(dir)
@@ -759,6 +782,35 @@ module VagrantPlugins
       def machine_config_ssh
         @machine.config.ssh
       end
+
+      protected
+
+      # Check if server supports given key type
+      #
+      # @param [String, Symbol] type Key type
+      # @return [Boolean]
+      # @note This does not use a stable API and may be subject
+      # to unexpected breakage on net-ssh updates
+      def supports_key_type?(type)
+        if @connection.nil?
+          raise Vagrant::Errors::SSHNotReady
+        end
+        server_data = @connection.
+          transport&.
+          algorithms&.
+          instance_variable_get(:@server_data)
+        if server_data.nil?
+          @logger.warn("No server data available for key type support check")
+          return false
+        end
+        if !server_data.is_a?(Hash)
+          @logger.warn("Server data is not expected type (expecting Hash, got #{server_data.class})")
+          return false
+        end
+
+        @logger.debug("server data used for host key support check: #{server_data.inspect}")
+        server_data[:host_key].include?(type.to_s)
+      end
     end
   end
 end