diff --git a/metropolis/cli/metroctl/format.go b/metropolis/cli/metroctl/format.go
new file mode 100644
index 0000000..aaf7984
--- /dev/null
+++ b/metropolis/cli/metroctl/format.go
@@ -0,0 +1,49 @@
+package main
+
+import (
+	"fmt"
+	"io"
+	"log"
+	"os"
+
+	"source.monogon.dev/metropolis/node/core/identity"
+	apb "source.monogon.dev/metropolis/proto/api"
+)
+
+type encoder struct {
+	out io.WriteCloser
+}
+
+func (e *encoder) writeNodeID(n *apb.Node) error {
+	id := identity.NodeID(n.Pubkey)
+	_, err := fmt.Fprintf(e.out, "%s\n", id)
+	return err
+}
+
+func (e *encoder) close() error {
+	if e.out != os.Stdout {
+		return e.out.Close()
+	}
+	return nil
+}
+
+func newOutputEncoder() *encoder {
+	var o io.WriteCloser
+	o = os.Stdout
+
+	// Redirect output to the file at flags.output, if the flag was provided.
+	if flags.output != "" {
+		of, err := os.Create(flags.output)
+		if err != nil {
+			log.Fatalf("Couldn't create the output file at %s: %v", flags.output, err)
+		}
+		o = of
+	}
+
+	if flags.format != "plaintext" {
+		log.Fatalf("Currently only the plaintext output format is supported.")
+	}
+	return &encoder{
+		out: o,
+	}
+}