Add output of client certificate

app.go

@@ -86,6 +86,7 @@ func main() {
 
 	server := &http.Server{
 		Addr:      ":" + port,
+		TLSConfig: &tls.Config{ClientAuth: tls.RequestClientCert},
 		Handler:   mux,
 	}
 
@@ -230,6 +231,13 @@ func whoamiHandler(w http.ResponseWriter, r *http.Request) {
 	}
 
 	_, _ = fmt.Fprintln(w, "RemoteAddr:", r.RemoteAddr)
+
+	if r.TLS != nil {
+		for i, cert := range r.TLS.PeerCertificates {
+			_, _ = fmt.Fprintf(w, "Certificate[%d] Subject: %v\n", i, cert.Subject)
+		}
+	}
+
 	if err := r.Write(w); err != nil {
 		http.Error(w, err.Error(), http.StatusInternalServerError)
 		return