// Die Signatur
Signature signature = Signature.getInstance( "SHA256WITHRSA");

// Lade des  privaten Schlüssels und Zertifikats
char[] keystorePasswort = new char[] {'n', 'o', 'M', 'o', 'r', 'e', 'S', 'e', 'c', 'r', 'e', 't', 's'};
FileInputStream fis = new FileInputStream("../rincewind.keystore");
KeyStore keystore = KeyStore.getInstance("JKS") ;
keystore.load(fis, keystorePasswort);

KeyStore.PrivateKeyEntry  entry = (KeyStore.PrivateKeyEntry) keystore.getEntry("rincewind", new KeyStore.PasswordProtection(keystorePasswort));

PrivateKey privateKey = entry.getPrivateKey();
signature.initSign(privateKey );

// Der Inhalt
String klartext = "Die Botschaft";

// Signature berechnen
signature.update( klartext.getBytes());
byte[] sig = signature.sign();

String sigB64 = new String(Base64.encodeBase64(sig));
System.out.println( "Signatur       " +  sigB64);

// Signatur prüfen
Certificate cert = entry.getCertificate();
signature.initVerify( cert);
signature.update(klartext.getBytes());
System.out.println("Wert unverändert " + signature.verify(sig));