How to sign string with private key

Question

How can I get the signature of a string using SHA1withRSA if I already have the Private Key as byte[] or String?

Answer 1

I guess what you say is you know the key pair before hand and want to sign/verify with that.

Please see the following code.

import java.security.KeyPair; import java.security.KeyPairGenerator; import java.security.NoSuchAlgorithmException; import java.security.Signature;  import sun.misc.BASE64Encoder;  public class MainClass {     public static void main(String[] args) throws Exception {          KeyPair keyPair = getKeyPair();          byte[] data = "test".getBytes("UTF8");          Signature sig = Signature.getInstance("SHA1WithRSA");         sig.initSign(keyPair.getPrivate());         sig.update(data);         byte[] signatureBytes = sig.sign();         System.out.println("Signature:" + new BASE64Encoder().encode(signatureBytes));          sig.initVerify(keyPair.getPublic());         sig.update(data);          System.out.println(sig.verify(signatureBytes));     }      private static KeyPair getKeyPair() throws NoSuchAlgorithmException {         KeyPairGenerator kpg = KeyPairGenerator.getInstance("RSA");         kpg.initialize(1024);         return kpg.genKeyPair();     } } 

Here you need to change the method getKeyPair() to supply your known key pair. You may load it from a java key store [JKS].

You can't just have an arbitrary byte array either as your public key or private key. They should be generated in relation.