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.

Answer 2

You first must create a public key from array of bytes

byte publicKeyBytes[] = .... your public key in bytes ... 
KeyFactory keyFactory = KeyFactory.getInstance("RSA"); 
X509EncodedKeySpec publicKeySpec = new X509EncodedKeySpec(publicKeyBytes)); 
PublicKey publicKey = keyFactory.generatePublic(publicKeySpec);

and after using the publicKey to encrypt

String data = "... data to be encrypted ....";
String alg = "RSA/ECB/PKCS1Padding";
Cipher cipher = Cipher.getInstance("RSA/ECB/PKCS1Padding");
cipher.init(Cipher.ENCRYPT_MODE, publicKey);
byte encryptedBytes[] = cipher.doFinal(data.getBytes());

Now only who have the privateKey can read your data

@rczajka: a publicKey is a key. You can use it to sign somethig that only the owner (that have the privateKey) can read.

Answer 3

public static String sign(String samlResponseString, String keystoreFile, String keyStorePassword, String privateKeyPassword, String alias)
            throws NoSuchAlgorithmException, UnsupportedEncodingException,
            InvalidKeyException, SignatureException {
        PrivateKey pkey=getPrivateKey(  keystoreFile,   keyStorePassword, privateKeyPassword,   alias);
        String signedString = null;
        Signature signature = Signature.getInstance("SHA256withRSA");
        signature.initSign(pkey);
        signature.update(samlResponseString.getBytes());
        byte[] signatureBytes = signature.sign();
        byte[] encryptedByteValue = Base64.encodeBase64(signatureBytes);
        signedString = new String(encryptedByteValue, "UTF-8");
        System.out.println(signedString);
        return signedString;
    }