Tutorial for interoperability with java signing

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This tutorial shows you how to verify a Java generated signature by jsrsasign.

Here is a sample Java code which loads PKCS#8 DER binary RSA private key, sign a string "aaa" by the private key with "SHA1withRSA" algorithm, then write a resulted signature value to a file as binary.

/*
 * TSign.java
 */
import java.lang.*;
import java.io.*;
import java.security.*;
import java.security.spec.*;

public class TSign {
    public TSign() {
	try {
	    byte[] prvKeyBytes = loadBytes("z1.prv.p8p.der"); // PKCS#8 private key

	    KeyFactory kf = KeyFactory.getInstance("RSA");
	    KeySpec keySpec = new PKCS8EncodedKeySpec(prvKeyBytes);
	    PrivateKey prvKey = kf.generatePrivate(keySpec);

	    Signature sig = Signature.getInstance("SHA1withRSA");
	    sig.initSign(prvKey);
	    sig.update("aaa".getBytes());
	    byte[] sigBytes = sig.sign();
	    System.out.println(sigBytes);

	    saveBytes(sigBytes, "TSign.sig.bin");
	    
	} catch (Exception ex) {
	    ex.printStackTrace();
	}
    }

    private static byte[] loadBytes(String fileName) {
	try {
	    FileInputStream fis = new FileInputStream(fileName);
	    byte[] data = new byte[fis.available()];
	    fis.read(data);
	    fis.close();
	    return data;
	} catch (Exception ex) {
	    ex.printStackTrace();
	}
	return null;
    }

    private static void saveBytes(byte[] data, String fileName) {
	try {
	    FileOutputStream fos = new FileOutputStream(fileName);
	    fos.write(data);
	    fos.close();
	} catch (Exception ex) {
	    ex.printStackTrace();
	}
    }

    public static void main(String[] args) {
	TSign t = new TSign();
    }
}

Here is another sample jsrsasign JavaScript program to verify signature. This script loads PKCS#8 PEM RSA public key and a binary signature value file, then verifies the signature value.

#!/usr/bin/env node
var rs = require("jsrsasign");
var rsu = require("jsrsasign-util");

var hSig = rsu.readFileHexByBin("TSign.sig.bin");
var pubpem = rsu.readFile("z1.pub.p8.pem");
var pubkey = rs.KEYUTIL.getKey(pubpem);

var sig = new rs.KJUR.crypto.Signature({alg: "SHA1withRSA"});
sig.init(pubkey);
sig.updateString("aaa");
var result = sig.verify(hSig);

console.log(result);

You'll see how easy to verify a signature with jsrsasign. KEYUTIL.getKey method provides very easy way to load a key. This program can load PKCS#8 public key as well as a PKCS#5 public key or a X.509 certificate by the same code.