import java.security.MessageDigest;

public class KeyStretching {

	public static void main(String[] args)
		throws Exception {

	String key = "";
    	String password = "0123456789ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz";
    	String salt = "0123456789ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz";

        //as this is purely an example, I'm going to use sha-256
        //you may want to use something less 'old' in production
        MessageDigest md = MessageDigest.getInstance("SHA-256");

        long start = System.currentTimeMillis();

        byte[] byteData;
        StringBuilder sb = new StringBuilder(64);
        for (int a = 0; a < 100000; a++) {
            md.update((key + password + salt).getBytes());
            byteData = md.digest();
            for (int i = 0; i < byteData.length; i++) {
            	sb.append(Integer.toString((byteData[i] & 0xff) + 0x100, 16).substring(1));
            }
            key = sb.toString();
            sb.delete(0, sb.length());
    	}

    	float elapsed = (System.currentTimeMillis() - start) / 1000F;

    	System.out.println("Time : " + elapsed);
    	System.out.println("Hex format : " + key);
    }
}