Example how to decrypt a protected message with a provided secret using the Data Encryption Standard algorithm. The messaged is readed from a provided file and the printed to the console. The provided secret is required to have exactly 8 characters.

MessageDESDecrypter.java

import java.io.ByteArrayOutputStream;
import java.io.FileInputStream;
import java.io.IOException;
import java.io.InputStream;
import java.io.OutputStream;
import java.security.InvalidKeyException;
import java.security.Key;
import java.security.NoSuchAlgorithmException;

import javax.crypto.Cipher;
import javax.crypto.CipherInputStream;
import javax.crypto.NoSuchPaddingException;
import javax.crypto.spec.SecretKeySpec;

public class MessageDESDecrypter {

    private static final String PASSWD = System.getProperty("passwd", "__secret");
    private static final String SOURCE = System.getProperty("source", "target/test.secured");

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

        ByteArrayOutputStream sink = new ByteArrayOutputStream();
        FileInputStream in = new FileInputStream(SOURCE);
        Cipher cipher = createCipher(PASSWD.getBytes());
        
        decode(in, cipher, sink);
        
        String decoded = new String(sink.toByteArray());
        System.out.println(decoded);
    }

    static Cipher createCipher(byte[] secret) throws NoSuchAlgorithmException, NoSuchPaddingException, InvalidKeyException {
        Cipher cipher = Cipher.getInstance("DES");
        Key key = new SecretKeySpec(secret, "DES");
        cipher.init(Cipher.DECRYPT_MODE, key);
        return cipher;
    }

    static void decode(InputStream in, Cipher cipher, OutputStream sink) throws IOException {

        try (CipherInputStream cis = new CipherInputStream(in, cipher)) {
            for (int b; (b = cis.read()) != -1;) {
                sink.write(b);
            }
        }
    }
}