Decodeing a encoded string in java?

Question

In my application I i'm encoding a string and then generating a MAC id (using javax.crypto.Mac).But i tried to decode it back ,but i couldn't.Can you please help me out.Can you point out where I have done wrong?

Code

String userid = "AmilaI";
String time = gmtFormat.format(now)+ "Z";

String algorithmKey = time + userid;

SecretKeySpec sks = new SecretKeySpec(algorithmKey.getBytes("UTF-8"), "HmacSHA1");
Mac mac = Mac.getInstance("HmacSHA1");
mac.init(sks);
byte[] hashBytes = mac.doFinal(route.getBytes("UTF-8"));

String hmac = Base64.encodeBase64String(hashBytes);
hmac = hmac.replaceAll("\r\n", "");
System.out.println("Encrypted "+ hmac );

byte[] decoded = Base64.decodeBase64(hmac);
System.out.println("Decrypted " + new String(decoded, "UTF-8") + "\n");

How can I reverse the path and get alogorithmKey back so I can get the user id by decrypting?

Nadir · Accepted Answer · 2014-09-16 08:43:16Z

6

HmacSHA1 is a hash so it means it works only one way, you cannot get the original value from it. You would need to use algorithm that is reversible.

answered Sep 16, 2014 at 8:43

Nadir

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Comments

Chirag Jain · Accepted Answer · 2014-09-16 08:52:27Z

Try this

 public class Crypto {

        private static final String engine = "AES";
        private static final String crypto = "AES/CBC/PKCS5Padding";
        private static Context ctx;

        public Crypto(Context cntx) {
            ctx = cntx;
        }

        public byte[] cipher(byte[] data, int mode)
                throws NoSuchAlgorithmException, NoSuchPaddingException,
                InvalidKeyException, IllegalBlockSizeException,
                BadPaddingException, InvalidAlgorithmParameterException {
            KeyManager km = new KeyManager(ctx);
            SecretKeySpec sks = new SecretKeySpec(km.getId(), engine);
            IvParameterSpec iv = new IvParameterSpec(km.getIv());
            Cipher c = Cipher.getInstance(crypto);
            c.init(mode, sks, iv);
            return c.doFinal(data);
        }

        public byte[] encrypt(byte[] data) throws InvalidKeyException,
                NoSuchAlgorithmException, NoSuchPaddingException,
                IllegalBlockSizeException, BadPaddingException,
                InvalidAlgorithmParameterException {
            return cipher(data, Cipher.ENCRYPT_MODE);
        }

        public byte[] decrypt(byte[] data) throws InvalidKeyException,
                NoSuchAlgorithmException, NoSuchPaddingException,
                IllegalBlockSizeException, BadPaddingException,
                InvalidAlgorithmParameterException {
            return cipher(data, Cipher.DECRYPT_MODE);
        }

        public String armorEncrypt(byte[] data) throws InvalidKeyException,
                NoSuchAlgorithmException, NoSuchPaddingException,
                IllegalBlockSizeException, BadPaddingException,
                InvalidAlgorithmParameterException {
            return Base64.encodeToString(encrypt(data), Base64.DEFAULT);
        }

        public String armorDecrypt(String data) throws InvalidKeyException,
                NoSuchAlgorithmException, NoSuchPaddingException,
                IllegalBlockSizeException, BadPaddingException,
                InvalidAlgorithmParameterException {
            return new String(decrypt(Base64.decode(data, Base64.DEFAULT)));
        }
    }

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