java - How to fix Invalid AES key length?

Question

I am working on a text encryption and decryption project (following Struts 2)

Whenever I enter the password and the plain text I get a Invalid AES Key Length error.

The Service Class

package com.anoncrypt.services;

import java.security.Key;
import javax.crypto.Cipher;
import javax.crypto.spec.SecretKeySpec;

import sun.misc.BASE64Decoder;
import sun.misc.BASE64Encoder;

public class SymAES
{
    private static final String ALGORITHM = "AES";
    private static byte[] keyValue= new byte[] { 'T', 'h', 'i', 's', 'I', 's', 'A', 'S', 'e', 'c', 'r', 'e', 't', 'K', 'e', 'y' };

     public  String encode(String valueToEnc) throws Exception {
        Key key = new SecretKeySpec(keyValue, ALGORITHM);
        Cipher c = Cipher.getInstance(ALGORITHM);
        c.init(Cipher.ENCRYPT_MODE, key);
        byte[] encValue = c.doFinal(valueToEnc.getBytes());
        String encryptedValue = new BASE64Encoder().encode(encValue);
        return encryptedValue;
    }

    public  String decode(String encryptedValue) throws Exception {
        Key key = new SecretKeySpec(keyValue, ALGORITHM);
        Cipher c = Cipher.getInstance(ALGORITHM);
        c.init(Cipher.DECRYPT_MODE, key);
        byte[] decordedValue = new BASE64Decoder().decodeBuffer(encryptedValue);
        byte[] decValue = c.doFinal(decordedValue);
        String decryptedValue = new String(decValue);
        return decryptedValue;
    }

    public  void start(String passcode)throws Exception
    {
        keyValue = passcode.getBytes();
    }
}

---

And this is the error

java.security.InvalidKeyException: Invalid AES key length: 6 bytes
    com.sun.crypto.provider.AESCrypt.init(AESCrypt.java:87)
    com.sun.crypto.provider.ElectronicCodeBook.init(ElectronicCodeBook.java:93)
    com.sun.crypto.provider.CipherCore.init(CipherCore.java:582)
    com.sun.crypto.provider.CipherCore.init(CipherCore.java:458)
    com.sun.crypto.provider.AESCipher.engineInit(AESCipher.java:307)
    javax.crypto.Cipher.implInit(Cipher.java:797)
    javax.crypto.Cipher.chooseProvider(Cipher.java:859)
    javax.crypto.Cipher.init(Cipher.java:1229)
    javax.crypto.Cipher.init(Cipher.java:1166)
    com.anoncrypt.services.SymAES.encode(SymAES.java:35)
    com.anoncrypt.actions.SymEncrypt.execute(SymEncrypt.java:24)

Answer 1

Things to know in general:

1. Key != Password
   • SecretKeySpec expects a key, not a password. See below
2. It might be due to a policy restriction that prevents using 32 byte keys. See other answer on that

In your case

The problem is number 1: you are passing the password instead of the key.

AES only supports key sizes of 16, 24 or 32 bytes. You either need to provide exactly that amount or you derive the key from what you type in.

There are different ways to derive the key from a passphrase. Java provides a PBKDF2 implementation for such a purpose.

I used erickson's answer to paint a complete picture (only encryption, since the decryption is similar, but includes splitting the ciphertext):

SecureRandom random = new SecureRandom();
byte[] salt = new byte[16];
random.nextBytes(salt);

KeySpec spec = new PBEKeySpec("password".toCharArray(), salt, 65536, 256); // AES-256
SecretKeyFactory f = SecretKeyFactory.getInstance("PBKDF2WithHmacSHA1");
byte[] key = f.generateSecret(spec).getEncoded();
SecretKeySpec keySpec = new SecretKeySpec(key, "AES");

byte[] ivBytes = new byte[16];
random.nextBytes(ivBytes);
IvParameterSpec iv = new IvParameterSpec(ivBytes);

Cipher c = Cipher.getInstance("AES/CBC/PKCS5Padding");
c.init(Cipher.ENCRYPT_MODE, keySpec, iv);
byte[] encValue = c.doFinal(valueToEnc.getBytes());

byte[] finalCiphertext = new byte[encValue.length+2*16];
System.arraycopy(ivBytes, 0, finalCiphertext, 0, 16);
System.arraycopy(salt, 0, finalCiphertext, 16, 16);
System.arraycopy(encValue, 0, finalCiphertext, 32, encValue.length);

return finalCiphertext;

---

Other things to keep in mind:

• Always use a fully qualified Cipher name. AES is not appropriate in such a case, because different JVMs/JCE providers may use different defaults for mode of operation and padding. Use AES/CBC/PKCS5Padding. Don't use ECB mode, because it is not semantically secure.
• If you don't use ECB mode then you need to send the IV along with the ciphertext. This is usually done by prefixing the IV to the ciphertext byte array. The IV is automatically created for you and you can get it through cipherInstance.getIV().
• Whenever you send something, you need to be sure that it wasn't altered along the way. It is hard to implement a encryption with MAC correctly. I recommend you to use an authenticated mode like CCM or GCM.