android加密

Question

I want to encrypt/decrypt some passwords in the SQLite database of my application. To do that I have searched on the internet and I have found the AES algorithm. I have this code:

public String encript(String dataToEncrypt)
        throws NoSuchAlgorithmException, NoSuchPaddingException,
        InvalidKeyException, IllegalBlockSizeException, BadPaddingException {
    // I'm using AES encription

    if(!dataToEncrypt.equals("")){
        String key = "FMVWf8d_sm#fz";

        Cipher c = Cipher.getInstance("AES");
        SecretKeySpec k;
        try {
            k = new SecretKeySpec(key.getBytes(), "AES");
            c.init(Cipher.ENCRYPT_MODE, k);
        } catch (UnsupportedEncodingException e) {
            e.printStackTrace();
        }


        return new String(c.doFinal(Base64.decode(dataToEncrypt)));
    }
    return "";
}

public String decript(String encryptedData)
        throws NoSuchAlgorithmException, NoSuchPaddingException,
        InvalidKeyException, IllegalBlockSizeException, BadPaddingException {

    if(!encryptedData.equals("")){
        String key = "FMVWf8d_sm#fz";

        Cipher c = Cipher.getInstance("AES");
        SecretKeySpec k = new SecretKeySpec(Base64.decode(key), "AES");
        c.init(Cipher.DECRYPT_MODE, k);
        return new String(c.doFinal(Base64.decode(encryptedData)));
    }
    return "";
}

After running this I get this error on encrypt method:

01-27 14:50:51.698: ERROR/ACTIVITY(782): java.security.InvalidKeyException: Key length not 128/192/256 bits.

I have seen some other cases here on stackoverflow but I want to give the key to the AES not to generate it...

Can somebody help me with this? If there is other encryption method to use but without using another jars or external classes and to let me give the key.

Thank you very much!

Answer 1

The error message makes it perfectly clear: your encryption key must be of certain size: 128, 192 or 256 bits. And your key is 104 bits. Note, that as you want to use only printable characters in your key, the length of the key should be 192 or longer bits, cause your alphabet (set of characters that you use) makes encryption weaker.

Answer 2

Usual practice is such:

1. Get password (in your case String key = "FMVWf8d_sm#fz"; )
2. Generate using some hash function key with length 128, 192 or 256
3. Put it into encryption algorithm
4. Have fun

So you are missing key generation stage. Do smth like:

       // Get the KeyGenerator

       KeyGenerator kgen = KeyGenerator.getInstance("AES");
       kgen.init(128); // 192 and 256 bits may not be available


       // Generate the secret key specs.
       SecretKey skey = kgen.generateKey();
       byte[] raw = skey.getEncoded();

       SecretKeySpec skeySpec = new SecretKeySpec(raw, "AES");


       // Instantiate the cipher

       Cipher cipher = Cipher.getInstance("AES");

       cipher.init(Cipher.ENCRYPT_MODE, skeySpec);

       byte[] encrypted =
         cipher.doFinal((args.length == 0 ?
          "This is just an example" : args[0]).getBytes());
       System.out.println("encrypted string: " + asHex(encrypted));

       cipher.init(Cipher.DECRYPT_MODE, skeySpec);
       byte[] original =
         cipher.doFinal(encrypted);
       String originalString = new String(original);
       System.out.println("Original string: " +
         originalString + " " + asHex(original));