Android: Define which encryption to use: AES 128 or AES 256

Question

In an android project I am using

import javax.crypto.Cipher;

ecipher = Cipher.getInstance("AES/CBC/PKCS5Padding"); 

I would like to ask: Whether AES128 or AES256 is used, is defined by the key that is used? For example key="012345678901234567890123456789012"; would have as a result AES256 to be used?

thanks
Thomas

Answer 1

Yes whether AES128 or AES256 is used is defined by the key that is in use.

However it's not the String length but the byte[] length that determines it. At some point in your code you should be converting String to byte[] . The size of resulting that byte[] is your key size. Without knowing how you convert "012345678901234567890123456789012" , it's not possible to know your encryption strength.

Alternatively you can use KeyGenerator :

    KeyGenerator keyGen = KeyGenerator.getInstance("AES");
    keyGen.init(128); // or 256

    // Generate the secret key specs
    SecretKey secretKey = keyGen.generateKey();
    byte[] byteArray = secretKey.getEncoded();