IllegalBlockSizeException when trying to encrypt and decrypt a string with AES

Question

I have a hardcoded key with which I want to encrypt a string before storing it in SharedPreferences. This is the code I have so far:

public class TokenEncryptor {

    private final static String TOKEN_KEY = "91a29fa7w46d8x41";

    public static String encrypt(String plain) {
        try {
            Cipher cipher = Cipher.getInstance("AES/CBC/PKCS5Padding");
            AlgorithmParameterSpec ivSpec = new IvParameterSpec(new byte[16]);
            SecretKeySpec newKey = new SecretKeySpec(TOKEN_KEY.getBytes(), "AES");
            cipher.init(Cipher.ENCRYPT_MODE, newKey, ivSpec);
            return new String(cipher.doFinal(plain.getBytes()));
        } catch (Exception e) {
            Ln.e(e);
            return null;
        }
    }

    public static String decrypt(String encoded) {
        try {
            Cipher cipher = Cipher.getInstance("AES/CBC/PKCS5Padding");
            AlgorithmParameterSpec ivSpec = new IvParameterSpec(new byte[16]);
            SecretKeySpec newKey = new SecretKeySpec(TOKEN_KEY.getBytes(), "AES");
            cipher.init(Cipher.DECRYPT_MODE, newKey, ivSpec);
            return new String(cipher.doFinal(encoded.getBytes()));
        } catch (Exception e) {
            Ln.e(e);
            return null;
        }
    }
}

It seems to be catching an exception at the end of decrypt method:

javax.crypto.IllegalBlockSizeException: error:0606506D:digital envelope routines:EVP_DecryptFinal_ex:wrong final block length

Can someone point me in the right direction? I have a feeling I'm doing something wrong instantiating IvParameterSpec.

Answer 1

When you encrypt a string with AES, you get an array of bytes back. Trying to convert those bytes directly to a string (new String(cipher.doFinal(plaintextBytes))) will cause all sorts of problems. If you require the output from your encryption method to be a string, then use Base64 rather than attempting a direct conversion. In your decryption method, convert the Base64 string back into a byte array before decrypting the byte array.

Also, do not use getBytes() since the output depends on the system defaults. Use getBytes("utf-8") or whatever. That eliminates ambiguity.