Srikanth Kandalam Srikanth Kandalam - 1 month ago 15
Java Question

Encrypt in python and decrypt in Java with AES-CFB

I am aware of a question very similar to this (How do I encrypt in Python and decrypt in Java?) but I have a different problem.

My problem is, I am not able to decrypt in Java correctly. Despite using the correct key and IV, I still get garbage characters after decryption. I don't have any compile/run-time errors or exceptions in Java so I believe I am using the right parameters for decryption.

Python Encryption Code -

from Crypto.Cipher import AES
import base64
key = '0123456789012345'
iv = 'RandomInitVector'
raw = 'samplePlainText'
cipher = AES.new(key,AES.MODE_CFB,iv)
encrypted = base64.b64encode(iv + cipher.encrypt(raw))


Java Decryption Code -

private static String KEY = "0123456789012345";
public static String decrypt(String encrypted_encoded_string) throws NoSuchAlgorithmException, NoSuchPaddingException,
InvalidKeyException, IllegalBlockSizeException, BadPaddingException {

String plain_text = "";
try{
byte[] encrypted_decoded_bytes = Base64.getDecoder().decode(encrypted_encoded_string);
String encrypted_decoded_string = new String(encrypted_decoded_bytes);
String iv_string = encrypted_decoded_string.substring(0,16); //IV is retrieved correctly.

IvParameterSpec iv = new IvParameterSpec(iv_string.getBytes());
SecretKeySpec skeySpec = new SecretKeySpec(KEY.getBytes("UTF-8"), "AES");

Cipher cipher = Cipher.getInstance("AES/CFB/NoPadding");
cipher.init(Cipher.DECRYPT_MODE, skeySpec, iv);

plain_text = new String(cipher.doFinal(encrypted_decoded_bytes));//Returns garbage characters
return plain_text;

} catch (Exception e) {
System.err.println("Caught Exception: " + e.getMessage());
}

return plain_text;
}


Is there anything obvious that I am missing?

Answer

The Cipher Feedback (CFB) mode of operation is a family of modes. It is parametrized by the segment size (or register size). PyCrypto has a default segment size of 8 bit and Java (actually OpenJDK) has a default segment size the same as the block size (128 bit for AES).

If you want CFB-128 in pycrypto, you can use AES.new(key, AES.MODE_CFB, iv, segment_size=128). If you want CFB-8 in Java, you can use Cipher.getInstance("AES/CFB8/NoPadding");.


Now that we have that out the way, you have other problems:

  • Always specify the character set you're using, because it can change between different JVMs: new String(someBytes, "UTF-8") and someString.getBytes("UTF-8"). When you do, be consistent.

  • Never use a String to store binary data (new String(encrypted_decoded_bytes);). You can copy the bytes directly: IvParameterSpec iv = new IvParameterSpec(Arrays.copyOf(encrypted_decoded_bytes, 16)); and cipher.doFinal(Arrays.copyOfRange(encrypted_decoded_bytes, 16, encrypted_decoded_bytes.length)).

  • In Java, you're assuming that the IV is written in front of the ciphertext and then encoded together, but in Python, you're never doing anything with the IV. I guess you posted incomplete code.

  • It is crucial for CFB mode to use a different IV every time if the key stays the same. If you don't change the IV for every encryption, you will create a multi-time pad which enables an attacker to deduce the plaintext even without knowing the key.

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