Encrypt in python and decrypt in Java with AES-CFB

Question

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 decry

Answer 1

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");.

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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.