.. _decryption:

============================
Media Server File Decryption
============================

.. contents::
   :local:
   :depth: 3

Overview
~~~~~~~~

Example script to decrypt a file downloaded from Acrobits Media Server.

Python Example
~~~~~~~~~~~~~~

.. code-block:: python
		
		#!/usr/bin/python3

		from cryptography.hazmat.primitives.ciphers import Cipher, algorithms, modes
		from cryptography.hazmat.backends import default_backend
		from binascii import unhexlify,crc32

		key = unhexlify('F4EC56A83CDA65B2C6DC11E2CF693DAA')
		nonce = b'\0'*len(key)

		cipher = Cipher(algorithms.AES(key), modes.CTR(nonce), backend=default_backend())
		dec = cipher.decryptor()

		file_in = open('encrypted', 'rb')
		file_out = open('decrypted.jpg', 'wb')

		crc = 0

		while True:
		  enc_block=file_in.read(10240)
		  if not enc_block:
		     break
		  print('.', end='')
		  dec_block = dec.update(enc_block)
		  crc = crc32(dec_block, crc)
		  file_out.write(dec_block)

		print()

		file_in.close()
		file_out.close()

		print("Hash: "+str(crc))

JavaScript Example
~~~~~~~~~~~~~~~~~~

The following example requires the latest version (16.x) of Node.js LTS. The `crc` module is 
3rd-party, and therefore optional. It is only required to verify the CRC of decrypted files, and 
any lines using it can be removed.

.. code-block:: javascript

    import { webcrypto } from 'crypto';
    import { readFile, writeFile } from 'fs/promises';

    // crc module is optional and only needed if crc calculation functionality is required
    // run `npm install crc` to install if needed
    import crc from 'crc'; // remove if not using crc


    const { subtle } = webcrypto;
    const { crc32 } = crc; // remove if not using crc

    const hexKey = 'F4EC56A83CDA65B2C6DC11E2CF693DAA';
    const keyArr = Uint8Array.from(hexKey.match(/.{1,2}/g).map(hex => parseInt(hex, 16)));
    const counter = new Uint8Array(keyArr.length);

    const key = await subtle.importKey('raw', keyArr, "AES-CTR", true, ['decrypt']);
    const input = await readFile('encrypted', { flag: 'r' });

    const decResult = await subtle.decrypt(
      {
        name: 'AES-CTR',
        counter,
        length: 128
      },
      key,
      input
    );

    const outBuf = Buffer.from(decResult);
    await writeFile('decrypted_js.jpg', outBuf);

    console.log(`Hash: ${crc32(outBuf)}`); // remove if not using crc



OpenSSL Example
~~~~~~~~~~~~~~~

.. code-block:: sh

	openssl enc -aes-128-ctr  -d  -K F4EC56A83CDA65B2C6DC11E2CF693DAA  -iv 00000000000000000000000000000000 -nopad -in encrypted -out decrypted_file.jpg

	crc32 decrypted_file.jpg

The encrypted version is available :download:`here <encrypted>`.

The decrypted version is available :download:`here <decrypted.jpg>`.

The hex-encoded key for this file is ``F4EC56A83CDA65B2C6DC11E2CF693DAA``

The unsigned decimal integer hash of the original is ``4488649``