Cryptography Workshop

Background

In today’s workshop you are going explore some cryptographic concepts with some hands-on exercise.

In general, you should prefer using existing cryptographic protocols where possible. Because any small mistake can leave it vulnerable. However, it is still good to have some experience in working with various algorithms to know how to use them and what pitfalls or limitations there are.

For the exercises to make sense you will be working in a setting where two applications are communicating over the network. Both applications can run on your own machine. Just pretend there is a malicious person sniffing the traffic between the applications.

Create your solution to the exercises as a console application. It is okay to hard-code the messages.

Sockets

I think the easiest way to get started is simply to use raw TCP sockets.

Since the majority of students in the class are working in .NET, then I’ve made a small wrapper around the built-in socket, that can make it a bit easier to work with. See SocketWrapper.cs.

You can layer JSON on top if you need more structure to the messages send/received.

You are not required to use a specific programming language in this course. So, you can use whatever programming language you prefer for the exercises. Here are some links to learn more about sockets in various programming languages.

• TCP Overview in .NET
• Python - Socket Programming HOWTO
• A Guide to Java Sockets
• Socket.IO (JavaScript)

You will need two programs. One that listens for connections (server). And another that connects to the first (client).

Once a connection have been established, client and server can send messages back and forth.

Client or server can simply take turns either sending or receiving messages.

Crypto libraries

You will need a crypto library that implements the various algorithms referred to in the exercises. Here are some:

.NET

• System.Security.Cryptography

Python

• PyCryptodome
• cryptography

Node.js

• Crypto

Prefer built-in libraries over 3rd party libraries. If an algorithm isn’t provided by built-in library then pick a popular 3rd party.

Exercises

Exercise 1

Send an encrypted message over the socket. The messages must be decrypted on the other end using a pre-shared symmetric key.

You can use AES-GCM as cipher. Remember IV needs to be unique each time you encrypt a message. Use a Cryptographically secure pseudorandom number generator such as RandomNumberGenerator.

For .NET see Authenticated Encryption in .NET with AES-GCM.

Exercise 2

The same as in previous exercise, but use an encryption key derived from a password instead.

In .NET there are two implementations of PBKDF2. KeyDerivation.Pbkdf2 and Rfc2898DeriveBytes.Pbkdf2.

For Argon2, scrypt and bcrypt you might need a 3rd party package.

Exercise 3

Same as previous, but use a variant of Diffie–Hellman key exchange instead of having a pre-shared key / secret.

Make a DHKE between the communicating parties. Then encrypt the message at one end using the resulting key. Decrypt at the other end.

You can use Elliptic Curve Diffie-Hellman. For .NET use ECDiffieHellman.

Note: you will need to export the public key and send it to the other end.

Exercise 4

Modify your solution from previous exercise such that a session key is used to encrypt the message.

On the end that is sending the message, you should:

1. Generate a random key SK (used as session key)
2. Use SK to encrypt the message
3. Encrypt SK using the shared secret from DHKE
4. Send the encrypted message + encrypted SK to receiver

On receiving end:

1. Decrypt SK using shared secret from DHKE
2. Decrypt message using SK

Elliptic Curve (EC) cryptography can not encrypt data directly. A hybrid crypto system is used to provide encryption with EC. Hybrid means combining symmetric and asymmetric cryptography like in this exercise.

Is your solution safe from man-in-the-middle attacks (MITM)? What will it take to make it resilient against MITM attacks?