2

The Go Challenge 2

Can you help me secure my company’s data transmission?

Last week, our competitor released a feature that we were working on in secret. We suspect that they are spying on our network. Can you help us prevent our competitor from spying on our network?

To get started

Check out the two files main.go and main_test.go here. These files are the starting point for this challenge.

3rd April 2015: Guillaume has made 2 changes to the test files:

• The first change is minor and concerns Window user with dual stack. He has changed the net.Listen("tcp", ":0") to net.Listen("tcp", "127.0.0.1:0"). This shouldn’t change anything for most people.
• The second is more problematic and would impact people using io.Copy. In TestReadWritePing, he has moved the w.Close() in the writing goroutine just like in the other tests.

Goal of the challenge

In order to prevent our competitor from spying on our network, we are going to write a small system that leverages NaCl to establish secure communication. NaCl is a crypto system that uses a public key for encryption and a private key for decryption.

Your goal is to implement the functions in main.go and make it pass the provided tests in main_test.go.

Steps involved

The first step is going to be able to generate the public and private keys. Next, we want to create an io.Writer and io.Reader that will allow us to automatically encrypt/decrypt our data.

Part 1

Implement the following helpers that will return our NACL Reader / Writer.

func NewSecureReader(r io.Reader, priv, pub *[32]byte) io.Reader
func NewSecureWriter(w io.Writer, priv, pub *[32]byte) io.Writer

Part 2

Now that we can encrypt/decrypt message locally, it would be interesting to do so over the network!

We are going to write a server that will exchange keys with the client in order to establish a secure communication.

In order to be able to encrypt/decrypt, we need to perform a key exchange upon connection.

For the sake of the exercise, performing the key exchange in plain text is acceptable. (In a production system, it would not be acceptable due to MITM risk!)

Unfortunately, everybody has already left for the day, so let’s write a secure echo server so we can test!

In order to test our echo server, we can do:

$> ./challenge2 -l 8080&
$> ./challenge2 8080 “hello world”
hello world

Requirements of the challenge

• Use the latest version of Go i.e. version 1.4.2
• Use only standard library and package(s) under golang.org/x/crypto/nacl.

Hints

• We consider that our messages will always be smaller than 32KB
• Most of the elements involved for encryption/decryption have fixed length
• encoding/binary can be useful for the variable parts

Further exploration

If you have liked this challenge, you can keep programming outside the main challenge. You can:

• create an actual user interface. It would be nice if both side had a prompt and could send more than one message at a time.
• Handle multiple client and group chat
• Add compression

Please keep in mind that cryptography is a complex subject matter, with many very subtle risks and mistakes to make. While it makes for a fun exercise, inventing your own crypto systems for production use is usually not a good idea and should be left to a handful of very talented people.