Creating a fake authentication server

SSL? Digital signatures? Who cares?!

András Rutkai

What is this about?

I've created a fake authentication server for Minecraft.

The reasons? Because I can do it and it is "safer" this way.

But let's start from the beginning...

What is Minecraft?

Minecraft Logo

Minecraft is a sandbox FPS/exploration/building game.

Almost endless possibilities in an almost endless world.

Written in Java ;)

Why is it special?

Usually you get a client and you can play on dedicated servers.

But in Minecraft you can get and maintain a server as well!

The motivation

I want to use the old client and server together without depending on the vendor.

So I need an authentication server for it (to stay in online mode).

Market research

Then I've found the API documentation of the auth server.


And I know the architecture!


So what do we have?

An API documentation with:

  • Endpoints
  • Parameters
  • Responses

This should be a piece of cake!

The Server

The endpoints are simple REST-JSON.

This makes the backend language obvious: Node.js (awesomely faaast)

The database will be MongoDB (future plan)

Node tests

But it's never that easy

The server's endpoints are hard-coded.

The server uses a signed HTTPS connection.

A few packets are signed with a private key.


How to redirect the data from a valid domain to another?

Easy: put a new entry in the hosts file!

Works like a charm. :)

What to do with SSL?

Untrusted certificate

We can create a self-signed certificate only.

Java doesn't accept a self signed certificate. :(

I had to find a way to sign that certificate somehow,

but how can I sign a certificate for a domain, I don't own? :)

Just trust me

Fortunately, you don't have to sign it.

You can simply trust it!

Solution: download the certificate and add it as a trusted certificate (keychain).

A small Java program to download and store it is created.

What about the packet signature?

There are a few endpoints which are signed with a private key.

Obviously, there is no way to obtain the private key.

Then how to sign it?!

The public key

Then I've found the public key in the jar file.

This should be easy now; I just have to replace the public key!

Public key in Jar

Make it easy

I want to create this server for the users, right?

Create a (relatively) easy-to-use tool, a Swiss army knife!

But in which language? We should use a cross platform language which is probably pre-installed on most machines.

Repacking the jars

We want to replace the private key. Which steps are required?

  • Downloading a vanilla jar
  • Unpacking it
  • Twice :@
  • Replacing the key and repacking it


Unfortunately not :( I had to face with this:

Manifest in jar

Replacing the SHA digest

So, during the repacking phase we have to calculate a new digest and replace it.

But still no... did we miss something?

Signatures in jar file

The missing part

More signatures!

Fortunately, Java doesn't mind if I skip "a few" files from the archive, e.g. the signatures.


Signatures everywhere!

However, I'm still not done yet.

I've found a public key in the server's jar and in a library too!

But the same replacement method worked everywhere. :)

My Swiss army knife


Was it boring?

What about a demo?

Thank you!

  • Github:
  • Blog post: