A Russian researcher, Efim Bushmanov has claimed to have reverse engineered most of Skype’s code and aims to make the project open source. He has written about his findings and has made the necessay binaries to compile the reverse engineered software available for download on his blog.
Bushmanov has further requested for helps from other developers to complete the project of reverse engineering. He further writes,
My aim is to make skype open source. And find friends who can spend many hours for completely reverse it.
Now, most of hard things already done(for 1.x/3.x/4.x versions of skype). Including rc4 and arithmetic compression.
You have unique chance to take a look on skype internal protocol and encryption. You will see what it uses strong AES and RSA encryption with public key infrastructure.
The works has raised some legal concerns. It is still not clear whether he is breaking any laws. It depends on how he has gone through his works. Many claim it to be impossible to replicate a software with clean hands.