I wouldn't venture to say this doesn't belong on HN since it really is
interesting (if it was actually done correctly), but the files available
for download are most likely illegal, were most likely created with
pirated tools (IDA Pro/Hex-Rays, and yes, as a customer of theirs for over a
dozen years I've reported it), and of course, the usual vilification of
reverse engineering.

If you're reading this on a desktop or laptop system (rather than a
phone), then you are most likely using an "IBM PC Compatible" even if
you're using an Intel based Apple, and hence, you're using the fruits of
completely legal reverse engineering.

The way to do reverse engineering legally is to have one team reverse
engineer the target and completely document how it works. Once it's
documented, another disconnected team writes a new implementation from
the documentation. This process is how you're using an IBM PC
Compatible today, so yes, reverse engineering for compatibility is
perfectly legal.

If there is a patented algorithm required, it's not a sure thing. There
are most likely compatible ways around the patent, but there's also the
fact that the patent is only valid in the US. With open source hosted in
some other country, who are you going to sue? The users in the US?
--Nope, users are the ones paying for skype.

You might say, "But we forbid reverse engineering in our license!!!"

Contract clauses forbidding reverse engineering are invalid in many
countries and jurisdictions, and of course, you also have to prove the
other party agreed to the contract/license. With this said, it's very
easy to create a international jurisdictional nightmare to render any
such contract clause tactically impossible to enforce.

The easiest way to think about this is security research. The folks
finding and reporting exploitable flaws in software are obviously
reverse engineering it. Occasionally companies have tried to legally go
after people who have published security research on their products, but
usually this ends very badly for the company. Additionally, doing
security research is protected use in some countries and jurisdictions.

In short, competition is good for markets, and competing by studying and
mimicking the competition is both normal and legal.

For the "rights" advocates out there, there are legal problems with the
three file downloads available:

1.) According to the first file name, the original binaries are being
redistributed which may be (and usually is) against the license terms
and default rights granted by copyrights.

2.) The IDA Pro database (most likely) contains the entire target
binary, so you do have (illegal) redistribution of a copyrighted work.
You can load only parts of a target binary into IDA, but that doesn't
matter since it is still a portion of the original work. As for whether
or not said portion could fall under fair use is debatable (i.e.
lawsuit). In general usage, the entire binary is loaded, since without
it, you're limited to static analysis (i.e. no debugging).

3.) Decompilation, and to a lesser degree disassembly, are equivalent to
"machine translation" in the sense of copyright. Creating a translation
is considered creating a "derivative work" and unless you have been
given rights to create derivative works, then you're in trouble. One of
the comments here on HN claims the "source code" file is the output of
the Hex-Rays Decompiler.

I've never used skype and I've never read their license so I don't know
if they specifically allow redistribution.

I have no love for skype or microsoft, but if this had been done
CORRECTLY by releasing written documentation so an entirely new
implementation could be written, then I'd have no problem with it.
There are right ways and wrong ways to legally create compatible (open
source) software through reverse engineering, and this is a perfect
example of the wrong way.