Skype protocol reverse engineered, source available

http://skype-open-source.blogspot.com/

by jbk • 15 years ago

451 126 15 years ago

skype-open-source.blogspot.com

iwwr• 15 years ago

Better start uploading this to torrents if its legit. Skype will issue takedowns any moment.

vdm• 15 years ago

Already done by OP.

michael_dorfman6• 15 years ago

My aim is to make skype open source.

Isn't that decision up to the folks who own the rights to the original code?

Personally, I'd be hesitant to get into a project that has more need for lawyers than coders.

crocowhile3• 15 years ago

I think that claim would be better off if rephrased as "my aim is to make a skype compatible open source software"

michael_dorfman6• 15 years ago

That still doesn't mitigate the need for lawyers. The protocol is still intellectual property, and reverse engineering it may not be without consequences.

dennis714• 15 years ago

It is looking like result of Hex-Rays decompiler.

crocowhile3• 15 years ago

Can you say more?

michael_dorfman6• 15 years ago

A simple Google search turns up this: http://www.hex-rays.com/decompiler.shtml

yread• 15 years ago

with identifiers like unpack_41_715680 and the_thing it is not exactly easy to read

zerd• 15 years ago

That's life for reverse engineered stuff. If you don't really know what a function does, how do you name it?

daeken• 15 years ago

You come up with a name based on the observed behavior. I'm honestly amazed that they've made it as far as they have without decent naming conventions -- the use of good identifiers in your IDBs cuts reversing time way, way down.

braindead_in3• 15 years ago

It seems that the encryption algorithm has been reverse engineered. I guess you'll still need the keys to decrypt the voice data using this algorithm, assuming it works. It's a big deal if has been done, because a lot of people have been trying to crack it. Some governments are going to love this. The Skype client itself has a lot of obfuscation to prevent something like this.

JoshTriplett4• 15 years ago

The ability to interoperate with Skype as one of the participants in a call has little to do with the ability to intercept and decrypt Skype calls as a non-participant in a call, unless Skype uses broken crypto (a very real possibility).

46Bit2• 15 years ago

I think it's fair to surmise that those intelligence agencies that care have probably had the algorithm for a long time and searched for weaknesses. Bear in mind that at one time they were complaining about it's use by criminals to avoid phonetaps.

braindead_in3• 15 years ago

There has been some speculation about a backdoor in Skype which it has shared with intelligence agencies. Never confirmed by Skype of course. But this could allow anyone to decrypt a Skype conversation stream. All you need is a Skype supernode to get started. Or some kind of spyware on the subject's computer which stores/transmits the data stream.

Animus7• 15 years ago

If the goal is to open up Skype, this isn't the way.

Even if some insane insomniac de-twiddles the pages upon pages of optimized indirection in this code (which I seriously doubt), all Skype has to do is tweak the protocol or encryption and the researcher is back to square one. It's a losing battle. And that's not even getting into the legality of it it all.

How about instead of trying to fruitlessly crack Skype, we spend the time making something that's both open and better?

JoshTriplett4• 15 years ago

Both seem quite valuable. Long-term, we want an open peer-to-peer encrypted communication system. Short-term, until Skype dies, it would help to have the ability to interoperate with people who use it. Similarly, while XMPP represents the right open standard for chat, existing Open Source chat programs still need to know how to interoperate with MSN, AIM, and Yahoo

werg• 15 years ago

If the system can successfully masquerade as an older Skype version it stands a chance. That is unless Skype has a baked-in not yet understood mechanism for pushing protocol changes to its clients. Skype would have to find discerning features, implement it on their servers and possibly even push it to their client updates. Whenever such discerning features were found it's a simple matter of arms race, i.e. a difficult but fair chance. Alternatively Skype can start blocking older versions which is rather unlikely.

Concerning the open and better issue; There definitely are open alternatives. None of them have quite the firewall-defying capabilities of Skype. Nor the user base for that matter. Building any kind of social network is fraught with chicken&egg problems and those first to reach mass have it made. Just check how one of the richest and most powerful technology companies Google is struggling to get a foot in Facebook's market.

jacobr• 15 years ago

There is WebRTC (http://sites.google.com/site/webrtc/) for Real Time Communication/Conferencing which Google just open sourced. I think that's the way forward rather than developing a separate "Skype-killer" protocol/app.

zhyder• 15 years ago

It's not that easy for Skype to tweak their protocol, since they have to ensure that slightly older installed versions continue to interoperate.

peterwwillis3• 15 years ago

> How about instead of trying to fruitlessly crack Skype, we spend the time making something that's both open and better?

There's value in making an open client that works with the existing network (for example, Microsoft recently killed the Skype integration plug-in for Asterix).

tene• 15 years ago

"Asterisk"

evanwolf6• 15 years ago

Skype is not yet part of Microsoft. Skype retired the little-used app on its own.

LiveTheDream• 15 years ago

You may be interested in this[1], open technology for voice and video on the web.

[1] http://news.ycombinator.com/item?id=2605390

noarchy• 15 years ago

Are they going to force an upgrade to the Skype client, in order to enforce a change to the protocol? That would not be a great move. A lot of us are using older clients, due to how much we loathe the newer versions.

VMG3• 15 years ago

How easy is it for skype to change the protocol?

eleusive• 15 years ago

Not very easy if they want to maintain compatibility with existing clients (especially considering they haven't updated their linux client for years now).

neworbit• 15 years ago

Backwards compatibility with linux clients probably isn't the top priority of Skype now that they're being rolled into MS, but I suspect embedded deployments on handheld devices might still keep them from deprecating.

ysangkok• 15 years ago

The latest Linux version is from April this year.

tttp• 15 years ago

Two major issues: http://skype-open-source.blogspot.com/2011/06/skype-under-rc... || Copyright (c) 2004-2009 by VEST Corporation. || All rights reserved. Strictly Confidential! The project is using existing code, and "All rights reserved" is not an approved OSI license I believe ;)

And things like compression algorithm are patented, and that's very likely skype is using some of it. Reminds me of a project by Intel of providing an implementation of g729 (a voice codec). The source was available, but it was "non commercial usage only" because of the patents mostly.

An interesting project, but doubt we'll see any usable implementation anytime soon IMO. And even if it does, skype will probably alter a bit the protocol to make it fail if it reaches a critical mass.

drdaeman• 15 years ago

Software patents are mostly US-only problem. As Efim's probably a Russian, this is not an issue.

And Skype can't really easily alter the protocol because of all sort of skype-compatible devices (IP phones and so on).

TerryS• 15 years ago

"All rights reserved" is not an obstacle to an OSI license. If you open source some code then you can still reserve all your rights to the code. This mean that you can redistribute/sell/license it under any other licensing schemes you wish to. Other people, of course, can only use it under the license you grant them.

Joeboy2• 15 years ago

From what I understand, the big unsolved problem open VOIP options have isn't the voice bit, it's negotiating a connection through NAT/firewalls/dodgy routers etc. I doubt this helps with those problems.

__rkaup__• 15 years ago

What makes VoIP different from any other protocols, like HTTP?

Joeboy2• 15 years ago

I won't pretend to have any actual expertise on the subject, but here is some relevant discussion by someone that does:

http://linux.slashdot.org/comments.pl?sid=2170464&cid=36...

code_duck• 15 years ago

I don't believe most people are thinking there's something exciting about Skype's technology. They just want to be able to make things compatible with Skype.

senko• 15 years ago

Relatedly: there was a presentation about (reversing) Skype internals on BlackHat Europe 2006 (warning, PDF): http://www.blackhat.com/presentations/bh-europe-06/bh-eu-06-...

Skype probably updated their client/protocol since, but still an interesting read.

foxhill2• 15 years ago

aside from the awesome technical exersize in hacking, i don't see this as any net benefit for VOIP.

the time would have been much better spent working on the GNU VOIP client, not only would improvements have been usable without legal issues, they would be there in an (ostensibly, perhaps) understandable format - working code.

pyre2• 15 years ago

They could reverse engineer and document the firewall/nat negotiation bit, which could then be incorporated into the GNU client.

foxhill2• 15 years ago

annoying and tedious that sort of code is, i agree, but not impossible?

besides, i was under the impression that audio/video chats went through skype servers? (i've never actually checked though)

bergie• 15 years ago

The right way to go about that would be to have a Telepathy provider that talks Skype.

That is actually how Skype works in N900, but the implementation there is obviously proprietary.

jcr10• 15 years ago

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.

EliRivers• 15 years ago

HN is an internationally read site. Things that are illegal in some countries are not illegal in others. As responsible citizens, it is up to the individual to not engage in illegal activities in the region said individual is in.

jcr10• 15 years ago

I think you misread my statement. I should have been more clear, but if you read it again, you'll see I agree with you. It is interesting and belongs here.

Create2• 15 years ago

You are mentioning Compaq BIOS. Think Samba though.

Even OOo/LO .doc support is based on 1-2 FTE revEngs (which btw is dumped mfc/w32 memory on a FAT, but read Sun/IBM anyway).

The team will write a public spec, print it out on paper, and another team Down and Under will scan it and create new code (think RSA patent export). The skype protocol has long been reverse engineered and is available to several parties.

VMG3• 15 years ago

Why do you assume the tools are pirated?

tommi• 15 years ago

The older IDA Pro 5.0 is licensed as freeware, free for non-commercial use.

http://www.hex-rays.com/idapro/idadown.htm

jcr10• 15 years ago

I think you actually want two answers, the question you asked and "why report it?"

The IDA Pro disassembler and the Hex-Rays decompiler are not only very expensive tools, but they are very difficult to purchase. Due to constant problems with piracy, these days they will only sell their products to three areas; (1) governments/law enforcement, (2) very well established corporations (typically well known security research people), (3) very well established university researchers.

Typically, they refuse to sell to individuals, but there is a fourth class of customers who are individuals; very old customers like me who have a perfect track record of maintaining possession of their copy of the software.

Every copy of the software is custom compiled and watermarked so it is traceable to a particular person. Every database created by the software is also watermarked, so when someone who is not a licensed customer publishes a database (.idb), the software can be traced and the account will be terminated (i.e. no further purchases allowed).

When someone does something blatantly stupid like disassembling and decompiling skype then publicly making all of the files available, it is fairly certain that they are using a illegal copy of the software. They do not understand what they're doing. They do not understand the tool they are using. And they don't have any respect for either the tool or the work of others. --All of this loudly screams PIRATE!

The pirates either don't know about or don't care about the watermarks in the databases they create. They don't realize that publishing a database is discouraged. I've never heard of a case where a database watermark was successfully forged (i.e. pin the blame on someone else), but a cracker named "Quine" once successfully removed the watermarking in IDA back in the late 90's.

The "correct" method to publicly share the research work done in IDA is to dump the database to an IDC script (an internal language), then provide the IDC script and the target binary. Customers know this, or at least they should. With that said, friends do toss databases back and forth on occasion, but that's a matter of trust between friends where both of them are customers. Some people in the InfoSec and AntiVirus crowds exchange databases, even across competing corporate lines since they're all working together towards the same goal and they've known each other for years.

MostAwesomeDude• 15 years ago

While you are correct about this particular instance of RE, I want to just take this opportunity to remind you that hands-on black-box RE is the technique used to create many of the drivers you see in Linux and BSD. Prior to AMD and Intel releasing video card documentation, every video card supported through community drivers was usually best-understood through RE experiments.

skypeopensource4• 15 years ago

Also, for 1) this is not original binaries. This is obfuscation and anti-debug features removed.

praptak2• 15 years ago

This does not change the legal status much. It is derived work (at best.)

rmc• 15 years ago

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.

So you'd Skype's co-operation to do this? They are able to prevent reverse engineering by not writing the documents?

jcr10• 15 years ago

If it is done correctly, co-operation from skype is not required. The team that does the reverse engineering will write the specs and documentation from what they learn by examining and analyzing the executable binaries.

pflats• 15 years ago

Not at all.

Suppose you and I work for the same company. I bust open Skype through decompilation, reading memory, the network, whatever trick I want. With that, I write documentation for how Skype's protocols work.

You read my documentation, and implement it in a new program. Since we haven't talked, and you've never seen a line of Skype's code, you haven't infringed on any copyrights.

It is important to note, though, that this does not necessarily protect us against a patent suit.

Create2• 15 years ago

They are writing spec documents...

http://developer.skype.com/resources/Skype_UVC_Specification...

it would be nice to see linphone be able to talk to skype people...

Iv• 15 years ago

"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."

Yes, it is a common silly practice that stems from the real madness that are copyright laws. Considering that the documentation passed between the two teams contain all the informations to make the software work correctly, I wonder what makes it different from a source code. I could easily write a code generator that would be fed a "documentation" file and generate the C code that creates the final program. Hell, a C program is a specification on how to generate a given binary code. I wonder how often this really happens behind the doors at these "clean room implementation" teams.

yuhong• 15 years ago

>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.

Not if you are booting via EFI, for example if you are booting Mac OS X on an Apple.

jrockway• 15 years ago

Who cares? It's done now. Time continues on; use it or don't use it.

Eternal• 15 years ago

I'm not a US citizen or a lawyer but is the separation of implementation and exploration really required by the copyright laws (ignoring patent issues)?

I can understand that exploration/implementation division as a preemptive "don't sue us" move, but do US copyright really provide that such strong protection that someone who has looked at a decompilation can't be writing an independent implementation? It seems to me the writing an implementation with a different structure or in another language ought to be different enough for copyright reasons.

dkalmnekfaiksmf• 15 years ago

Mirror available at http://thepiratebay.org/torrent/6442887

tibbon• 15 years ago

Pastebin of the unpack-4142.c : http://pastebin.com/AY3abgEJ

ezioamf• 15 years ago

Skype superiority will finish when IPV6 be fully adopted. When this will happen? Never?

gcb2• 15 years ago

So microsoft really bought skype and is already playing the bait-and-switch game?!

mihaelamj• 15 years ago

Microsoft bought Skype to kill Asterisk (since Cisco is not a competition really). Skype for Asterisk project killed, will stop working in 2 years

evanwolf6• 15 years ago

>Microsoft bought Skype to kill Asterisk (since Cisco is not a competition really)

Um, how does this make any sense?

gcb2• 15 years ago

...i was mostly making a joke. but this indeed is a classical bait-and-switch move. no clue what microsoft is trying to kill. but i bet it's google voice.

nextparadigms• 15 years ago

I haven't tried Skype since 4.0, and I just tried it again now. When did it become such bloatware? I don't think I'll ever want to use it again if it stays this way. The new interface looks pretty confusing, too.

evanwolf6• 15 years ago

Has anyone actually read the revealed code? Aside from the commented copyright text at the top, can you explain what the code does well enough to document Skype protocols? Is this in any way useful if you want to talk to Skype servers or clients?

jduran• 15 years ago

this is just shady

skypeopensource4• 15 years ago

Oh, yes.

nikcub• 15 years ago

two things:

1. I would love it if MSFT, as the new owner, gave up on the cat-and-mouse of security through obscurity and obfuscation and settled on a published and peer-reviewed protocol. I am sick of the memory footprint and cpu spikes in having to run skype clients because 70% of its resources are dedicated to hiding what is really going on. I would love a nice, clean, light version

2. we can well assume that if this is happening in the public domain then it was probably done a few years ago behind closed doors at the NSA et al

skypeopensource4• 15 years ago

rofl! http://skype-open-source.blogspot.com/2011/06/my-interview-t...

Skype protocol reverse engineered, source available

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