------ Forwarded Message From: David Pollak <dpp@athena.com> Date: Sun, 30 Jan 2005 07:44:21 -0800 To: <dave@farber.net> Cc: <slg@ex.com> Subject: Re: FW: [IP] more on Simson Garfinkel analyses Skype - Open Society Institute Dave, I've been following the Simson/Skype thread on IP and I've read the Columbia analysis of the Skype protocol (http://www1.cs.columbia.edu/~library/TR-repository/reports/reports-2004/cuc s-039-04.pdf) I've known Simson for 14 or so years and have a ton of respect for his technical skills. However, I think there are some significant Skype vulnerabilities and associated legal ramifications that Simson did not discuss in his article. Security is based on trust of the parties exchanging information that they are who they claim and that the data exchanged appears to be random to an untrusted observer. While Skype's use of encryption supports the second part of the definition, it does not support the first. Because it does not support the first, it is very easy to use the Skype network to intercept communications between any user or to pose as any user. This presents a problem as against both third parties and governmental agencies. A critical part of the Skype network is the "super-nodes." According to the Columbia paper, super-nodes perform 3 functions: * Designating the login authority * Media packet forwarding * Routing user search requests Super-nodes appear to "volunteer" to perform the function. Or put another way, they are nodes that are not under the control of Skype, but they perform all the routing functions necessary to discover a user and exchange information with the user. Super nodes run on any machine running the Skype program and the machines under Skype control have no way to determine if the super nodes are running unmodified Skype code. If one were skilled in reverse engineering x86 code and one were willing to violate Skype's user agreement, one could create a Skype node that volunteered to be a super-node. It would appear to all other Skype nodes as a normal super-node. It would perform all the functions of a Skype super-node. However, it would do a little bit more. Let's call one of these super-nodes a "bad seed." The bad seed could point users to another authentication server. Thus, the user would exchange username and authentication information with a "bad relay proxy" rather than the Skype server. That permits the "bad relay proxy" to deny Skype access to a user that I designate. Okay a denial of service attack is not great stuff, but for businesses that rely of Skype (http://news.com.com/No-cost+Skype+strikes+chord+with+businesses/2100-7352_3 -5553053.html ), having the prospect of a DoS attack could be an issue. Further, the "bad relay proxy" could collect username/password challenge (I'm assuming that Skype's not sending the actual password, but performing a challenge/response method of verifying the password) data and do dictionary attacks on the passwords. This isn't a hardcore vulnerability you say. Yep... I agree. The bad seed routes some of the media requests if the two peers cannot see each other directly. Skype claims that because the packets are encrypted, the super-nodes are just routing agents. This is true unless the bad seed is part of the key exchange (see the next paragraph.) If the bad seed is part of the exchange of private AES keys that are used to encrypt the voice data, then they are able to decrypt the audio or text streams. Yikes. If a bad seed was a man in the middle of the private key exchange, then the bad seed could record your conversation with another user. Okay, that's not good. Given that any Skype node can become a super-node just by raising its hand and a skilled hacker can re-engineer a Skype node to perform bad acts, then if you connect to the Skype network, you don't know which nodes are listening in on your conversation. But wait, you say, the requirement for the above bit of scariness is doing a man-in-the-middle attack on the encryption key exchange. You're right. And here's where the Skype network is totally insecure. One function of the super-node in the Skype network is to route and respond to user search requests. If I want to connect to "other_dude" on the Skype network, my client sends out search requests to a series of super-nodes. The super-nodes either respond with the address of "other_dude" or forward the requests to other super-nodes. If one of the super-nodes is "bad seed", that node can respond that it is "other_dude." Because there is no cryptographic trust or any form of trust authority in the Skype network, any super node that returns the information about "other_dude" is trusted by my node. An aside... SSL certificates are signed by a trusted third party. That third party validates that the certificate is held by the organization that claims to hold the certificate. Using SSL insures that the party that I'm communicating with is the party that they claim to be within the bounds that I trust the signer of the SSL certificate *and* that once the connection is established that no one can understand the data exchanged with this party. That initial trust of the signed certification is a critical part of the security of the overall communication. If I do a session key exchange with an unknown party, the communication is *not* secure. This is the case with Skype. The Skype network relies on trusting the super-node. A bad seed can perform a man in the middle attack during the session key exchange by posing as the party being contacted (or forwarding the information of another compromised node) to a caller. So, my bad seed is able to route call requests to an untrusted node and do a man-in-the-middle during the key exchange and snoop into my call. The only question is how many bad seeds to you need in order to capture a significant percentage of the routing requests that go over the Skype network. My guess is that the number is in the hundreds. So, with a hundred machines located around the world, I could intercept any Skype call and record it. Pretty scary. The PSTN primarily uses pairs of copper wires to transmit voice communications from my house to the phone company central office. I can gain physical access to those copper pairs very easily as long as I have physical proximity to the location of the person I want to snoop on. It's not hard to do. Yeah, you have to paint a white van with "Verizon" or "SBC" so the police don't hassle you. But you can do it. If the government wants to do it, it's somewhat harder. The government has to get a warrant to listen to your phone conversations. Once they obtain a warrant, they present it to the phone company which makes an entry into their switch to record the call or send a real-time copy of it to the government. SIP is different. SIP supports encryption, but most SIP providers do not make use of it. The Microsoft SIP client libraries have the option of communicating with the SIP server via TLS (TLS is like SSL, but uses the same IP port for both encrypted and unencrypted traffic.) Additionally, the media portion of a SIP call can be encrypted by setting a flag in the media descriptor. While most SIP providers do not use this functionality, it's part of the SIP spec and can be turned on. Note that the machines that could play man-in-the-middle with an encrypted SIP call are controlled by your SIP provider (rather than any machine running Skype.) Thus, you can trust the security of your call as much as you trust your SIP provider. With unencrypted SIP calls, if you are able to intercept packets, then you can tap the call. Anybody on your LAN can listen into your call. This level of security is no different than anyone in your house can listen in on your phone calls and anyone in your office can probably do the same. Anyone who can intercept the packet stream outside your LAN can also listen in on the conversation. This is more of a challenge. UDP packets (the stuff that the media stream goes over) may or may not be routed through the same backbone during all parts of the conversation. There is a certain amount of security with the packets going over the backbone. The ability to snoop on an unencrypted SIP call is marginally more difficult that snooping on a PSTN call. For the government, it's more of a challenge. Because the media portion of a SIP call goes directly between the end points without going through the SIP providers network. This raises an interesting issues: http://news.com.com/2100-7352-5296417.html This is interesting for two reasons. First, SIP "telephone" companies like Vonage will have to provide a flag to allow them to intercept the media stream and decode it if the government has a warrant. Second, the government has acknowledged that SIP callers have the same expectation of privacy that copper-pair PSTN callers have. This is really important. Users of peer-to-peer file sharing programs don't have an expectation of privacy in their use of P2P programs. That's why so many folks are being sued (http://news.com.com/RIAA+files+754+new+file-swapping+suits/2110-1027_3-5494 259.html?tag=nl ) Skype touts themselves as a P2P voice communications system (http://skype.com/products/explained.html ) That means that if you use Skype, you have the same expectation of privacy as a P2P user. Given that the government has the resources to build bad seeds and that P2P users have no expectation of privacy, you can bet that there are government run Skype nodes looking for Skype communications between Osama911 and Sleeper_in_Seattle and that the government doesn't have a warrant for these activities. To conclude my long rant, the Skype network is radically insecure because it relies on untrusted super-nodes to perform trusted functions, most notably user look-up. It's easy to build a compromised super-node (a bad seed.) With a limited number of bad seeds, the communications between any users can be intercepted or denied. It's something that a person with the resources to rent 100 servers in collocation facilities around the world could do (that's about $10,000 per month investment.) Given that Skype is a P2P network and users such networks are not afforded the same expectation of privacy that users of the PSTN and other telephone networks are afforded, the government could use such a mechanism to listen to Skype-based calls and have a reasonable legal argument that they do not need a warrant to do so. That's my 2 cents. Thanks, David PS -- I was CTO and VP Engineering for an Internet security company for a number of years and I'm a member of the Rhode Island bar. Annette Hurst wrote:
-----Original Message----- From: owner-ip@v2.listbox.com [mailto:owner-ip@v2.listbox.com] On Behalf Of David Farber Sent: Saturday, January 29, 2005 2:11 AM To: Ip Subject: [IP] more on Simson Garfinkel analyses Skype - Open Society
Institute
------ Forwarded Message From: "Jonathan S. Shapiro" <shap@eros-os.org> <mailto:shap@eros-os.org> Date: Fri, 28 Jan 2005 22:03:48 -0500 To: <dave@farber.net> <mailto:dave@farber.net> Subject: Re: [IP] I more on Simson Garfinkel analyses Skype - Open Society Institute
I'm going to attempt to chime in on this, because I think Brad is saying something that I feel is badly wrong.
The most important element of an encryption scheme is that there must be
some
well-founded basis for a well-defined degree of confidence. The encryption may be well done or poorly done. It may be sufficiently protective or it may not. The thing is that the user has a right and a need to know where on the spectrum it falls.
The other alternative is ignorance. The first problem with this is that *your* bad choices can have the effect of disclosing things that have negative consequences for someone else! The second problem is that it describes the majority of real users.
In the case of Skype, the argument Brad is making is simply absurd. The question is not whether something is better than nothing. The question is why Skype chose to implement an undocumented and unqualified proprietary encryption scheme at considerable expense rather than use one of the many existing schemes that are well known, well characterized, and free for the taking.
When viewed from a business perspective, the only plausible rationale is immediately apparent. Skype's objective isn't to protect conversations. It is to render Skype users a captive audience by impeding interoperability.
It is hardly a new precedent. I seem to remember AT&T trying to use allegedly proprietary interfaces to impede the attachment of Tom Carter's Hush-a-Phone in 1956 or so. Different method, same basic strategy.
Jonathan Shapiro
On Fri, 2005-01-28 at 20:53 -0500, David Farber wrote:
------ Forwarded Message From: Brad Templeton <btm@templetons.com> <mailto:btm@templetons.com> Organization: http://www.templetons.com/brad Date: Fri, 28 Jan 2005 17:22:29 -0800 To: David Farber <dave@farber.net> <mailto:dave@farber.net> Cc: <daw@cs.berkeley.edu> <mailto:daw@cs.berkeley.edu> ,
<adam@shostack.com>
<mailto:adam@shostack.com> , <simsong@csail.mit.edu> <mailto:simsong@csail.mit.edu> Subject: Re: [IP] Simson Garfinkel analyses Skype - Open Society Institute
I'm sorry to pick nits, but I have to stand by my statement. No matter how atrociously bad other systems may be, I don't see any basis for saying that Skype is any better. It might be better, or it might be just as bad. We don't know.
While I fully agree that one can have much more confidence in a security system which can be independently analysed and verified as secure, it is exactly the attitude above, common in the security community, which I believe has stopped us from deploying security.
"Some" security, even things like DES (which our own foundation proved can be crackable), poorly chosen keys, algorithms with flaws, protocols that are vulnerable to men in the middle, and proprietary encryption systems -- all of these are often declared to be "no better" than having no encryption at all.
And so, people, buying that argument, often give us no encryption at all, because encryption is hard to do well, and if people keep telling you that you have to do it perfectly or you might as well not bother -- then people don't bother.
The trut is, most people's threat models are not the same as a security consultants. They accept that if the NSA wants to man-in-the-middle them, the NSA is going to succeed.
Skype has resisted basic efforts by skilled reverse engineers to look at its protocols. That doesn't mean they are secure, but it does mean they are secure from basic efforts. If I wanted to listen in your your skype call and had a tap on your ethernet, I would at least have to put a lot of work into it, and possibly could not do it at all. That is a _lot_ more than what is true with in-the-clear SIP, where I could slap a packet sniffer on your net and hear your call fairly trivially, and with certainty that I would succeed.
This is, in fact, a huge difference. Encryption is really about how hard you make it for the attacker. Because above a certain level of hardness there are a lot of easier ways into your network and computer.
So yes, let's decry that we can't verify Skype's encryption and must take their word that it is resistent to attack. But let's not promote this attitude that it is no better than nothing.
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