The known unknowns of Skype interception

Over the past few weeks, the technical blogosphere, and most recently, the mainstread media have tried to answer the question: What kind of assistance can Skype provide to law enforcement agencies?

Most of the stories have been filled with speculation, sometimes informed, but mostly not. In an attempt to paint as clear a picture as possible, I want to explain what we do and don't know about Skype and surveillance.

Skype has long provided assistance to governments

The Washington Post reported yesterday that:

Skype, the online phone service long favored by political dissidents, criminals and others eager to communicate beyond the reach of governments, has expanded its cooperation with law enforcement authorities to make online chats and other user information available to police

The changes, which give the authorities access to addresses and credit card numbers, have drawn quiet applause in law enforcement circles but hostility from many activists and analysts.

To back up its claim, the post cites interviews with "industry and government officials familiar with the changes" who "poke on the condition of anonymity because they weren’t authorized to discuss the issue publicly." Ugh.

However, a quick Google search for "Skype law enforcement handbook" quickly turns up an official looking document on the whistleblower website cryptome.org, dated October 2007, which makes it clear that Skype has long been providing the assistance that the Post claims is new.

From Skype's 2007 law enforcement handbook:

In response to a subpoena or other court order, Skype will provide:
• Registration information provided at time of account registration
• E-mail address
• IP address at the time of registration
• Financial transactions conducted with Skype in the past year, although details of the credit cards used are stored only by the billing provider used (for instance, Bibit, RBS or PayPal)
• Destination telephone numbers for any calls placed to the public switched telephone network (PSTN)
• All service and account information, including any billing address(es) provided, IP address (at each transaction), and complete transactional information

While Skype's law enforcement handbook suggests that the company does not have access to IP address session logs, high-profile criminal case from 2006 suggests that the company does.

Kobi Alexander, the founder of Comverse, was nabbed in Negombo, Sri Lanka yesterday by a private investigator. He is wanted by the US government in connection with financial fraud charges. He is accused of profiting from some very shady stock-option deals, to the detriment of Comverse shareholders. Once the deals became public and he was indicted, he resigned as CEO and fled the US.

Alexander was traced to the Sri Lankan capital of Colombo after he placed a one-minute call using Skype. That was enough to alert authorities to his presence and hunt him down.

This makes sense. Skype clients connect to Skype's central servers (so that users can make calls to non Skype users, and learn which of their friends are online and offline), and so the servers naturally learn the IP address that the user is connecting from. This is not surprising.

Skype voice call encryption

So while it is clear that Skype can provide government agencies with basic subscriber information and IP login info, what remains unclear is the extent to which governments can intercept the contents of Skype voice calls.

Skype has always been rather evasive when it comes to discussing this issue. Whenever questions come up, the company makes it a point to mention that it provides end to end encryption, but then dodges all questions about how it handles encryption keys.

Skype's strategy is genius - most journalists, even those that cover tech, know very little about the more granular aspects of cryptography. When Skype says it provides end to end call encryption, journalists then tell their readers that Skype is wiretapping proof, even though Skype never made that specific claim. Conveniently enough, Skype never bothers to correct the many people who have read a tad bit too much into the company's statements about security.

As Seth Schoen from EFF told Forbes recently, "my view is that Skype has gotten a reputation for impregnable security that it has never deserved." Exactly. Consumers think the service is secure, and Skype has absolutely no incentive to correct this false, yet positive impression.

The mud puddle test

Last year, I directed a bit of a media firestorm at Dropbox, after I filed an FTC complaint alleging that the company had been misleading its customers about the "military grade" security it used to protect the files uploaded by users. Earlier this year, the tech press started to ask similar questions about the cryptography and key management used by Apple's iCloud service.

Soon after, crytographer Matt Green proposed the 'mud puddle test' for easily determining if a cloud based storage solution has unencrypted access to your data.

1. First, drop your device(s) in a mud puddle.
2. Next, slip in said puddle and crack yourself on the head. When you regain consciousness you'll be perfectly fine, but won't for the life of you be able to recall your device passwords or keys.
3. Now try to get your cloud data back.
Did you succeed? If so, you're screwed. Or to be a bit less dramatic, I should say: your cloud provider has access to your 'encrypted' data, as does the government if they want it, as does any rogue employee who knows their way around your provider's internal policy checks.

Both Dropbox and iCloud fail the mud puddle test. If a user's laptop is destroyed and they forget their password, both services permit a user to reset the password and then download all of their data that was stored with the service. Both of these companies have access to your data, and can be forced to hand it over to the government. In contrast, SpiderOak, a competing online backup service (which I use) passes the test. If a SpiderOak user forgets their password, they lose their data.

What about Skype? After all, the company isn't an online backup service, but rather a communications service, right?

Well, as an initial matter, if you forget your password, Skype sends you a reset link by email, which lets you into your account, maintaining the same account balance and restoring your full contact list. Likewise, if you install Skype on a new computer, your contact list is downloaded, and you can conduct conversations that, to the other caller, will not in any way reveal that you recently installed Skype on a new device, or reset your password. It just works.

Encrypted communications require encryption keys.

Some protocols, like Off The Record (built into several Instant Messaging clients, but not to be confused with Google's fake, unencrypted Off The Record), random keys are created by the IM client, and then users are expected to exchange and verify them out of band (usually, by phone, or in person).

The OTR developers realized that users don't like manually verifying random alpha-numeric crypto fingerprints, and so the developers introduced a slightly easier method of verifying OTR keys in recent versions that uses secret questions or shared secrets selected by users (obviously, this is less secure, but more likely to be actually followed by users).

Another scheme, the ZRTP encrypted VOIP protocol, created by Phil Zimmermann of PGP fame avoids the static fingerprint method, and instead requires users to verify a random phrase at the beginning of each conversation. ZRTP (which is also used by Whisper Systems' RedPhone and the open source Jitsi chat tool) can rely on these pass phrase exchanges, because users presumably know each others' voices. Text based IM schemes don't have this voice recognition property, and so slightly heavier weight verification schemes are required there.

While these key/identity verification methods are a pain for users, they are important. Encryption is great, but without some method of authentication, it is not very helpful. That is, without authentication, you can be sure you have encrypted session, but you have no idea who is at the other end (someone pretending to be your friend, a government device engaging in a man in the middle interception attack, etc). The key verification/exchange methods used by OTR and ZRTP provide a strong degree of authentication, so that users can be sure that no one else is snooping on their communications.

Thanks for the crypto lesson

In contrast to the complex, user-visible fingerprint exchange and verification methods employed by OTR and ZRTP, Skype does nothing at all. Skype handles all the crypto and key exchange behind the scenes. When a Skype user installs the software on a brand new device and initiates a conversation with a friend already in their contact list, that friend is not told that the caller's device/software has a new crypto key and that it should be verified. Instead, the call just connects.

While we don't know the full details of how Skype handles its key exchange, what is clear is that Skype is in a position to impersonate its customers, or, should it be forced, to give a government agency the ability to impersonate its customers. As Skype acts as the gatekeeper of conversations, and the only entity providing any authentication of callers, users have no way of knowing if they're directly communicating with a friend they frequently chat with, or if their connection is being intercepted using a man in the middle attack, made possible due to the disclosure of cryptographic keys by Skype to the government.

I suspect that Skype does not create a new private encryption key for each device running Skype. Instead, my guess is that it creates a key once, when the user sets up their account, and then stores this online, along with the user's contact list. When the user installs Skype on a new device, the key is downloaded, along with all of their other account data. The user's public/private key pair would then be used to authenticate a session key exchange. If this is the design that Skype uses, the company can be compelled to disclose the private crypto keys it holds, allowing the government to impersonate users, and perform active man in the middle interception attacks against their communications.

One alternate, but equally insecure approach would be for the Skype clients to create a new public/private keypair each time the a user installs Skype on their computer and for Skype to digitally sign the user's public key using a certificate pre-installed in all Skype clients. In that scenario, while Skype the company won't have access to your private key, it will be able to sign public keys in your name for other people (including the government) that other Skype clients will accept without complaint. Such impersonation methods can then be used to perform man in the middle attacks.

Whatever the key exchange method that Skype uses, as long as users rely on Skype for all caller authentication, and as long as the company provides account access after a forgotten password, and seamless communications after the installation of Skype on a new computer, the company will fail the mud puddle test. Under such circumstances, Skype is in a position to give the government sufficient data to perform a man in the middle attack against Skype users.

Government agencies and encryption keys

Ok, so Skype has access to users' communications encryption keys (or can enable others to impersonate as Skype users). What does this mean for the confidentiality of Skype calls? Skype may in fact be telling the truth when it tells journalists that it does not provide CALEA-style wiretap capabilities to governments. It may not need to. If governments can can impersonate Skype users and perform man in the middle attacks on their conversations (with the assistance of broadband ISPs or wireless carriers), then they can decrypt the voice communications without any further assistance from Skype.

Do we know if this is happening? No. But that is largely because Skype really won't comment on the specifics of its interactions with governments, or the assistance it can provide. However, privacy researchers (pdf) have for many years speculated about governments compelling companies to hand over their own encryption keys or provide false certificates (pdf) for use in MiTM attacks. In such cases, when the requests come, there isn't really anything that companies can do to resist.

We need transparency

I suspect that 99% of Skype's customers have never given a moment's thought to the ease or difficulty with which government agencies can listen to their calls. Most likely use the service because it is free/cheap, easy, and enables them to talk to their loved ones with a minimum of hassle. There are, however, journalists, human rights activists and other at-risk groups who use Skype because they think it is more secure. In terms of Skype's hundreds of millions of users, these thousands of privacy-sensitive users are a tiny rounding error, a drop in the bucket.

Skype is not transparent about its surveillance capabilities. It will not tell us how it handles keys, what kind of assistance it provides governments, under what circumstances, or which governments it will and won't assist. Until it is more transparent, Skype should be assumed to be insecure, and not safe for those whose physical safety depends upon confidentiality of their calls.

Skype of course can't talk about the requests for assistance it has received from intelligence agencies, since such requests are almost certainly classified. However, Skype could, if it wished to, tell users about its surveillance capabilities. It doesn't.

I personally don't really care if Skype is resistant to government surveillance or not. There are other schemes, such as ZRTP, which are peer reviewed, open, documented protocols which activists can and should use. What I would like though, is for Skype to be honest. If it is providing encryption keys to governments, it should tell its customers. They deserve the truth.