Entries Tagged "VoIP"

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NSA Wants Help Eavesdropping on Skype

At least, according to an anonymous “industry source”:

The spybiz exec, who preferred to remain anonymous, confirmed that Skype continues to be a major problem for government listening agencies, spooks and police. This was already thought to be the case, following requests from German authorities for special intercept/bugging powers to help them deal with Skype-loving malefactors. Britain’s GCHQ has also stated that it has severe problems intercepting VoIP and internet communication in general.

Skype in particular is a serious problem for spooks and cops. Being P2P, the network can’t be accessed by the company providing it and the authorities can’t gain access by that route. The company won’t disclose details of its encryption, either, and isn’t required to as it is Europe based. This lack of openness prompts many security pros to rubbish Skype on “security through obscurity” grounds: but nonetheless it remains a popular choice with those who think they might find themselves under surveillance. Rumour suggests that America’s NSA may be able to break Skype encryption — assuming they have access to a given call or message — but nobody else.

The NSA may be able to do that: but it seems that if so, this uses up too much of the agency’s resources at present.

I’m sure this is a real problem. Here’s an article claiming that Italian criminals are using Skype more than the telephone because of eavesdropping concerns.

Posted on February 23, 2009 at 6:51 AMView Comments

Man-in-the-Middle Attacks

Last week’s dramatic rescue of 15 hostages held by the guerrilla organization FARC was the result of months of intricate deception on the part of the Colombian government. At the center was a classic man-in-the-middle attack.

In a man-in-the-middle attack, the attacker inserts himself between two communicating parties. Both believe they’re talking to each other, and the attacker can delete or modify the communications at will.

The Wall Street Journal reported how this gambit played out in Colombia:

“The plan had a chance of working because, for months, in an operation one army officer likened to a ‘broken telephone,’ military intelligence had been able to convince Ms. Betancourt’s captor, Gerardo Aguilar, a guerrilla known as ‘Cesar,’ that he was communicating with his top bosses in the guerrillas’ seven-man secretariat. Army intelligence convinced top guerrilla leaders that they were talking to Cesar. In reality, both were talking to army intelligence.”

This ploy worked because Cesar and his guerrilla bosses didn’t know one another well. They didn’t recognize one anothers’ voices, and didn’t have a friendship or shared history that could have tipped them off about the ruse. Man-in-the-middle is defeated by context, and the FARC guerrillas didn’t have any.

And that’s why man-in-the-middle, abbreviated MITM in the computer-security community, is such a problem online: Internet communication is often stripped of any context. There’s no way to recognize someone’s face. There’s no way to recognize someone’s voice. When you receive an e-mail purporting to come from a person or organization, you have no idea who actually sent it. When you visit a website, you have no idea if you’re really visiting that website. We all like to pretend that we know who we’re communicating with — and for the most part, of course, there isn’t any attacker inserting himself into our communications — but in reality, we don’t. And there are lots of hacker tools that exploit this unjustified trust, and implement MITM attacks.

Even with context, it’s still possible for MITM to fool both sides — because electronic communications are often intermittent. Imagine that one of the FARC guerrillas became suspicious about who he was talking to. So he asks a question about their shared history as a test: “What did we have for dinner that time last year?” or something like that. On the telephone, the attacker wouldn’t be able to answer quickly, so his ruse would be discovered. But e-mail conversation isn’t synchronous. The attacker could simply pass that question through to the other end of the communications, and when he got the answer back, he would be able to reply.

This is the way MITM attacks work against web-based financial systems. A bank demands authentication from the user: a password, a one-time code from a token or whatever. The attacker sitting in the middle receives the request from the bank and passes it to the user. The user responds to the attacker, who passes that response to the bank. Now the bank assumes it is talking to the legitimate user, and the attacker is free to send transactions directly to the bank. This kind of attack completely bypasses any two-factor authentication mechanisms, and is becoming a more popular identity-theft tactic.

There are cryptographic solutions to MITM attacks, and there are secure web protocols that implement them. Many of them require shared secrets, though, making them useful only in situations where people already know and trust one another.

The NSA-designed STU-III and STE secure telephones solve the MITM problem by embedding the identity of each phone together with its key. (The NSA creates all keys and is trusted by everyone, so this works.) When two phones talk to each other securely, they exchange keys and display the other phone’s identity on a screen. Because the phone is in a secure location, the user now knows who he is talking to, and if the phone displays another organization — as it would if there were a MITM attack in progress — he should hang up.

Zfone, a secure VoIP system, protects against MITM attacks with a short authentication string. After two Zfone terminals exchange keys, both computers display a four-character string. The users are supposed to manually verify that both strings are the same — “my screen says 5C19; what does yours say?” — to ensure that the phones are communicating directly with each other and not with an MITM. The AT&T TSD-3600 worked similarly.

This sort of protection is embedded in SSL, although no one uses it. As it is normally used, SSL provides an encrypted communications link to whoever is at the other end: bank and phishing site alike. And the better phishing sites create valid SSL connections, so as to more effectively fool users. But if the user wanted to, he could manually check the SSL certificate to see if it was issued to “National Bank of Trustworthiness” or “Two Guys With a Computer in Nigeria.”

No one does, though, because you have to both remember and be willing to do the work. (The browsers could make this easier if they wanted to, but they don’t seem to want to.) In the real world, you can easily tell a branch of your bank from a money changer on a street corner. But on the internet, a phishing site can be easily made to look like your bank’s legitimate website. Any method of telling the two apart takes work. And that’s the first step to fooling you with a MITM attack.

Man-in-the-middle isn’t new, and it doesn’t have to be technological. But the internet makes the attacks easier and more powerful, and that’s not going to change anytime soon.

This essay originally appeared on Wired.com.

Posted on July 15, 2008 at 6:47 AMView Comments

Eavesdropping on Encrypted Compressed Voice

Traffic analysis works even through the encryption:

The new compression technique, called variable bitrate compression produces different size packets of data for different sounds.

That happens because the sampling rate is kept high for long complex sounds like “ow”, but cut down for simple consonants like “c”. This variable method saves on bandwidth, while maintaining sound quality.

VoIP streams are encrypted to prevent eavesdropping. However, a team from John Hopkins University in Baltimore, Maryland, US, has shown that simply measuring the size of packets without decoding them can identify whole words and phrases with a high rate of accuracy.

The technique isn’t good enough to decode entire conversations, but it’s pretty impressive.

Posted on June 19, 2008 at 6:27 AMView Comments

Swiss Police to Use Trojans for VoIP Tapping

At least they’re thinking about it:

Swiss authorities are investigating the possibility of tapping VoIP calls, which could involve commandeering ISPs to install Trojan code on target computers.

VoIP calls through software services such as Skype are encrypted as they are passed over the public Internet, in order to safeguard the privacy of the callers.

This presents a problem for anyone wanting to listen in, as they are faced with trying to decrypt the packets by brute force — not easy during a three-minute phone call. What’s more, many VoIP services are not based in Switzerland, so the authorities don’t have the jurisdiction to force them to hand over the decryption keys or offer access to calls made through these services.

The only alternative is to find a means of listening in at a point before the data is encrypted.


In order to install the application on the target computer, the Swiss authorities
envisage two strategies: either have law enforcement surreptitiously install it locally, or have the telco or ISP which provides Internet access to that computer install it remotely.

The application, essentially a piece of Trojan code, is also able to turn on the microphone on the target PC and monitor not just VoIP conversations, but also any other ambient audio.

Posted on October 18, 2006 at 2:26 PMView Comments

Skype Call Traced

Kobi Alexander fled the United States ten days ago. He was tracked down in Sri Lanka via a Skype call:

According to the report, Alexander was located after making a one-minute call via the online telephone Skype service. The call, made from the Sri Lankan capital Colombo, alerted intelligence agencies to his presence in the country.

Ars Technica explains:

The fugitive former CEO may have been convinced that using Skype made him safe from tracking, but he — and everyone else that believes VoIP is inherently more secure than a landline — was wrong. Tracking anonymous peer-to-peer VoIP traffic over the Internet is possible (PDF). In fact, it can be done even if the parties have taken some steps to disguise the traffic.

Let this be a warning to all of you who thought Skype was anonymous.

Posted on August 24, 2006 at 1:45 PMView Comments

Broadening CALEA

In 1994, Congress passed the Communications Assistance for Law Enforcement Act (CALEA). Basically, this is the law that forces the phone companies to make your telephone calls — including cell phone calls — available for government wiretapping.

But now the government wants access to VoIP calls, and SMS messages, and everything else. They’re doing their best to interpret CALEA as broadly as possible, but they’re also pursuing a legal angle. Ars Technica has the story:

The government hopes to shore up the legal basis for the program by passing amended legislation. The EFF took a look at the amendments and didn’t like what it found.

According to the Administration, the proposal would “confirm [CALEA’s] coverage of push-to-talk, short message service, voice mail service and other communications services offered on a commercial basis to the public,” along with “confirm[ing] CALEA’s application to providers of broadband Internet access, and certain types of ‘Voice-Over-Internet-Protocol’ (VOIP).” Many of CALEA’s express exceptions and limitations are also removed. Most importantly, while CALEA’s applicability currently depends on whether broadband and VOIP can be considered “substantial replacements” for existing telephone services, the new proposal would remove this limit.

Posted on July 28, 2006 at 11:09 AMView Comments

Sidebar photo of Bruce Schneier by Joe MacInnis.