Compression lets attackers tap VoIP calls

New research suggests current encryption and compression methods are inadequate

A common compression technique can make internet telephone calls significantly more susceptible to bugging, according to recent research from Johns Hopkins University.

Internet telephony has become widely used through consumer-centric applications such as Skype, and is becoming more common in enterprises.

The new research suggests, however, that standard encryption and compression methods, when used together, are not sufficiently secure.

VoIP calls are commonly encrypted using a technique that preserves the lengths of voice patterns in the original, unencrypted conversation, the researchers said.

Such calls are relatively difficult to listen in on, they said. But when length-preserving encryption is used with the variable bitrate (VBR) compression technique, the combination leaks a significant amount of information about the conversation, they found.

"Previous work has shown that combining VBR compression with length-preserving encryption leaks information about VoIP conversations," the researchers said in the report. "We show that this information leakage is far worse than originally thought."

In such conversations, particular phrases could be identified within encrypted VoIP calls with more than 90 percent accuracy. Even in decoding entire conversations, accuracy was significant, the research found.

"On average, our method achieves recall of 50 percent and precision of 51 percent for a wide variety of phonetically rich phrases spoken by a diverse collection of speakers," the researchers said.

The problem arises because VBR compression alters the compression rate based on the type of sound being compressed, using higher compression for simpler sounds and lower compression for more complex sounds.

The resulting audio stream makes spoken sounds easy to identify based on bit patterns, and these patterns are preserved in length-preserving encryption.

The researchers used a dictionary of spoken sounds to reconstruct the original conversation without the need to crack its encryption. The reconstructed conversation could then be analyzed as a full conversation or automatically scanned for particular keywords.

The technique could, for instance, be particularly effective in identifying certain predictable phrases used in professional business conversations, the researchers said.

The research analyzed the impact of noise, dictionary size and word variation on the performance of the technique.

"The results of our study show that an attacker can spot a variety of phrases in a number of realistic settings," the researchers said in the paper.

For mitigating such attacks, padding could be used to make the bit patterns less recognizable, the researchers argued.

However, none of the default encryption transforms of the Secure Real-time Transport Protocol, a standard for secure VoIP calls, specify the use of padding, the researchers pointed out.

"Although padding could introduce inefficiencies into real-time protocols, our analysis indicates that it offers significant confidentiality benefits for VoIP calls," added the researchers.

The paper (PDF), called "Spot me if you can: Uncovering spoken phrases in encrypted VoIP conversations", was presented by five Johns Hopkins researchers last month at the 2008 IEEE Symposium on Security and Privacy in Oakland, California.

Join the newsletter!

Error: Please check your email address.
Rocket to Success - Your 10 Tips for Smarter ERP System Selection
Keep up with the latest tech news, reviews and previews by subscribing to the Good Gear Guide newsletter.

Matthew Broersma

Show Comments



Sansai 6-Outlet Power Board + 4-Port USB Charging Station

Learn more >



Back To Business Guide

Click for more ›

Most Popular Reviews

Latest Articles


PCW Evaluation Team

Louise Coady

Brother MFC-L9570CDW Multifunction Printer

The printer was convenient, produced clear and vibrant images and was very easy to use

Edwina Hargreaves

WD My Cloud Home

I would recommend this device for families and small businesses who want one safe place to store all their important digital content and a way to easily share it with friends, family, business partners, or customers.

Walid Mikhael

Brother QL-820NWB Professional Label Printer

It’s easy to set up, it’s compact and quiet when printing and to top if off, the print quality is excellent. This is hands down the best printer I’ve used for printing labels.

Ben Ramsden

Sharp PN-40TC1 Huddle Board

Brainstorming, innovation, problem solving, and negotiation have all become much more productive and valuable if people can easily collaborate in real time with minimal friction.

Sarah Ieroianni

Brother QL-820NWB Professional Label Printer

The print quality also does not disappoint, it’s clear, bold, doesn’t smudge and the text is perfectly sized.

Ratchada Dunn

Sharp PN-40TC1 Huddle Board

The Huddle Board’s built in program; Sharp Touch Viewing software allows us to easily manipulate and edit our documents (jpegs and PDFs) all at the same time on the dashboard.

Featured Content

Latest Jobs

Don’t have an account? Sign up here

Don't have an account? Sign up now

Forgot password?