Feeds

Linguists use sounds to bypass Skype crypto

And you thought grammar was useless…

Using blade systems to cut costs and sharpen efficiencies

Decryption is difficult and computationally expensive. So what if, instead of decrypting the content of a message, you found a correlation between the encrypted data and its meaning – without having to crack the code itself?

Such an approach has been demonstrated by a group of University of North Carolina linguists working with computer scientists on encrypted Skype calls. While their research paper only managed to partially recover conversations, an encryption scheme that leaks even some of the data it’s meant to protect is no longer secure.

It works like this: spoken English has a set of known – and quite settled – rules for its phonetic grammar.

For non-linguists, this means the order in which we can and cannot put different sounds together. The “ds” sound, or phoneme, at the end of sounds is fairly common at the end of English words, but doesn’t occur at the beginning.

Systems like speech-to-text converters use these rules to break strings of sounds into individual words; they match sounds against a dictionary of legal phoneme combinations and map these into words. What the researchers discovered is that encryption leaves a pattern that can be subjected to this kind of analysis – without decrypting the data.

When you encode spoken English for VoIP using (in the case of Skype) CELP (code excited linear projection), you will end up with patterns in the data that match the patterns in the sounds. In particular, those patterns end up being reflected in the size of the data frame: the more complex the sound that’s being encoded, the larger the frame, resulting in a correlation between frame size and the original sounds spoken.

When the data created by CELP is encrypted, it retains the original frame size – and that means that even encrypted Skype data will retain the correlation between the size of the data frame and the original phonemes.

The technique gets another helping hand: at least some of the time, boundaries between sounds correspond to sudden changes in frame size, hinting at the difference between “Han Solo” and “Hans Solo”.

The researchers mapped the size of encrypted data frames in the Skype stream back to likely patterns of phonemes, and used that mapping – which they called “Phonetic Reconstruction” – to reconstruct the call, without decrypting the data.

So how well does it work? Not so well that we should all abandon Skype tomorrow. However, the researchers noted that if an encryption scheme is to be considered secure, “no reconstruction, even a partial one, should be possible; indeed, any cryptographic system that leaked as much information as shown here would immediately be deemed insecure.”

Bigger phoneme-word dictionaries (covering more dialects and languages) and faster processing would improve the accuracy of this kind of analysis ®

The smart choice: opportunity from uncertainty

More from The Register

next story
Yorkshire cops fail to grasp principle behind BT Fon Wi-Fi network
'Prevent people that are passing by to hook up to your network', pleads plod
HIDDEN packet sniffer spy tech in MILLIONS of iPhones, iPads – expert
Don't panic though – Apple's backdoor is not wide open to all, guru tells us
NEW, SINISTER web tracking tech fingerprints your computer by making it draw
Have you been on YouPorn lately, perhaps? White House website?
LibreSSL RNG bug fix: What's all the forking fuss about, ask devs
Blow to bit-spitter 'tis but a flesh wound, claim team
Black Hat anti-Tor talk smashed by lawyers' wrecking ball
Unmasking hidden users is too hot for Carnegie-Mellon
Attackers raid SWISS BANKS with DNS and malware bombs
'Retefe' trojan uses clever spin on old attacks to grant total control of bank accounts
Manic malware Mayhem spreads through Linux, FreeBSD web servers
And how Google could cripple infection rate in a second
Don't look, Snowden: Security biz chases Tails with zero-day flaws alert
Exodus vows not to sell secrets of whistleblower's favorite OS
prev story

Whitepapers

Seven Steps to Software Security
Seven practical steps you can begin to take today to secure your applications and prevent the damages a successful cyber-attack can cause.
Consolidation: The Foundation for IT Business Transformation
In this whitepaper learn how effective consolidation of IT and business resources can enable multiple, meaningful business benefits.
Designing a Defense for Mobile Applications
Learn about the various considerations for defending mobile applications - from the application architecture itself to the myriad testing technologies.
Build a business case: developing custom apps
Learn how to maximize the value of custom applications by accelerating and simplifying their development.
Consolidation: the foundation for IT and business transformation
In this whitepaper learn how effective consolidation of IT and business resources can enable multiple, meaningful business benefits.