Eavesdropping on Encrypted Calls

Here is an interesting article about a technique for detecting phonems/words/phrases even in encrypted audio:

http://technology.newscientist.com/article/dn14124-compressed-web-phone-calls-are-easy-to-bug.html

The gist of the article is:

"Spot me if you can: Uncovering spoken phrases in encrypted VoIP", was given by Charles Wright of Johns Hopkins. Charles began by stating that VoIP offers comparable quality and better security than typical land lines, although it may be possible to deduce some information from encrypted traffic by sampling certain characteristics. If the attacker's goal is to recover information about the word content of a VoIP stream, then there are considerable challenges that must be surmounted; most notable are the large potential vocabulary and natural variability of human speech. Charles proceeded with the claim that despite these challenges, such information can be deduced due to the fact that the efficient variable bitrate encoding used by VoIP encodes different phonemes at distinct bitrates. He then showed how a hidden markov model can be used to recover spoken word content at recall rates of approximately 50% for reasonable precision rates. He concluded by pointing out that VoIP packets can be padded with null content to thwart such an attack.