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Variable bitrate compression, already used in some VoIP applications and planned by many more, uses an algorithm to compress different sounds in different ways. Certain syllables can be represented more efficiently than others, leading to different sized packets for different sounds made during a conversation.

Researchers at John Hopkins University have discovered that analysing the size of these packets can help decipher parts of the message, including whole words and phrases.

Currently the team cannot decrypt entire conversations, but can take a text string and search a conversation for its inclusion. Approaching the problem this way is less computationally intensive, as the phrase can be encoded with variable bitrate compression to show how the search phrase would appear in a VoIP packet.

Finding short words currently has an accuracy rate of 50%, while longer and more distinctive words can be found with an accuracy of up to 90%.

The team presented a paper on their research at the 2008 IEEE Symposium on Security and Privacy in California last month.