Analysis of encrypted VoIP traffic by packet size

You can successfully listen to VoIP conversations even if the VoIP traffic is securely encrypted. The fact is that modern sound encryption algorithms are specially optimized for voice encoding with maximum quality with a narrow bandwidth. Therefore, some services use VBR compression. The abbreviation VBR means “variable bitrate”, that is, changing the bitrate depending on the content of the content. This “advanced” technology has not yet gained wide acceptance, but many VoIP services are planning to implement it in the future.

When using VBR, long complex sounds (oy) are encoded with a large bit rate, and small simple sounds (t) - with a small one to save bandwidth. So, scientists have found a way to restore the original content by analyzing the bitrate and linguistic analysis using a database of phonemes and words.

Specialists from Johns Hopkins University have shown that a simple analysis of packet sizes already gives enough information to restore the original words and phrases. This can be passwords and other sensitive information.
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Phrase recognition accuracy is 50%, and long compound words - 90%.