What makes the technique feasible is that each keystroke makes a relatively distinct sound, however subtle, when hit. Typical users type about 300 characters per minute, leaving enough time for a computer to isolate the sounds of individual keystrokes and categorize the letters based upon the statistical characteristics of English text. For example, the letters “th” will occur together more frequently than “tj,” and the word “yet” is far more common than “yrg.”

This sounds a lot like Solar Designers technique for analysing SSH traffic, as presented at Hackers At Large in 2001…