I saw a talk at Defcon many years ago showing this. I think they just used statistics. I can't find the video, but here is the PDF of their presentation: Sniff

I saw a talk at Defcon many years ago showing this. I think they just used statistics. I can't find the video, but here is the PDF of their presentation: Sniff Keystrokes With Lasers/Voltmeters - Side Channel Attacks Using Optical ... https://media.defcon.org/DEF%20CON%2017/DEF%20CON%2017%20presentations/DEF%20CON%2017%20-%20barisani-bianco-sniff_keystrokes.pdf https://media.defcon.org/DEF%20CON%2017/DEF%20CON%2017%20pre...