size change. A coil or capacitor which emits acoustic noise will, conversely, also be microphonic, and the high-end audio industry takes steps with coils and capacitors to reduce these microphonics (immissions) because they can muddy a hi-fi amplifier's sound. In March 2015, it was made public that some inkjet printers using ultrasonic heads can be read back using high frequency MEMS microphones to record the unique acoustic signals from each nozzle and using timing reconstruction with known printed data, that is, "confidential" in 12-point font. Thermal printers can also be read using similar methods but with less fidelity as the signals from the bursting bubbles are weaker. The hack also involved implanting a microphone, chip storage IC and burst transmitter with long-life Li+ battery into doctored cartridges substituted for genuine ones sent by post to the target, typically a bank, then retrieved from the garbage using challenge-response RFID chip. A similar work on reconstructing printouts made by dot-matrix printers was publicized in 2011. A new acoustic cryptanalysis technique discovered by a research team at Israel's Ben-Gurion University Cybersecurity Research Center allows data to be extracted using a computer's speakers and headphones. Forbes published a report stating that researchers found a way to see information being displayed, by using microphone, with 96.5% accuracy. In 2016, Genkin, Shamir, and Tromer published another paper that described a key extraction attack that relied on the acoustic emissions from laptop devices during the decryption process. They demonstrated the success of their attack with both a simple mobile phone and a more sensitive microphone. Countermeasures This kind of cryptanalysis can be defeated by generating sounds that are in the same spectrum and same form as keypresses. If sounds of actual keypresses are randomly replayed, it may be possible to totally defeat such kinds of attacks. It is advisable to use at least 5 different recorded variations (36 x 5 = 180 variations) for each keypress to get around the issue of FFT fingerprinting. Alternatively, white noise of a sufficient volume (which may be simpler to generate for playback) will also mask the acoustic emanations of individual keypresses. See also TEMPEST ACOUSTINT References Cryptanalysis Cryptographic attacks Side-channel attacks