Due to changes in power usage when performing different operations, your computer generates a varying high-pitched tone, which when properly detected and decoded can be correlated with the compute activity.  Noticing that RSA decryption operations induce different sound patterns, University of Tel Aviv researchers have shown that it is possible to recover 4096-bit secret keys on a computer running GnuPG’s implementation of RSA, within an hour,  with the aid of a simple microphone placed next to the computer.