Researchers have now demonstrated that even with modern laptop, desktop, and server computers, an inexpensive attack can harvest 4,096-bit encryption keys using a parabolic microphone within 33 feet — or even from 12 inches away, using a cellphone microphone.
An anonymous reader quotes this article from The Register:
In both cases it took an hour of listening to get the 4,096-bit RSA key… As a computer’s processor churns through the encryption calculations, the machine emits a high-frequency “coil whine” from the changing electrical current flowing through its components… The team recommends encryption software writers build in “blinding” routines that insert dummy calculations into cryptographic operations. After discussions with the team, GNU Privacy Guard now does this.

Read more of this story at Slashdot.