Modern encryption techniques are tested every time someone makes a purchase over the Internet or spends electronic cash stored in smart cards. These strategies rely on so-called one-way functions, which are easy to execute in one direction (for instance, multiplying two prime numbers) but difficult to reverse (factoring a large number into two primes). With ever-increasing computer power and advances in quantum computing, however, such methods may soon become breakable. According to a report published in the current issue of the journal Science, researchers have developed a new approach to cryptography--built around a piece of plastic the size of a stamp--that is hard to crack and nearly impossible to forge.

Ravikanth Pappu of the Center for Bits and Atoms at the Massachusetts Institute of Technology and his colleagues designed a physical device capable of computing a one-way function. Instead of using mathematics, the setup relies on the scattering of light to encrypt data. After light from a laser passes through an epoxy chip containing micron-size spheres of glass in a random arrangement, the resulting interference pattern is projected onto a two-dimensional grid. The intensity of light in each square allows the team to manufacture an encryption key that is 2,400 bits long. "Remembering that information is physical often allows us to do things in surprising ways that could not be done using digital systems alone," Pappu notes.

The tokens cost only a penny to produce, but the scientists are confident that making an unauthorized copy would be extremely difficult using current manufacturing methods. For instance, minor changes in the positioning of the spheres leads to detectable changes in the interference pattern: one bead moved less than a micron changes about half the bits in the key. In addition, multiple keys can be generated from the same token by changing the angle of the incident light, so even if one key were cracked, the token could still guard information. The authors thus conclude that the "introduction of physical one-way functions greatly expands where, and how, information can be protected."