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.