e data within it without harming the quality of the picture," he says. Although the technique is currently implemented in software, implemen- tation in hardware or firmware in trusted devices where image integrity is crit- ical to the application could be possible. For instance, the technique could be used in a trusted digital camera used to gather forensic evidence to be later used at a trial. If information is embedded in the images captured with the camera using the new algorithms, any subsequent manipulations of the pic- tures could be detected and the area where they occurred pinpointed. 7.8 QUANTUM CRYTOGRAPHY Quantum cryptography is a codemaker's Holy Grail. The idea is to use a rapid series of light pulses (photons) in one of two different states to transmit infor- mation in an unbreakable code. Quantum cryptography differs from other code schemes in that the attempt by a third party to intercept a code's key itself alters the key. It is as though the very act of listening in on a conversa- tion makes the eavesdropper known. Let us say that Bill wants to send a secret message to Judie. For the message to be secret, Bill has to employ some scheme to encode the message. And Judie needs a key to the scheme to decode the message. The crucial communication for the sake of preserving secrecy is not the message but the key. Therefore, Bill sends a string of single photons whose polarizations successively contain the key. If a third party, John, tries to detect the singly transmitted photons, the act of detection causes an irreversible change in the wave function of the system. ("Wave function" denotes the quantum mechanical state of a physical system.) If John then tries to send the key on to Judie, the key will, in effect, bear the imprint of John's intermediate detection. THE UNBLINKING EYE-SECURITY AND SURVEILLANCE 7.8.1 Quantum Dots Utilizing the power of quantum cryptography, a National Institute of Stan- dards and Technology (NIST) scientist has demonstrated efficient production of single photons-the small