Nilesh.org Notes on technology and everything else

Two of the main goals of cryptography are the encryption of messages to render them unintelligible to third parties and their authentication to certify that they have not been modified. These goals can be accomplished if the sender (“Alice”) and recipient (“Bob”) both possess a secret random bit sequence known as “key” material, which they use as a parameter in a cryptographic algorithm. It is essential that Alice and Bob acquire the key material with a high level of confidence that any third party (“Eve”) does not have even partial information about the random bit sequence. If Alice and Bob communicate solely through classical messages it is impossible for them to generate a certifiably secret key owing to the possibility of passive eavesdropping. However, secure key distribution becomes possible if they use the single-photon communication technique of quantum cryptography, or more accurately, quantum key distribution (QKD).

The security of QKD is based on the inviolability of the laws of quantum mechanics and provably secure (information theoretic) data handling protocols. Eve can neither “tap” the key transmissions owing to the indivisibility of quanta nor copy them because of the quantum "no-cloning" theorem. At a deeper level, QKD resists interception and retransmission by an eavesdropper because in quantum mechanics, in contrast to the classical world, the result of a measurement cannot be thought of as revealing a “possessed value” of a quantum state. A unique aspect of quantum cryptography is that Heisenberg’s uncertainty principle ensures that if Eve attempts to intercept and measure Alice's quantum transmissions, her activities must produce an irreversible change in the quantum states (“collapse of the wavefunction”) that are retransmitted to Bob. These changes will introduce an anomalously high error rate in the transmissions between Alice and Bob, allowing them to detect the attempted eavesdropping. In particular, from the observed error rate Alice and Bob can put an upper bound on any partial knowledge that an eavesdropper may have acquired by monitoring their transmissions. This bound allows the intended users to apply conventional information theoretic techniques to distill a secret error free key.