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Quantum Cryptography. Scott Roberts CSE 599 03/01/2001. Overview. Classical Cryptography Quantum Cryptography BB84 Quantum Key Distribution Scheme Demo. Classical Cryptography. Private-key cryptography Based on secrecy of the key Problem: key distribution Public-key cryptography
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Quantum Cryptography Scott Roberts CSE 599 03/01/2001
Overview • Classical Cryptography • Quantum Cryptography • BB84 Quantum Key Distribution Scheme • Demo
Classical Cryptography • Private-key cryptography • Based on secrecy of the key • Problem: key distribution • Public-key cryptography • Based on one-way functions • Problem: Based on unproven mathematical assumptions • In danger due to recent advances in quantum computing – namely Shor’s algorithm.
Quantum Cryptography(a.k.a. Quantum Key Distribution) • Enables Alice to send a key to Bob and detect eavesdropping. • Really about key distribution (QKD). • QKD based on an unchangeable law of physics: the Heisenberg Uncertainty Principle. • For any two incompatible observables, A and B, reducing the uncertainty of A forces the uncertainty of B to increase, and vice versa. • i.e., measuring one of the observables interferes with the measurement of the other • This is key to QKD
QKD (cont’d) • QKD uses photons to transmit key data • If Eve attempts to measure a photon that is in transit from Alice to Bob, and Eve uses a measurement (rectilinear or diagonal) that is incompatible with the polarity of the photon, that measurement will randomly change the polarity of that photon. • Eve cannot measure the photon both rectilinearly and diagonally due to Heisenberg • Therefore, eavesdropping is easy to detect.
BB84 Scheme for QKD • Created by Bennett and Brassard in 1984. • Uses a quantum channel for transmitting the key • Uses a public channel for key reconciliation. • In the real world, Alice and Bob share a small secret key that is used for authentication in public channel.
BB84 Steps • Alice sends Bob photons with polarizations chosen at random. • For each photon, Bob chooses at random the type of measurement – rectilinear or diagonal. • Bob records the results of his measurements but keeps them a secret.
BB84 Steps (cont’d) • Bob publicly announces the type of measurements (not polarities) he made and Alice tells him which measurements were of the correct type. • Alice and Bob keep all cases in which Bob measured the correct type. • Should have same photon polarities. • These are translated into bits (0º and 45º = 0, 90º and 135º = 1.
BB84 Steps (cont’d) • Bob and Alice test for eavesdropping by publicly comparing and discarding a randomly selected subset of their polarization data. • Problem: can result in a small key • As an alternative Alice and Bob could test the parity of a randomly chosen subset of the key data. • This allows for a much larger key.
BB84 Demo • Demonstrates the BB84 QKD scheme. • Allows user to see the effects that eavesdropping has on BB84. • Can test both standard technique and parity modification.
Conclusion • Classical Cryptography has served us well thus far but is inherently flawed. • Private-key: keys must be private • Public-key: based on unproven mathematical assumptions • In danger due to Shor’s algorithm for factoring large integers • Quantum Cryptography is totally secure • Based on unbreakable laws of physics • Easily able to detect eavesdropping
