http://www.umbc.edu/˜lomonaco A TALK ON QUANTUM CRYPTOGRAPHY OR HOW ALICE OUTWITS EVE VERSION 1.6 SAMUEL J. LOMONACO, JR. Abstract. Alice and Bob wish to communicate without the archvil- lainess Eve eavesdropping on their conversation. Alice, decides to take two college courses, one in cryptography, the other in quantum mechan- ics. During the courses, she discovers she can use what she has just learned to devise a cryptographic communication system that automat- ically detects whether or not Eve is up to her villainous eavesdropping. Some of the topics discussed are Heisenberg’s Uncertainty Principle, the Vernam cipher, the BB84 and B92 cryptographic protocols. The talk ends with a discussion of some of Eve’s possible eavesdropping strate- gies, opaque eavesdropping, translucent eavesdropping, and translucent eavesdropping with entanglement. Contents 1. Preface 1 1.1. The Unique Contribution of Quantum Cryptography 1 1.2. A Note to the Reader 2 2. Introduction 2 3. A Course on Classical Cryptography 2 3.1. Alice’s enthusiastic decision 2 3.2. Plaintext, ciphertext, key, and ... Catch 22 2 3.3. Practical Secrecy 3 3.4. Perfect Secrecy 4 3.5. Computational Security 5 4. A Course on Quantum Mechanics 7 4.1. Alice’s Reluctant Decision 7 4.2. The Classical World – Introducing the Shannon Bit 7 4.3. The Quantum World – Introducing the Qubit 8 4.4. Where do qubits live? 8 4.5. Some Dirac notation – Introducing kets 9 Date : January 27, 2001. Partially supported by ARL Contract #DAAL01-95-P-1884, ARO Grant #P-38804- PH-QC, the Computer Security Division of NIST, and the L-O-O-P Fund. This paper is a revised version of a paper published in “Coding Theory, and Cryptography: From Geheimscheimschreiber and Enigma to Quantum Theory,” (edited by David Joyner), Lec- ture Notes in Computer Science and Engineering, Springer-Verlag, 1999 (pp. 144-174). It has been reproduced with the permission of Springer-Verlag. 1