


Steklov Institute St. Petersburg

Technische Universität München

State University St. Petersburg

Joint Advanced Student School (JASS)
Course 1: Algorithms in IT Security
St. Petersburg  Wednesday, March 30 through Saturday, April 9, 2005
Andreas Würfl
Alternative Approaches in Cryptography
Abstract
Several encryption algorithms have been discussed in the course of this seminar. This
paper will introduce two alternative approaches. A change in the concept of provable security
leads to a cipher which is shown to be "perfect with high probability". Assuming that
the memory capabilities of a potential adversary are limited a stronglyrandomized cipher is
devised which uses a publiclyaccessible string of random bits. In this cipher the secret key is
short but the plaintext can be very long.
The second approach is based on quantum physics. Heisenberg's uncertainty principle guarantees
the security of quantum cryptography. The first experiment which demonstrated the
practicability of the only known encryption unbreakable by law of nature was conducted in
1989. The physical and technical fundamentals are introduced as well as the Quantum Key
Exchange (QKE) protocol. We will demonstrate the effects of attempted eavesdropping and
its detection. Finally the rapid development of the last 15 years and the currently available
implementations will be presented.
