University of Virginia, Department of Computer Science
CS200: Computer Science, Spring 2002

Notes: Wednesday 20 March 2002
Schedule

Computability
Proof: A proof of S in an axiomatic system is a sequence of strings, T0, T1, ..., Tn where: Halting Problem

Input: a procedure P (described by a Scheme program)
Output: true if P always halts (finishes execution), false otherwise.

Malicious Code Problem

Is the Malicious Code Problem decidable? (think about this before Friday)

Input: a procedure P
Output: #t if P is would do something bad, #f otherwise.

Assume we have a precise definition of what something bad means (for example, format your hard drive).

Links
There is a remarkably close parallel between the problems of the physicist and those of the cryptographer. The system on which a message is enciphered corresponds to the laws of the universe, the intercepted messages to the evidence available, the keys for a day or a message to important constants which have yet to be determined. The correspondence is very close, but the subject matter of cryptography is very easily dealt with by discrete machinery, physics not so easily.
Alan Turing

CS 655 University of Virginia
Department of Computer Science
CS 200: Computer Science
David Evans
evans@cs.virginia.edu
Using these Materials