CS150 Notes 25 (21 October 2005)

Upcoming Schedule

• By today: Read rest of GEB part I (Chapters 2-4 and 6-9, in addition to Chapters 1 and 5 you have already read). See the study guide questions on Notes 18. I recommend reading a chapter every few days rather than trying to read it all just before October 21.
• (Ideally as soon as possible) Read GEB, Aria with Diverse Variations and Chapter 13. This chapter proves that the halting problem is not decidable, and introduces the Church-Turing Thesis (which we will explore more in later classes). You will not be assigned to read Chapter XIV, but it goes into more depth on Gödel's proof and is recommended.
• Monday, 31 October: Problem Set 6

(define (halts? procedure input) ...?...)

(define (contradict-halts input) (if (halts? contradict-halts null) (infinite-loop) 150))

Malicious Code Problem

Input: a procedure P
Output: true if P is would do something bad, false otherwise.

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

(define (halts? P input)
  (is-virus? `(begin ((vaccinate P) input)
		     virus-code)))

Is it possible to write a virus scanner that detects all malicious code? (tricky, think carefully)

Links

• Mark W. Eichin and Jon A. Rochlis, With Microscope and Tweezers: An Analysis of the Internet Virus of November 1988, IEEE Symposium on Security and Privacy, 1989.
• The Art of Computer Virus Research and Defense, Peter Szor (One Page Review in IEEE Security and Privacy, by Nathanael Paul, UVa student)