Distributed and Concurrent Systems                      11.4.2008

Exercise 5

No X-exercise this week.


25) Let us simulate a distributed system by students an a classroom.
    Each student has one or more neighbours. All communication is done
    via delivering paper notes to neighbors. You can assume that all
    students can act concurrently, i.e., send and receive notes with any
    neigbour. Notes can only contain numbers, and you recognize the
    person who wrote them from the figures.

    Task is to count the number of students.

    Let us assume that one student (but we don't know who) tries to
    sabotage the count. What kind of tricks (s)he can do? Is it possible
    to make an algorithm/protocol that always count correctly whatever
    tricks the single person does? If you think so, sketch such
    algorithm. If you think that reliable count is impossible, show the
    situation and the tricks that make count go wrong.

26) Let us continue the exercise 5 (week 1), but increase the group to a
    bit larger (>= 10) and exploit also multicast the same message to
    all members of the group. Let us assume that an ordinary message
    costs 10c. What would be the cheapest protocol if the multicast
    costs a) 10c, b) 20c, c) N*5c, where N is the number of
    participants.


27) Let us continue the exercise 5 (week 1), but make it for a secret
    closed club. We'll use emails instead of SMS messages. What kind of
    security measures would ensure confidentiality is new members are
    taken only at physical meetings? a) Let us assume that all members
    participate all meetings. b) Part of the members may be absent from
    part of meetings.

28) Convert the previous task so that new members can be accepted via
    recommendations from three exiting members using emails only. What
    security protocols could used now?


Take skeletons for next exercises CryptExample and FileExample) from
www-page.

29) Write a Java program that counts and prints in hex the MD5 hash of the
    given file.

30) Write a Java program that encrypts the given file using Blowfish and
    a key asked from the user (twice).