Appendix G: Exercises - Codes: The Guide to Secrecy from Ancient to Modern Times - page 588

G.9 Chapter 9 Exercises

9.1. Suppose we have a large codebook that consists of short words with

corresponding 6-digit numbers. Devise a means of choosing passphrases
from this list based upon the roll of dice.

9.2. Assume that you have a password p and each time you logon to a WWW
site the host uses a one-way function f to calculate f ( p ), and compares
this with a stored value. Devise a means using only this function, that
will prompt you to change your password after, say, 100 logins.

9.3. The SSH protocol presented on pages 334-339, has a significant additional
feature called port-forwarding , which means that either Alice or the server
can bind a socket to a collection of specified ports. In practice, what this
means is that when Alice, say, connects to one of these ports, the call is
relayed to the other end of this particular SSH call, from which another
call is made to some other predetermined port. Effectively, this is an SSH
built-in tunneling mechanism (see page 337 for details). Discuss the pros
and cons of such tunnels.
( Hint: Consider setting up such tunnels to avoid firewalls. )

9.4. Explain how a basic wireless system operates. (See Section 9.2.)

9.5. Explain the relationship between frequency of a signal and the size of

an antenna. In particular, why is it possible for cell phones to have such
small antennas?
( Hint: The higher the frequency, the smaller the wavelength, so the closer
the antenna the same size is to the wavelength it receives, the better the
reception. In practice, an antenna is some fraction of the wavelength, such
as half, say. )

9.6. A cell phone has internal memory that contains the wireless phone num-
ber, a system identification number or SID, which identifies the phone to
the system to which it subscribes, and an electronic serial number identi-
fying that specific phone as a measure against fraud. When a cell phone
is turned on, it compares its SID with any overhead signals and when a
match occurs, the phone knows it is operating within the subscriber net-
work. How would a cell phone operate when it is away from its subscriber
network and needs roaming mode ?

9.7. Cell phone networks are made up of cells each of which is associated with
a base station. These cells overlap, so when one is travelling, there needs
to be a hand-off (see page 352), from one cell to another. How would a
mobile switching center perform this hand-off?
( Hint: A mobile switching station continuously monitors the power level
of a given cell phone, together with the power level of the base station.
Speculate how the switching station would react when a cell is getting close
to the edge of the current base station's cell. )