Due Date: Tuesday, November 8, 2005
(25 points) In Firday's class, I mentioned the rsh
program. This program, and the related programs rlogin and
rcp, allow a user (call her ursula) on one system(call
it origin) to connect to another system (call it destin)
without supplying a password. Basically, Ursula logs into destin
and sets up a file, called a .rhosts file, that names host
origin and user ursula. This tells the host destin
that whenever ursula tries to log in from host origin,
the remote host origin is trusted to have authenticated
ursula properly, and no further authentication--like asking
for a password--is to be done. What assumption(s) is (are) being
made? When is installing these three programs a good idea, and when
is it a bad idea?
(20 points) A cryptographer once stated that cryptography
could provide complete security, and that any other computer security
controls were unnecessary. Why is he wrong? (Hint: Think
about using a cipher. What parts or aspects of using the cipher
does the cipher itself not protect? Could you protect those aspects
using a different cipher? If you could, what does that cipher
(20 points) Let k be the number corresponding to the
encipherment key for a Caesar cipher. (So, in English, "A" is 0,
"B" is 1, and so forth.) The decipherment key differs; it is (26 -
k) mod 26. (Thus, the decipherment key corresponding to the
encipherment key "B", or 1, is (26 - 1) mod 26 = 25, or "Z".) One
of the characteristics of a public key system is that the encipherment
and decipherment keys are different. Why then is the Caesar cipher
a classical cryptosystem, not a public key cryptosystem? Be specific.
(10 points) Please decipher the following Cæsar cipher: TEBKFKQEBZLROPBLCERJXKBSBKQP.
(25 points) In class, we discussed system names at the
transport layer (host names), the network layer (IP addresses), and
the data link layer (MAC addresses). Now consider a file system
data. The file system is associated with the host on which
it resides (call this host reside). But when another host
(call it remote) is given access to that file system, users
on that host refer to the file system by its name, not by
the name of the system on which it resides.
Assume that the system administrator of remote must first
mount the file system by saying something like "give my users
access to file system data on reside". Please
describe how one might implement this. In other words, if a user
reads a file on file system data, what happens?
Now assume the system administrator need do nothing (no mounting
required). How might one implement this; if a user reads a file on
file system data, what happens the first time? The next time?
(25 points) Solve the following Vigenère cipher: TSMVM
MPPCW CZUGX HPECP RFAUE IOBQW PPIMS FXIPC TSQPK SZNUL OPACR DDPKT
SLVFW ELTKR GHIZS FNIDF ARMUE NOSKR GDIPH WSGVL EDMCM SMWKP IYOJS
TLVFA HPBJI RAQIW HLDGA IYOUX.
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