Homework 3

Due Date: Tuesday, November 8, 2005
Points: 100

1. (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?
2. (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 assume?)
3. (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.
4. (10 points) Please decipher the following Cæsar cipher: TEBKFKQEBZLROPBLCERJXKBSBKQP.
5. (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.
   1. 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?
   2. 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?

Extra Credit

1. (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.