Homework #3 -- Due October 5

The client/server application is the one described in class on 13 September. The server models an eight-slice roulette wheel. The client places bets by naming an amount and a number between 0 and 7. It the client guess the correct number, the client wins seven times its bet. Otherwise, the client loses its bet.

In order to prevent the server from cheating, i.e., picking a number after the client makes its pick, we use a verification method based on number theory. The server chooses a number v between 0 and 999,999,999. The outcome, r, of the next spin of this number is calculated, using C integer division, as

Before the client makes its ``call'', the server provides the client with the number w, the square of v modulo 2,000,000,137, a prime number. The abstract protocol loop is

  1. the server chooses v, and consequently w and r;
  2. the server transmits w to the client;
  3. the client places its bet by sending a message to the server;
  4. the server transmits v and r to the client; and, finally,
  5. the client verifies v, w, and r.

Of course, there are certain other details, such as packet format and minimum bets. This are illustrated in the following sample run of the client and server shown in the left column. The server's responses are shown in boldface.

BALANCE 1000000Initial client balance is 1,000,000
VERIFIER SQUARED 919696766Server transmits w
WHAT COMMANDSClient sends a bad packet
INVALID INPUTServer critiques the packet
BALANCE 875000Server extracts a bad packet penalty
VERIFIER SQUARED 784423615Server starts again
BET 50 ON 4
BAD BET AMOUNT 50Minimum bet is 100,000
BALANCE 765625Another bad packet penalty
VERIFIER SQUARED 710391967
BET 100000 ON 10
BAD BET NUMBER 10Bets are from 0 to 7
BALANCE 669922The penalties are mounting up
VERIFIER SQUARED 473746622
Client transmits a blank line
INVALID INPUT
BALANCE 586182Yep, another one
VERIFIER SQUARED 544028316
BET 500000 ON
INVALID INPUT
BALANCE 512910
VERIFIER SQUARED 178991952
BET 500000 ON 7A good bet at last!
NUMBER 7
YOU WINClient has won!
VERIFIER 932968576Server provides v
BALANCE 4012910
VERIFIER SQUARED 261884547
BET 4000000 ON 7
NUMBER 7
YOU WIN
VERIFIER 893211251
BALANCE 32012910
VERIFIER SQUARED 633585890
EXITClient quits while well ahead
CLIENT EXITServer acknowledges
BALANCE 32012910
EXITING

Right now the server is running on penrose.cs.unca.eduat port 7295. You can actually telnet to it directly by typing 

    telnet penrose 7295

Your assignment is to write a user-friendly client for this application. The client should prompt the user for a bet number and amount and then transmit the appropriate packets to the server. Your client should also verify that the server didn't cheat, by checking out the numbers returned in the VERIFIER SQUARED and VERIFIER packets. There are a few subroutines stored in the directory /usr/local/csci/363/roul that you will find very helpful.

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