Section for Week 10, Tuesday, 3/16/99
Note:
  Seventh HW solutions are posted.
  Eighth will be posted when the HW becomes past-due.
  Thursday's section will be a review for the final.  Be there or be square.

1.  NUMBERS
    Diffie-Hellman key exchange: 
      Pick p (~1024 bits), g<p (generator).  Pick private keys x_A and x_B < p.
      Public keys are y_A=g^(x_A) mod p and y_B=g^(x_B) mod p.  To generate
      secret key, raise either public key to the other's private one mod p:
      z=(y_A)^(x_B) = (y_B)^(x_A) mod p
    Make sure "mod p" is clear.  Additive and exponential properties.

2.  XML
    Part a of 8.3 should be straightforward. (Q's?)
    Part b issues:
    a) Trying to convert something like:
       <?XML VERSION="1.0">
       <roster>
         <student>
           <SID>902378210</SID>
           <name>CHEN, KENNETH YU</name>
           <email>kennych@ucla.edu</email>
           <major>PECO-INT</major>
         </student>
       </roster>
       to:
       902378210:CHEN, KENNETH YU:kennych@ucla.edu:PECO-INT

       Simplest way to not have to deal with newlines: slurp in the whole
       file as one string.  To do this we undefine the input record
       seperator.  By default the input record seperator is a newline which
       is why when we usually do $data=<> we get one line into $data.
       However, doing:
       undef $/;
       $data=<>;
       Will have the effect of taking the whole input as one long string
       (containing newlines etc.) and putting it in $data.
       What then?  
       Need to get rid of the newlines and spacing from indentation, so we
       do:
       $data =~ s/\s+/ /g;
       What does this do?  What does the 'g' do?

       Lengthy explanation of using while(/<...>.*?<...>/g) {} construct
       (specifically, the use of the 'g' modifier).

       Once you think you have both parts done, try piping the output of one
       into the other:
       8.1a h/rosterx |8.1b 
       should generate the rosterx file back at you.

3.  The password Suite
    "create an empty user_data file" -- you can do this either by typing
    "pico user_data" and then using ctrl-O (to write out the file, without
    typing anything in) or else do:
    "cat /dev/null > user_data"
    
    The file locking outlined in the assignment is a bit messy (and makes
    possible a race condition).  Instead, use &lock before the first file
    access and &unlock after the last file access.