MISC: DH key xchange

[Karl Lui Barrus <klbarrus@owlnet.rice.edu>]

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Earlier somebody (Samuel Pigg?) asked for a D-H key exchange reference.
It should be in any modern crypto textbook, and is easy to follow.

Alice and Bob want to exchange keys over a hostile channel.  They pick
a prime p, a random number a, and exchange this information.  

Alice picks Ra, a random number less than p, and keeps it secret.  Bob picks
Rb, also a random number less than p.  Alice calculates Ya = a^Ra mod p, and 
sends the result to Bob.  Bob calculates Yb = a^Rb mod p, and sends the 
result to Alice.  To recover the common key Alice and Bob will now use with
each other, they raise the result the other person sent them to their secret
random number, and take the result modulo p.  That is, Alice calculates
Yb^Ra mod p, and Bob calculates Ya^Rb mod p.

Even if an eavesdropper gets a, p, and the intermediate Ya and Yb, the final
key cannot be determined (due the difficulty of the discrete logarithm).

Example:

1) Alice and Bob pick a = 11, p = 347

2) Alice picks Ra = 240
   Bob picks Rb = 39

   Alice and Bob keep Ra and Rb secret

3) Alice calculates Ya = a^Ra mod p
                       = 11^240 mod 347 = 49

   Bob calculates   Yb = a^Rb mod p
                       = 11^39 mod 347 = 285

4) Alice sends Bob Ya = 49
   Bob sends Alice Yb = 285


5) Alice calculates Yb^Ra mod p = 285^240 mod 347
                                = 268

   Bob calculates   Ya^Rb mod p = 49^39 mod 347
                                = 268

   Now Alice and Bob can communicate using their common key.  Even if an
   enemy intercepts a = 11, p = 347, Ya = 49, and Yb = 285, the common
   key cannot be calcuated.  (Well, they can here since I'm using small
   numbers, but with large numbers the discrete log problem is intractable).





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Karl L. Barrus: klbarrus@owlnet.rice.edu         
keyID: 5AD633 hash: D1 59 9D 48 72 E9 19 D5  3D F3 93 7E 81 B5 CC 32 

"One man's mnemonic is another man's cryptography" 
  - my compilers prof discussing file naming in public directories