Version 0 of Diffie-Hellman Key Agreement Protocol

Updated 2004-06-17 13:02:40 by SRIV

2004-06-17 SRIV The Diffie-Hellman key agreement protocol (also called exponential key agreement) was developed by Diffie and Hellman in 1976. The protocol allows two users to exchange a secret key over an insecure medium without any prior secrets.

Because the numbers calculated in the program are larger than what stock tcl can handle, you must load a bignum extension. For a simple pure tcl demonstration, I chose to use the {MPA] extension to handle the large integers. This operates relatively slow, so keep your numbers small, perhaps around 4 digits maximum.

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You would typically run this program on two separate computers, although for testing just run it two times on the same computer.

  lappend auto_path lib
  package require mpa

  #Can be any prime number  
  set prime_num 571
  #Has to be a number that is less than prime_num, typically small for speed
  set number 3

  puts -nonewline "Enter a secret number:< "
  flush stdout
  gets stdin private_val

  #Calculate public value 3 ^ $private_val mod $prime_num
  set        public_val [::mpa::int::mod [::mpa::int::pow $number $private_val] $prime_num]
  puts "This is a public value that you send to your partner site:> $public_val"

  puts -nonewline "Enter the public value that you received from your partner site:< "
  flush stdout
  gets stdin public_val

  #Calculate your common key value $public_val ^ $private_val % $prime_num
  set        common_val [::mpa::int::mod [::mpa::int::pow $public_val $private_val] $prime_num]

  puts "Your commmon key value to use for encryption is:> $common_val"

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Sample results from two instances

Machine #1:

 Enter a secret number:< 123
 This is a public value that you send to your partner site:> 343
 Enter the public value that you received from your partner site:< 217
 Your commmon key value to use for encryption is:> 277

Machine #2:

 Enter a secret number:< 321
 This is a public value that you send to your partner site:> 217
 Enter the public value that you received from your partner site:< 343
 Your commmon key value to use for encryption is:> 277

Note how both machines derive a common number, 277, without revealing enough information to actually calculate that common number.