The Diffie-Hellman Protocol

• Can be used to establish common secret key over insecure channel
• Requires commutative encryption function E(E(M, Ka), Kb) = E(E(M, Kb), Ka)
• Procedure
  1. Alice chooses a key M
  2. Alice sends Bob E(M, Ka)
  3. Bob sends Alice E(E(M, Ka), Kb)
  4. ... which is E(E(M, Kb), Ka)
  5. Alice decrypts it into E(M, Kb) and sends it to Bob
  6. Bob decrypts it to M
  7. M can then be used as a shared secret key

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