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Diffie–Hellman by hand

Level: 201 · working knowledge

Stub — an outline, not a lesson. There is no runnable example behind this page yet, so nothing on it has been through the check that backs every other claim in this library. The bullets below are the questions the finished page has to answer.

One line: Two people agree on a shared secret over a wire an attacker is reading in full — with numbers small enough to check on paper.

The questions this page has to answer

  • Work the whole exchange with p = 23, g = 5, and see both sides land on the same number.
  • What exactly does the eavesdropper have, and what is the problem they cannot solve? (Discrete log.)
  • Why does this give no authentication at all, and what is the man-in-the-middle attack in two lines?
  • Why are real parameters 2048-bit or elliptic-curve, and what is x25519 doing instead?
  • What is forward secrecy, and why is it a property of the exchange rather than of the cipher?

See also