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Intermediate Topic 21 of 51 Security & Cryptography

Diffie-Hellman Key Exchange

Agreeing on a shared secret over an open channel.

How it works

Diffie-Hellman lets two parties establish a shared secret over a public channel without having met. Each picks a private value, computes a public value from a common generator and prime modulus or elliptic-curve base point, and exchanges public values; each then combines their private value with the other's public value to arrive at the same secret. An eavesdropper sees only the public values and faces the discrete logarithm problem. Ephemeral variants provide forward secrecy.

Step-by-step walkthrough

  1. They agree on public numbers g and p.
  2. Alice computes A from her secret a and sends A.
  3. Bob computes B from his secret b and sends B.
  4. Alice computes B to the power a, Bob computes A to the power b.
  5. Both get the same shared secret and an eavesdropper cannot easily compute it.

Where it is used

TLS, SSH, IPsec, Signal and WireGuard use Diffie-Hellman, usually in elliptic-curve form such as X25519, to derive session keys.

Common pitfall

Unauthenticated exchange is vulnerable to man-in-the-middle attacks, so it must be combined with signatures or certificates. Weak or reused parameters, small groups and invalid public values enable attacks, as in Logjam. Always use standardized groups or curves, validate received points and derive keys with a key derivation function.

Where it fits in the course

Diffie-Hellman Key Exchange is topic 21 of 51 in Security & Cryptography, rated Intermediate. It follows Public Key Infrastructure. The next topic is Block Cipher Modes.

Practice and explore

How to study this topic

  1. Read the explanation above and restate the definition in your own words.
  2. Open the interactive lesson and answer its mini quiz.
  3. Read the common pitfall, then write a short example that avoids it.
  4. Check your understanding in the Security & Cryptography quiz, and revisit the topic if you miss a question.

Frequently asked questions

What is Diffie-Hellman Key Exchange?
Agreeing on a shared secret over an open channel. It belongs to Security & Cryptography and is an intermediate-level topic.
Where do people use Diffie-Hellman Key Exchange?
TLS, SSH, IPsec, Signal and WireGuard use Diffie-Hellman, usually in elliptic-curve form such as X25519, to derive session keys.
What is the most common mistake with Diffie-Hellman Key Exchange?
Unauthenticated exchange is vulnerable to man-in-the-middle attacks, so it must be combined with signatures or certificates.
What should I learn before this topic?
Start with Public Key Infrastructure, the topic just before it in Security & Cryptography.
How can I practice it?
Answer the mini quiz in the interactive lesson, then take the Security & Cryptography quiz.
Is Diffie-Hellman Key Exchange hard to learn?
It is an Intermediate topic. Be comfortable with the Beginner topics in this area first, then read the common pitfall carefully, since it is where most mistakes happen.
How long does it take to learn Diffie-Hellman Key Exchange?
Most learners can read the explanation and finish the interactive lesson in under an hour. Fluency comes from applying the idea, so plan a few short practice sessions with the quiz.
Which topics are related to Diffie-Hellman Key Exchange?
Public Key Infrastructure, Block Cipher Modes, Digital Signatures are closely connected. The full list is in the related topics section of this page.
What should I study after Diffie-Hellman Key Exchange?
Continue with Block Cipher Modes, the next topic in Security & Cryptography.

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