Diffie-Hellman
The Diffie-Hellman key exchange: public parameters, exchanged values, the shared secret, what an eavesdropper sees, the discrete logarithm that protects the exchange, and the man-in-the-middle attack.
- 23 cards
- 6 topics
- Free
The outline, topic by topic
Each topic is weighted the way the exam weighs it.
- Concepts
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- Scenarios
A few cards from the deck
Key exchange: what is it?
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A protocol by which two parties converge on the same secret while exchanging only messages that anyone can read.
The resulting secret then serves as the key of a fast symmetric cipher: the parties never had to meet to agree on a key.
Bob has chosen b = 15 and receives A = 8 (p = 23). What does this program print?
```python
p = 23
b = 15
A = 8
print(pow(A, b, p))`2191208
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2Bob computes A^b mod p = 8¹⁵ mod 23 = 2, the shared secret. 19 is his own public value 5¹⁵ mod 23, and 120 comes from A × b.
Alice sends A = … and Bob sends B = …; each then obtains the same secret g^(ab) mod p.
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Alice sends A = g^a mod p and Bob sends B = g^b mod p; each then obtains the same secret g^(ab) mod p.
Alice computes B^a = g^(ba) and Bob computes A^b = g^(ab): the two exponents multiply in either order, so the result is the same.