1. Diffie-Hellman
Secure Key Exchange
1976

2. Whitfield Diffie Martin Hellman

3. Alice & Bob Agree on 2 numbers n and g g is primitive relative mod (n)
For each x < n, there is an a such that
ga = x mod (n)
These do not have to kept secret

4. Alice Chooses a large random number x Calculates
X = gx mod (n)
Sends X, g, and n to Bob.

5. Bob Chooses a large random number y Calculates Sends Y to Alice.
Y = gy mod (n)
Sends Y to Alice.

6. Alice
Calculates
k = Yx mod (n)

7. Bob
Calculates
k’ = Xy mod (n)

8. The Key k’ = k is the shared key Nobody can calculate k given
k = Yx mod (n) = (gy )x mod (n) = gyx mod (n)
k’ = Xy mod (n) = (gx )y mod (n) = gxy mod (n)
Nobody can calculate k given
n, g, X, and Y

9. The Key Only Alice and Bob know k Good for only one session
Can’t be sure connected to the same person
Used if you only want a symmetric key
No authentication