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Diffie-Hellman

Secure Key Exchange 1976. Diffie-Hellman. Whitfield Diffie Martin Hellman. 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 g a = x mod (n) These do not have to kept secret. Alice.

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Diffie-Hellman

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  1. Secure Key Exchange 1976 Diffie-Hellman

  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 = gxmod (n) • Sends X, g, and n to Bob.

  5. Bob • Chooses a large random number y • Calculates Y = gymod (n) • Sends Y toAlice.

  6. Alice • Calculates k = Yxmod (n)

  7. Bob • Calculates k’ = Xymod (n)

  8. The Key • k’ = k is the shared key k = Yxmod (n) = (gy )xmod (n) = gyxmod (n) k’ = Xymod (n) = (gx )ymod (n) = gxymod (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

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