The University of SydneyPage 42Goal: avoid playback attack
Drawbacks?nonce: number (R) used only once-in-a-lifetimeap4.0: to prove Alice “live”, Bob sends Alice nonce, R. Alicemust return R, encrypted with shared secret key
“I am Alice”RK (R)A-BAlice is live, and only Alice knows key to encrypt nonce, so it must be Alice!Authentication: yet another try