PU - Public Key PK - Private Key SK - Secret Key SID - Session ID P - Payload PU1 --> <-- PU2/PU1(SK)/SK(SID)/PK2(P) SK(P)/PK1(SID+P) --> <-- SK(f(P))/PK2(SID+f(P)) PU(SK)/SK(P)/PK(P) --> <-- SK(f(P))/PK(f(P))