Introduction to Information Security


Authentication and Key-Exchange Protocols



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Authentication and Key-Exchange Protocols

  • Combine authentication and key-exchange
  • Assume Carla and Diane are on opposite ends of a network and want to talk securely
    • Want to agree on a new session key securely
    • Want to each be sure that they are talking to the other and not an intruder
  • Wide-Mouth Frog
    • Assumes a trusted third-party, Sam, who shares a secret keys, KC and KD, respectively, with Carla and Diane

Authentication and Key-Exchange Protocols

  • Wide-Mouth Frog
  • C: => S(C,Encrypt((D,KCD,TC),KCS));

    S: => D(Encrypt((C, KCD, TS), KDS));

  • Observations:
    • Reliance on synchronized clocks to generate timestamps
    • Depends on a third-party that both participants trust
    • Initiator is trusted to generate good session keys

Authentication and Key-Exchange Protocols

  • Yahalom
  • C => D (C,NC);

    D => S (D,Encrypt((C,NC,ND),KD));

    S => C (Encrypt((D,KCD,NC,ND),KC),Encrypt((C,KCD),KD));

    C => D (Encrypt((C,KCD),KD),Encrypt(ND,KCD));

  • Note: Diane is the first one to contact Sam who only sends one message to Carla

Authentication and Key-Exchange Protocols

  • Denning and Sacco (public-key)
    • Carla sends a message to Sam including her name and Diane’s name
    • Sam replies with signed copies of both Carla and Diane’s public key
    • C: => S(C,D);

      S: => C(Encrypt((C,CPublic,TS),SPriavte),Encrypt((D,DPublic,TS),SPriavte));

      C: => D(Encrypt((C,CPublic,TS),SPriavte),Encrypt((D,DPublic,TS),SPriavte));

    • Carla generates the session key, KCD, and signed a message containing it and a timestamp with her private key
    • C: => D(Encrypt(Encrypt((KCD,TC),CPrivate),DPublic));


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