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sec-2 - Take-home points Distributed Systems Security II...

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Distributed Systems Security II 15-440 Take-home points What does using public-key for our authentication buy us? Compare kerberos (needham-schroeder) and SSL with a certificate authority Metrics: Scaling, robustness, timeliness Motivate & understand perfect forward secrecy and diffie-hellman A touch of research: Perspectives SSL auth vs. CA auth Remember digital signatures From last time... Shared key crypto with key K AB : Intuition: Hash them together HMAC(K AB , m) = H( (K..) | H(K ... | m)) Public key crypto with K A , K -1 A : Intuition: “signing” is encryption using the private key. But pub key operations are expensive: To make it practical, hash first so that the message is small, fixed-size. E(K -1 A , H(m)) Today: Auth protocols Needham-Schroeder - basis of Kerberos authentication Goal: Secure, usable authenticaiton system without needing public-key cryptography Idea: Everyone shares a key with a trusted third party server If A wants to talk to B, on demand, that server generates key K AB and shares it with (and only with) A and B.
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Needham-Schroeder and Kerberos • In following diagrams: –Client C initiates a connection to server S • Authentication server A generates “session key” K SC for them to use to talk to each other. Only A, S, and C will know this key. –Each entity shares a private key with the authentication server: C and A share a secret key K AC S and A share secret key K AS –Nobody else knows either of those two keys. Needham-Schroeder and Kerberos • Messages: 1: C to A: C,S,n Authentication server A Server S Client C 1 A nonce: a “number used once.” In Kerberos this is usually the time.
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