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Handshake Protocols COEN 350

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Simple Protocol Alice: Hi, I am Alice. My password is “fiddlesticks”. Bob: Welcome, Alice.

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Simple Protocol Vulnerable to sniffing and replay attack. Alice: Hi, I am Alice. My password is “fiddlesticks”. Bob: Welcome, Alice.... Mallory: Hi, I am Alice. My password is “fiddlesticks”. Bob: Welcome, Alice.

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Shared Secret Alice and Bob share a secret key K. Alice: I am Alice. Bob: Encrypt R. Alice: E K (R) Bob (calculates E K (R) as well.): Welcome Alice.

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Shared Secret Vulnerable to DOS attack. while(1){ Mallory: I am Alice. Bob: Encrypt R. Mallory: X. Bob (E K (R) != X): Access denied. }

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Shared Secret Vulnerable to sniffing and replay attack if R is not random or if R is repeated.

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Shared Secret, use of clock Alice: I am Alice, E K (clock). Bob calculates clock, compares with his value: Welcome Alice.

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Shared secret, use of clock Man in the Middle + replay attack: Mallory to Bob: KILL, KILL, KILL, KILL. Alice: Hi, I’m Alice. E K (clock). Mallory to Alice: KILL, KILL, KILL, KILL. Mallory to Bob: Hi, I’m Alice. E K (clock). Bob: Hi, Alice.

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Public Key Alice: “I’m Alice.” Bob: “R”. Alice: “E Alice (R)”. Bob calculates “D Alice E Alice (R) == R: Hi Alice.

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Public Key Alice: “I’m Alice.” Bob creates random challenge R: “E Alice (R)”. Alice: “R”. Bob checks R == R: Hi Alice.

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Public Key: DOS attack Trudy: “I’m Alice.” Bob: “R”. Trudy: “X” Bob calculates “D Alice E Alice (X) != R: Access Denied. Bob spends much more time computing than Trudy!

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Mutual Authentication: Shared Secret Alice: “I am Alice” Bob: “R B ” Alice: E K (R B ). R A. Bob calculates E K (R B ) himself: E K (R A ). Hi Alice. Alice calculates E K (R A ) herself: Hi Bob.

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Mutual Authentication with less messages? Alice: I am Alice. R A Bob: R B. E K (R A ). Alice: Hi Bob. E K (R B ). Bob: Hi Alice.

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Mutual Authentication with less steps is vulnerable to the replay attack Session 1 Trudy: I am Alice. R A. Session 1 Bob: R B. E K (R A ). Session 2 Trudy: I am Alice. R B. Session 2 Bob: R B’. E K (R B ). Session 1 Trudy: Hi Bob. E K (R B ). Session 1 Bob: Hi Alice.

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Warning Signals Requestor should authenticate herself first. Don’t have requestor and requestee do exactly the same thing. (E.g. use different key pairs.) If you provide encryption service, you set yourself up for a key guessing attack.

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Public Key: Simple Mutual Authentication Alice: “I am Alice. R A ” Bob:“E Bob (R A ). R B ” Alice D Bob E Bob (R A )=R A : Hello Bob. E Alice (R B ). Bob: D Alice E Alice (R B ) = R B : Hello Alice.

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Key Distribution Centers Maintains a shared secret for each registered user. To set-up a connection requires the KDC to set up a session key.

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Key Distribution Center Original Algorithm Alice to KDC: Alice wants Bob. KDC to Alice: Here is your session key. KDC to Bob: Here is your session key. This needs to be modified.

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Key Distribution Center: Needham Schroeder Protocol Alice to KDC: N1, Alice wants Bob. KDC to Alice: K A (N1,K S,Bob,Ticket), where Ticket=K B (K S,Alice). Alice to Bob: Ticket, K S (N2). Bob to Alice: K S (N2-1,N3). Alice to Bob: K(N3-1). N1, N2, N3 are nonces to prevent replay attacks.

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Key Distribution Center: Needham Schroeder Protocol Variant Alice to KDC: N1, Alice wants Bob. KDC to Alice: K A (N1,K S,Bob,Ticket), where Ticket=K B (K S,Alice). Alice to Bob: Ticket, K S (N2). Bob to Alice: K S (N2-1),K S (N3). Alice to Bob: K(N3-1). N1, N2, N3 are nonces to prevent replay attacks.

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Replay attack on modified NS Alice to KDC: N1, Alice wants Bob. KDC to Alice: K A (N1,K S,Bob,Ticket), where Ticket=K B (K S,Alice). Alice to Bob: Ticket, K S (N2). Bob to Alice: K S (N2-1),K S (N3). Alice to Bob: K S (N3-1). Trudy as Alice to Bob: Ticket, K S (N 2 ) Bob to Alice, but intercepted by Trudy: K S (N 2 -1), K S (N 4 ) Trudy as Alice to Bob: Ticket, K S (N 4 ). Bob to Alice, but intercepted by Trudy. K S (N4-1), K S (N 5 ). Trudy as Alice to Bob: K S (N4-1).

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Key Distribution Center Assume that Alice’s key has become compromised. Trudy can now present herself as Alice to Bob with an old ticket. Tickets need to have an expiration date!!!!!!!!!!!

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