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CS470, A.SelcukCryptographic Authentication1 Cryptographic Authentication Protocols CS 470 Introduction to Applied Cryptography Instructor: Ali Aydin Selcuk

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CS470, A.SelcukCryptographic Authentication2 Password authentication subject to eavesdropping Alternative: Cryptographic challenge-response –Symmetric key –Public key

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CS470, A.SelcukCryptographic Authentication3 Symmetric Key Challenge-Response Alice Bob I’m Alice a challenge R F(K AB,R) An example protocol:

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CS470, A.SelcukCryptographic Authentication4 Limitations: Authentication not mutual (login only) Subject to connection hijacking (login only) Subject to off-line password guessing (if K is derived from password) Bob’s database has keys in the clear

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CS470, A.SelcukCryptographic Authentication5 Symmetric Key Challenge-Response Alice Bob I’m Alice, K AB {timestamp} A one-message protocol:

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CS470, A.SelcukCryptographic Authentication6 Easy integration into password-sending systems More efficient: Single message, stateless Care needed against replays Care needed if key is common across servers Clock has to be protected as well Alternatively, with a hash function, send, I’m Alice, timestamp, H(K AB,timestamp)

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CS470, A.SelcukCryptographic Authentication7 Public Key Challenge-Response Alice Bob I’m Alice R [R] A By signature:

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CS470, A.SelcukCryptographic Authentication8 Public Key Challenge-Response Alice Bob I’m Alice {R} A R By decryption:

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CS470, A.SelcukCryptographic Authentication9 Problem: Bob (or Trudy) can get Alice to sign/decrypt any text he chooses. Solutions: –Never use the same key for different purposes (e.g., for login and signature) –Have formatted challenges

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CS470, A.SelcukCryptographic Authentication10 Mutual Authentication Both Alice and Bob authenticate each other An example protocol: Alice Bob I’m Alice R1 F(K AB,R1) R2 F(K AB,R2)

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CS470, A.SelcukCryptographic Authentication11 Alice Bob I’m Alice, R2 R1, F(K AB,R2) F(K AB,R1) Some saving:

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CS470, A.SelcukCryptographic Authentication12 Reflection attack: Trudy Bob I’m Alice, R1 R3, F(K AB,R1) Trudy Bob I’m Alice, R2 R1, F(K AB,R2) F(K AB,R1)

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CS470, A.SelcukCryptographic Authentication13 Solutions: –Different keys for Alice and Bob –Formatted challenges, different for Alice and Bob Principle: Initiator should be the first to prove its identity

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CS470, A.SelcukCryptographic Authentication14 Another weakness: Trudy can do dictionary attack against K AB acting as Alice, without eavesdropping. Solution against both problems: (Dictionary attack still possible if Trudy can impersonate Bob.) Alice Bob R1 F(K AB,R1), R2 F(K AB,R2) I’m Alice

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CS470, A.SelcukCryptographic Authentication15 Mutual authentication with PKC: Problem: How can the public/private keys be remembered by ordinary users? They can be stored in an electronic token (USB), or can be retrieved from a server with password-based authentication & encryption. Alice Bob I’m Alice, {R2} B R2, {R1} A R1

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CS470, A.SelcukCryptographic Authentication16 Session Key Establishment A session key is needed to authenticate/encrypt the rest of the session. The session key must be –different for each session –unguessable by an eavesdropper –not K AB {x} for some x predictable/extractable by an attacker Good if both sides contribute –avoids replays (“freshness guarantee”) –works if either side has a good random number generator

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CS470, A.SelcukCryptographic Authentication17 Nonces Nonce: Something created for one particular occasion Nonce types: –Random numbers –Timestamps –Sequence numbers Random nonces needed for unpredictability Obtaining random nonces from timestamps: encryption with a secret key.

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CS470, A.SelcukCryptographic Authentication18 Protocol Performance Comparison Computational Complexity: (to minimize CPU time, power consumption) –Number of private-key operations – “ “ public-key “ – “ “ bytes encrypted with secret key – “ “ bytes hashed Communication Complexity: –Number of message rounds –Bandwidth consumption

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