Authentication Deniable Authentication Protection Against Dictionary Attacks Isidora Petreska Dimitar Gosevski and

Contents Introduction to Authentication Deniable Authentication Deniable authentication protocols Adaptive Multi-Trapdoor Commitment (AMTC) Scheme ATMC – based authenticators Decisional Diffie-Hellman (DDH) Scheme Passwords and AuthenticationDeniable Authentication Countermeasures against dictionary attacks and their weaknesses Reveres Turing Test (RTT) Basic User Authentication Protocol Solving Protocol Drawbacks Security Analysis Analysis for a user account Setting the parameters

Introduction to Authentication Formal definition Authentication technologies Concerns to: –Deniable authentication –Password security

Deniable Authentication Property of deniability Concept of deniable authentication –Privacy concerns of the sender Need for deniable authentication: –in private key cryptography? –in public key cryptography?

Deniable authentication protocols Example of deniable protocol What if the sender changes his/her mind? Need to forward deniability Proposal of new schemes based on: –Adaptive Multi-Trapdoor Commitment and –Decisional Diffie-Hellman protocols

Adaptive Multi-Trapdoor Commitment (AMTC) Scheme Notion of commitment Trapdoor Commitment Scheme (TCS) Adaptive Multi-Trapdoor Commitment (AMTC) Scheme: –CKG - a master key generation algorithm –Sel - given a master public key (PK), it outputs an equivalent key (pk) –Tkg - having a triple (PK, pk, TK) it outputs a trapdoor information (tk) –Com - verify a commitment Com(PK, pk, M, R) –Equiv - opening of a commitment C

ATMC – based authenticators (1/2)

ATMC – based authenticators (2/2)

Decisional Diffie-Hellman (DDH) Scheme (1/2)

Decisional Diffie-Hellman (DDH) Scheme (2/2)

Passwords and Authentication Passwords as authentication method Passwords convenient for both service providers and users Dictionary attacks against passwords Password eavesdropping

Countermeasures against dictionary attacks and their weaknesses Countermeasures –Delayed response –Account locking procedure Drawbacks of the countermeasures –Global password attacks –Denial of Service Attacks –Customer service cost

Reveres Turing Test (RTT) Found by M.Naor Distinguish between human and automated program –Automated generation –Easy for Humans –Hard for machines –Small probability of guessing the answer correctly

RTT (Cont..) Used by large IT companies –Yahoo –AltaVista –PayPal Possible drawbacks of RTTs –Based on the visual capabilities of the human Improvement of RTTs –Audible RTTs

Basic User Authentication Protocol Combines RTT with any password based authentication system –Slow down the execution of the automated programs tying to break in the system Drawbacks of the Protocol –Usability difficult for the user to answer RTT in every login attempt –Scalability not easy to generate and serve RTT per login attempt

Solving Protocol Drawbacks Limited set of computer used by the user –Small possibility of dictionary attack from this computes –Identify specific computer web browser by using cookies –No need of solving RTT by this computers RTT required only for a fraction of the login attempts

Security Analysis User Server Interaction –Feedback no. 1 Invalid username or password –Feedback no. 2 First answer RTT than you will be inform if the username/password pair is correct –Whether to ask for RTT is deterministic function from username /password pair –Same time delay regardless if the entered password is correct or not

Analysis for a user account To verify fraction of correct or incorrect passwords a RTT mast be pass first Assume that all passwords has the same probability to be correct Randomly chosen passwords Wining Ticket Game

Setting the parameters Steps to designee a successful authentication protocol: –Estimating the benefit that the attacker gain from breaking into account –Estimating the size of the domain of passwords –Estimating the cost of solving single RTT by the attacker –The cost of breaking an account should be higher than the potential gain from the break

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