Acknowledgments. CSE565: Computer Security Lectures 16 & 17 Authentication & Applications
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1 CSE565: Computer Security Lectures 16 & 17 Authentication & Applications Shambhu Upadhyaya Computer Science & Eng. University at Buffalo Buffalo, New York Lec 16.1 Acknowledgments Material for some slides is drawn from Lawrie Brown s slides Lec
2 Authentication Protocols Used to convince parties of each other s identity and to exchange session keys May be one-way or mutual Key issues are confidentiality to protect session keys timeliness to prevent replay attacks Lec 16.3 Replay Attacks Where a valid signed message is copied and later resent simple replay repetition that can be logged repetition that cannot be detected backward replay without modification Countermeasures include use of sequence numbers (generally impractical) timestamps (needs synchronized clocks) challenge/response (using unique nonce) Lec
3 Using Symmetric Encryption Can use a two-level hierarchy of keys Usually with a trusted Key Distribution Center (KDC) each party shares own master key with KDC KDC generates session keys used for connections between parties master keys used to distribute these to them Lec 16.5 Using Public-Key Encryption Have a range of approaches based on the use of public-key encryption Need to ensure that you have correct public keys for other parties Using a central Authentication Server (AS) Various protocols exist using timestamps or nonces Lec
4 Remote Authentication Authentication functions developed d to support applicationlevel authentication & digital signatures Kerberos a privatekey remote authentication service X.509 directory authentication service Lec 16.7 Kerberos Trusted key server system from MIT Provides centralized private-key third-party authentication in a distributed network allows users access to services distributed through network without needing to trust all workstations rather all trust a central authentication t server Two versions in use: 4 & 5 Lec
5 Kerberos Requirements First published report identified its requirements as: security reliability transparency scalability Implemented using an authentication protocol based on Needham-Schroeder Lec 16.9 Needham-Schroeder Protocol Original third-party key distribution protocol For session between A, B mediated by KDC Protocol overview is: 1. A KDC: ID A ID B N 1 2. KDC A: E Ka [Ks ID B N 1 E Kb [Ks ID A ] ] 3. A B: E Kb [Ks ID A ] 4. B A: E Ks [N 2 ] 5. A B: E Ks [f(n 2 )] Lec
6 Problems with NS- Protocol & Solutions Used to securely distribute a new session key for communications between A & B But is vulnerable to a replay attack if an old session key has been compromised then message 3 can be resent convincing B that it is communicating with A, but actually it is a masquerader Modifications o to address this require: e timestamps (Denning 81) using an extra nonce (Neuman 93) Lec Kerberos 4 Overview A basic third-party authentication scheme Have an Authentication Server (AS) users initially negotiate with AS to identify self AS provides a non-corruptible authentication credential (ticket granting ticket TGT) Have a Ticket Granting server (TGS) users subsequently request access to other services from TGS on basis of users TGT Lec
7 Kerberos 4 Overview Lec Kerberos Realms A Kerberos environment consists of: a Kerberos server a number of clients, all registered with server application servers, sharing keys with server This is termed ed a realm typically a single administrative domain If have multiple realms, their Kerberos servers must share keys and trust Lec
8 Kerberos Version 5 Developed in mid 1990 s Provides improvements over v4 addresses environmental shortcomings encryption alg, network protocol, byte order, ticket lifetime, authentication forwarding, interrealm auth and technical deficiencies double encryption, non-std mode of use, session keys, password attacks Specified as Internet standard RFC 1510 Lec X.509 Authentication Service Part of CCITT X.500 directory service standards distributed servers maintaining some info database Defines framework for authentication services directory may store public-key certificates with public key of user signed by certification authority Also defines authentication protocols Uses public-key crypto & digital signatures algorithms not standardised, but RSA recommended Lec
9 X.509 Certificates Issued by a Certification Authority (CA), containing: version (1, 2, or 3) serial number (unique within CA) identifying certificate signature algorithm identifier issuer X.500 name (CA) period of validity (from - to dates) subject X.500 name (name of owner) subject public-key info (algorithm, parameters, key) issuer unique identifier (v2+) subject unique identifier (v2+) extension fields (v3) signature (of hash of all fields in certificate) Notation CA<<A>> denotes certificate for A signed by CA Lec X.509 Certificates Lec
10 Obtaining a Certificate Any user with access to CA can get any certificate t from it Only the CA can modify a certificate Because it cannot be forged, certificates can be placed in a public directory Lec CA Hierarchy If both users share a common CA then they are assumed to know its public key Otherwise CA's must form a hierarchy Use certificates linking members of hierarchy to validate other CA's each CA has certificates for clients (forward) and parent (backward) Each client trusts parents certificates Enable verification of any certificate from one CA by users of all other CAs in hierarchy Lec
11 CA Hierarchy Use Lec Certificate Revocation Certificates have a period of validity May need to revoke before expiry, eg.,: 1. user's private key is compromised 2. user is no longer certified by this CA 3. CA's certificate is compromised CA s maintain list of revoked certificates the Certificate Revocation List (CRL) Users should check certs with CA s CRL Lec
12 Authentication Procedures X.509 includes three alternative authentication ti ti procedures: One-Way Authentication Two-Way Authentication Three-Way Authentication All use public-key signatures Lec One-Way Authentication 1 message ( A->B) used to establish the identity of A and that message is from A message was intended for B integrity & originality of message Message must include timestamp, nonce, B's identity ty and is signed by A Lec
13 Two-Way Authentication 2 messages (A->B, B->A) which also establishes in addition: the identity of B and that reply is from B that reply is intended for A integrity & originality of reply Reply includes original nonce from A, also timestamp and nonce from B Lec Three-Way Authentication 3 messages (A->B, B->A, A->B) which enables above authentication ti ti without t synchronized clocks Has reply from A back to B containing signed copy of nonce from B Means that timestamps need not be checked or relied upon Lec
14 X.509 Version 3 Has been recognized that additional information is needed in a certificate /url, policy details, usage constraints Rather than explicitly naming new fields defined a general extension method Extensions consist of: extension identifier criticality indicator extension value Lec Certificate Extensions Key and policy information convey info about subject & issuer keys, plus indicators of certificate policy Certificate subject and issuer attributes support alternative names, in alternative formats for certificate subject and/or issuer Certificate path constraints allow constraints on use of certificates by other CA s Lec
15 Summary We have considered: Kerberos trusted key server system X.509 authentication and certificates Lec
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