Cryptography and Network SecurityChapter 14 – Authentication ApplicationsAuthentication ApplicationsKerberosKerberos RequirementsKerberos 4 OverviewSlide 7Kerberos RealmsKerberos Version 5X.509 Authentication ServiceX.509 CertificatesSlide 12Obtaining a CertificateCA HierarchyCA Hierarchy UseCertificate RevocationAuthentication ProceduresOne-Way AuthenticationTwo-Way AuthenticationThree-Way AuthenticationX.509 Version 3Certificate ExtensionsSummaryCryptography and Network SecurityThird Editionby William StallingsLecture slides by Lawrie BrownChapter 14 – Authentication ApplicationsWe cannot enter into alliance with neighboring princes until we are acquainted with their designs.—The Art of War, Sun TzuAuthentication Applications•will consider authentication functions•developed to support application-level authentication & digital signatures•will consider Kerberos – a private-key authentication service•then X.509 directory authentication serviceKerberos•trusted key server system from MIT •provides centralised 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 server•two versions in use: 4 & 5Kerberos Requirements•first published report identified its requirements as:–security–reliability–transparency–scalability•implemented using an authentication protocol based on Needham-SchroederKerberos 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 TGTKerberos 4 OverviewKerberos 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 a realm–typically a single administrative domain•if have multiple realms, their Kerberos servers must share keys and trustKerberos 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 1510X.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 recommendedX.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 CAX.509 CertificatesObtaining a Certificate •any user with access to CA can get any certificate from it •only the CA can modify a certificate •because cannot be forged, certificates can be placed in a public directoryCA 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 hierarchyCA Hierarchy UseCertificate Revocation•certificates have a period of validity•may need to revoke before expiry, eg:1. user's private key is compromised2. user is no longer certified by this CA3. CA's certificate is compromised•CA’s maintain list of revoked certificates–the Certificate Revocation List (CRL)•users should check certs with CA’s CRLAuthentication Procedures•X.509 includes three alternative authentication procedures: •One-Way Authentication •Two-Way Authentication •Three-Way Authentication •all use public-key signaturesOne-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 and is signed by ATwo-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 BThree-Way Authentication•3 messages (A->B, B->A, A->B) which enables above authentication without 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 uponX.509 Version 3•has been recognised that additional information is needed in a certificate –email/URL, policy details, usage constraints•rather than explicitly naming new fields defined a general extension method•extensions consist of:–extension identifier–criticality indicator–extension valueCertificate 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’sSummary•have considered:–Kerberos trusted key server system–X.509 authentication and