Computer Security CS477Lecture 11Ching-Hua ChuanOctober 8st2008Administration Homework #2 October 13th Test review October 13th Test #2 October 15thToday’s Outline Authentication Applications X.501 (Chap 4.2) Public key infrastructureX.509 Authentication Service X.509 is part of the X.500 series of recommendations that define a directory service. A directory: a server or distributed set of servers that maintains a database about users. Each certificate contains the public key of a user and is signed with the private key of a CA. X.509 is used in S/MIME, IP Security, SSL/TLS and SET. RSA is recommended to use.CertificatesCertificateCA<<A>> = CA{V, SN, AI, CA, TA, A, Ap}whereY<<X>> = certificate of user X signed by CA YY{I} = the signing I of Y. It consists of I with an encrypted hash code appended.The Typical Digital Signature ApproachCA userObtaining a User’s Certificate Characteristics of certificates generated by CA: Any user with access to the public key of the CA can recover the user public key that was certified. No part other than the CA can modify the certificate without this being detected. Because of the unforgeable characteristics, certificates can be: put in a directory without special protection transmitted between users directly In a Large Community Subscribing to a common CA is infeasible: Everyone needs to have CA’s public key. The public key must be securely delivered to every user. Solution: multiple CAs Each CA securely provides its public key to some fraction of users.A Simple ScenarioCA X1 CA X2user A user BPBX1PBX21X1<<X2>>2X2<<B>>3A chain of certificates: X1<<X2>>X2<<B>>X.509 Hierarchy Hierarchy CAs: U, V, …, Z Users: A, B, C Forward certificate Certif. of X generated by other CAs. Reverse certificate Certif. generated by X that are the certif of other CAs.X.509 HierarchyA acquires B’s certificate:X<<W>>W<<V>>V<<Y>>Y<<Z>>Z<<B>>B acquires A’s certificate:Z<<Y>>Y<<V>>V<<W>>W<<X>>X<<A>>Revocation of Certificates Reasons for revocation User’s PR key is compromised User is no longer certified by this CA CA’s certificate is compromised Certificate Revocation List (CRL) Every CA has one Keep revoked but not expired certificates, issued by this CAAuthentication ProceduresOne-wayTwo-wayThree-waySigned by APublic Key Infrastructure X.509 (PKIX)PKIX Architecture ModeloptionalSummary Authentication Applications X.501 (Chap 4.2) Public key infrastructureCertificate content, chain of certificate, hierarchy, authentication proceduresArchitecture model, management
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