Foundations of Computer SecurityLecture 54: CertificatesDr. Bill YoungDepartment of Computer SciencesUniversity of Texas at AustinLecture 54: 1 CertificatesWeb of TrustMuch of what happens on-line, particularly e-commerce, dependson establishing a web of trust relationships among the parties.Question: Why should A trust B with whom he’s never previouslydealt?Possible Answer: A might rely on a known third party to “vouchfor” B.The Chamber of Commerce, Better Business Bureau, creditreporting agencies, friends all function in part as certificationauthorities for some commercial transactions.Lecture 54: 2 CertificatesNeed for TrustWith a public key infrastructure (PKI), if A knows B’s public key,then A can:send a message securely to B;be assured that a message from B really originated with B.But, how does A know that the public key B presents is really B’spublic key and not someone else’s?The most common circumstance in which trust is needed in adistributed on-line context is reliably binding a public key to anidentity.Lecture 54: 3 CertificatesCertificatesA certificate is the electronic equivalent of a “letter ofintroduction.”A certificate is constructed with digital signatures and hashfunctions.A public key and a user’s identity are bound together within acertificate, signed by a certification authority, vouching for theaccuracy of the binding.Lecture 54: 4 CertificatesHow it Might WorkSuppose X is the president of a company; Y is her subordinate.Each have an RSA public key pair.1Y securely passes message {Y , KY} to X .2X produces a cryptographic hash of the message, i.e.,h({Y , KY}).3X produces {Y , KY, {h({Y , KY})}K−1X}.This last then becomes Y ’s certificate, signed by X .Lecture 54: 5 CertificatesValidating the CertificateSuppose Y presents to Z the certificate:{Y , KY, {h({Y , KY})}K−1X}What does Z do with this? What does Z learn?The message certifies the binding of Y and KY.X is the certifying authority.Data items Y and KYwere not altered or corrupted.This scheme assumes that Z has a trustworthy public key for X , toverify X ’s signature.Lecture 54: 6 CertificatesLessonsCertificates are needed to establish a web of trust in adistributed environment.A trusted individual can “vouch for” another party bycertifying the binding of identity to public key.A third party can check the validity of the binding.Next lecture: Certificates IILecture 54: 7
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