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CMSC 414 Computer and Network Security Lecture 18. Jonathan Katz. Administrative. Exam April 22 Based on material through April 15 Next HW out later today, will be Exercises rather than a project One more project (buffer overflows) later in the semester. WireShark demonstration.
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CMSC 414Computer and Network SecurityLecture 18 Jonathan Katz
Administrative • Exam April 22 • Based on material through April 15 • Next HW out later today, will be Exercises rather than a project • One more project (buffer overflows) later in the semester
WireShark demonstration • NYTimes sends the password in the clear • View SSL/TLS session • Old (insecure) Yahoo! email authentication
PKI overview • In our discussion of public-key crypto, we have assumed users know each others’ public keys • But how can public keys be reliably distributed? • Download from web page insecure against man-in-the-middle attack • Can be obtained from CD-ROM or in person, but this is impractical in general • One solution: bootstrap new public keys from public keys you already know! • Certificates vouch for binding of public keys to names
PKA PKB Certificates • One party can vouch for the public key of another • Cert(AB) = SignSKA(“B”, PKB, info) • “info” can contain expiration time, restrictions, etc. • Can view this as a directed edge in a graph: • If you know A’s public key (and trust its certification), you can learn B’s public key
Cert(AB) Cert(BC) PKA PKB PKC Transitivity/“certificate chains” • Can learn keys via multiple hops: • Semantics are slightly different here: you may trust A to certify B, but do you trust A to certify that B can certify others?
PKA PKB PKC Transitivity • Can also infer trust from multiple (disjoint?) paths to the same public key for the same identity • Edges may also have weights indicating level of trust • A difficult problem in general PKD PKE Public keys I already know
Usage of certificates • “Trust anchors” = set of public keys already known (and trusted to certify others) • How to obtain certificates? • Some possibilities: • B “collects” certificate(s) for itself, sends these all when starting a connection • A finds certificates/certificate chains beginning at its own trust anchors and terminating at B • A tells B its trust anchors, B (finds and) sends certificates or certificate chains beginning at those trust anchors and terminating at itself
PKI components • Certificates • Distributing the keys of the “trust anchors” • (Means for retrieving certificates) • (CAs) • (Naming conventions) • (Trust model/method for evaluating a certificate chain) • (Revocation)
CAs and certificates • A certificate authority (CA) is just a widely used trust anchor • CA authentication policy determines the level of authentication needed to identify the principal before the certificate is issued • CA issuance policy describes the principals to whom the CA will issue certificates • A single CA can “act” as multiple CAs, each with their own policies… • Use distinct public keys (with different security)
Example: Verisign (1996) • Three levels of authentication • Verification of valid email address • Verification of name/address • Background check • Different authentication policies; same issuance policy (individuals only) • Another issuance policy was for issuing certificates to corporations/web servers
Naming • Identifiers correspond to principals • Must uniquely identify the principal • (Real) names alone are not enough! • Need disambiguation • A principal may have multiple identifiers • Depending on that principal’s roles • E.g., work/personal
E.g., X.509 certificates • Distinguished names identify a principal • Series of fields, each with key and value • E.g. /O=University of Maryland/OU=College Park/OU=Computer Science/CN=J. Katz • “O” - organization; “OU” - organizational unit; “CN” = common name
What does identity mean? • Ultimately, identity is proved using physical means • Driver’s license, fingerprints, etc. • If these are compromised, then certificates are irrelevant! • Certificate is just a binding between external identity and (DN, PK)
Trust • How much to trust a particular certificate? • Based on: • CA authentication policy • Rigor with which policy is followed • Assumptions inherent in the policy
Trust models • Define valid trust anchors, how a verifier chooses trust anchors, and what certification paths create a legal chain from trust anchor to target
Monopoly model • A single CA certifies everyone • Drawbacks • Single point of failure • Not very convenient • Complete monopoly… • Pure monopoly not used in practice
Monopoly + RAs… • The CA can appoint RAs • RAs check identities and vouch for keys, but the CA does all actual signing • Certainly more convenient • Not necessarily more secure • (Note: RAs can be integrated into other models as well)
Monopoly + delegated CAs • CA can issue certificates to other CAs • Vouch for their key and their trustworthiness as a CA • Delegated CA can sign certificates itself • Users must now obtain a certificate chain • (Note: delegation can be incorporated into other models as well)
CA hierarchy • Hierarchical structure of CAs • Nodes correspond to CAs • Children of a CA are constrained by the policies of their parents
Conflicts • What if two CAs have the same distinguished name? • What if two different CAs issue certificates for the same distinguished name (but to different principals)? • An easy way to address these is to have hierarchical names for CAs, and to incorporate CA distinguished name into issued certificates
Oligarchy • Multiple trust anchors • E.g., multiple keys pre-configured in software • User can add/remove trust anchors • Issues: • Less resistant to compromise! • Who says the user trusts the trust anchors? • Can users be tricked into using “bad” trust anchors? • Issuer name may be bogus • Can public keys of “good” trust anchors be changed in the software?
Anarchy model • Users responsible for defining the trust anchors they want to use • Drawbacks • Scalability/usability? • How much trust to place in a certificate chain
PKI in practice • PKIs are implemented in web browsers • A certificate is meaningless without verifying the name in the certificate • A certificate from an unknown CA is useless • “Trust” is only as good as your trust anchors • Do you know who your trust anchors are? • PGP “web of trust” model • PGP keyserver