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Public Key Infrastructures

Public Key Infrastructures. CS 470 Introduction to Applied Cryptography Instructor: Ali Aydin Selcuk. Public Key Infrastructure. A system to securely distribute & manage public keys. Important for wide-area trust management (e.g., for e-commerce) Ideally consists of

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Public Key Infrastructures

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  1. Public Key Infrastructures CS 470 Introduction to Applied Cryptography Instructor: Ali Aydin Selcuk PKI

  2. Public Key Infrastructure • A system to securely distribute & manage public keys. • Important for wide-area trust management (e.g., for e-commerce) • Ideally consists of • a certification authority • certificate repositories • a certificate revocation mechanism (CRLs, etc.) • Many models possible: monopoly, oligarchy, anarchy, etc. PKI

  3. Monopoly Model • Single organization is the CA for everyone • Shortcomings: • no such universally-trusted organization • requires everyone to authenticate physically with the same CA • compromise recovery is difficult (due to single embedded public key) • once established, CA can abuse its position (excessive pricing, etc.) • requires perfect security at CA PKI

  4. Monopoly with Registration Authorities • CA trusts other organizations (RAs) to check identities, do the initial authentication • Solves the problem of physically meeting the CA. Other problems remain. • RAs can be incorporated into other models too PKI

  5. Delegated CAs • Root CA certifies lower-level CAs to certify others • All verifiers trust the root CA & verify certificate chains beginning at the root (i.e., the root CA is the trust anchor of all verifiers) • E.g., a national PKI, where a root CA certifies institutions, ISPs, universities who in turn certify their members • Limitations are similar to monopoly with RAs PKI

  6. Oligarchy • Many root CAs exists trusted by verifiers • The model of web security • Solves the problems of single authority (e.g., excessive pricing) • Disadvantages: • n security-sensitive sites instead of one. Compromise of any one compromises the whole system • users can easily be tricked into trusting fake CAs. (depending on implementation) PKI

  7. Anarchy • Each user decides whom to trust & how to authenticate their public keys • Certificates issued by arbitrary parties can be stored in public databases, which can be searched to find a path of trust to a desired party • Works well for informal, non-sensitive applications (e.g., PGP) PKI

  8. Revocation • Mechanisms to cancel certificates compromised before expiration • Certificate Revocation List (CRL): list of revoked certificates, published periodically by the CA • Delta CRLs: Only the changes since the last issue are published • Online Revocation Servers: No CRL is published. Verifier queries a central server to check if a certificate has been revoked. PKI

  9. Finding Certificate Chains • Can be started with the subject sending its certificates to the verifier (e.g., SSL) • A directory naming structure can be followed (e.g., DNSsec) PKI

  10. X.509 Certificates • Common standard for certificate format • PKIX: Internet standard for X.509-based PKI • Fields (X.509 v3): • version • serial number • signature algorithm identifier • issuer • validity period • subject • subject public key information • signature • standard extensions (key usage limitation, etc.) • other extensions (application & CA specific) PKI

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