CS 513 - System Security
Lecture 11

Lecturer: Professor Fred B. Schneider
Notes by: Vicky Weissman
Lecture Date: 3/2/00

Today's Topic - Certification Authorities

Public Key Distribution

Public key cryptography relies on a trusted service, called the Certificate Authority (CA), to distribute public keys. The CA's public key is distributed in software packages like Netscape.

The CA maintains a database of certificates; a certificate is a binding signed by the CA between a principal and its public key.

The CA's vulnerability is reduced by keeping the service off-line.

Key Revocation

When the CA learns that a private key has been compromised, it must revoke the corresponding public key.

To facilitate revocation, each certificate contains an expiration date and a serial number. At regular intervals, the CA signs and distributes a certification revocation list (CRL). The CRL contains the serial number and date of issue for every compromised key that has a valid expiration date. A principal will not use a public key that has either expired or is in the CRL.

Rather than using expiration dates and CRLs, certificates could be re-evaluated at the time of use. This approach, however, requires that the CA is on-line which would increase the service's vulnerability.

A discussion on multiple CAs was started at the end of this lecture and completed during the next class. Please refer to Lec12 for the complete notes.