CS703 - Advanced Operating Systems By Mr. Farhan Zaidi

Lecture No. 40

Overview of today’s lecture User authentication Password based authentication UNIX password scheme One-time password schemes Challenge response authentication Biometrics and other authentication schemes Access control and authorization Access control matrix

Authentication Usually done with passwords. This is usually a relatively weak form of authentication, since it’s something that people have to remember Empirically is typically based on wife’s/husband’s or kid’s name, favorite movie name etc. Passwords should not be stored in a directly-readable form Use some sort of one-way-transformation (a “secure hash”) and store that if you look in /etc/passwords will see a bunch of gibberish associated with each name. That is the password Problem: to prevent guessing (“dictionary attacks”) passwords should be long and obscure unfortunately easily forgotten and usually written down.

Authentication (2) Unix password security Encrypt passwords One time passwords Lamport’s clever scheme (Read Tanenbaum for details) Challenge-Response based authentication Used in PPP and many other applications

Authentication alternatives Badge or key Does not have to be kept secret. usually some sort of picture ID worn on jacket (e.g., at military bases) Should not be forgeable or copy-able Can be stolen, but the owner should know if it is (but what to do? If you issue another, how to invalidate old?) This is similar to the notion of a “capability” that we’ll see later

Biometrics Biometrics Example features: Authentication of a person based on a physiological or behavioral characteristic.  Example features: Face, Fingerprints, Hand geometry, Handwriting, Iris, Retinal, Vein, Voice. Strong authentication but still need a “Trusted Path”.

Decide whether user can apply operation to resource Access control Context System knows who the user is User has entered a name and password, or other info Access requests pass through gatekeeper OS must be designed so monitor cannot be bypassed ? Resource User process Reference monitor Decide whether user can apply operation to resource

Access control matrix [Lampson] Subjects Objects File 1 File 2 File 3 … File n User 1 read write - User 2 User 3 User m

Two implementation concepts Access control list (ACL) Store column of matrix with the resource Capability User holds a “ticket” for each resource File 1 File 2 … User 1 read write - User 2 User 3 User m Access control lists are widely used, often with groups Some aspects of capability concept are used in Kerberos, …