1. CS703 - Advanced Operating Systems
By Mr. Farhan Zaidi

2. Lecture No. 40

3. 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

4. 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.

5. 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

6. 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

7. 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”.

8. 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

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

10. 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, …