Advanced Operating Systems: Lecture 40 - Mr. Farhan Zaidi

CS703 ­ Advanced 
Operating Systems
By Mr. Farhan Zaidi

 

 


Lecture No. 
40


Overview of today’s lecture
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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
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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)
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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

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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

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(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

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Biometrics
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Biometrics
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Example features:
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Authentication of a person based on a physiological or behavioral
characteristic.
Face, Fingerprints, Hand geometry, Handwriting,
Iris, Retinal, Vein, Voice.

Strong authentication but still need a “Trusted Path”.


Access control
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Context
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System knows who the user is
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User has entered a name and password, or other info

Access requests pass through gatekeeper
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OS must be designed so monitor cannot be bypassed

User 
process

Reference
monitor

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Resource

Decide whether user can apply operation to resource


Access control matrix    [Lampson]
Objects

Subjects

…

File 1

File 2

File 3

File n

User 1

read

write

-

-

read

User 2

write

write

write

-

-

User 3

-

-

-

read read

write

read

write read

…
User m read


Two implementation concepts
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Access control list (ACL)

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Store column of matrix
with the resource
Capability
 User holds a “ticket” for
each resource
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File 1 File 2

…

User 1

read

write

-

User 2

write

write

-

User 3

-

-

read

read

write

write

…
User m

Access control lists are widely used, often with groups
Some aspects of capability concept are used in Kerberos, …