Security in ComputingSecurity in Computing

Protection in General-Purpose Operating Systems

I.I. Object protectionObject protection

A. Memory protection1. Methods:

a) Fence – in single-user OS prevent faulty user program destroying resident OS; confine user program to one side of the boundary;

Fence implementation (1) – Fixed fence; OS resides in predefined memory space and user on another

Fence implementation (2) – Variable fence register (base register); contains address of the end of OS; cannot protect one user against another

Figure 4-1  Fixed Fence.

Figure 4-2  Variable Fence Register.

I.I. Object protectionObject protectionb) Relocation – OS size change overtime;

programs relocated to starting address by relocation factor

Fence register used as a hardware device; fence register contents added to program address

c) Base/Bounds Registers – needed for multiuser environment

Base register provides only lower bound (starting address)

Bound register provide upper address limit; each user program reside within base and bound addresses; OS employs context switching

Figure 4-3  Pair of Base/Bounds Registers.

I.I. Object protectionObject protectiond) Tags – needed because base/bounds

registers create an all or nothing for sharing data

A tagged architecture provides for one or more extra bits in each word of memory to identify access rights – R-W-RW-X

e) Segmentation – simply dividing a program into separate pieces with logical memory addressing

f) Paging – alternative to segmentation – program divided into equal pieces called pages and memory into equal sized units called page frames

Figure 4-5  Example of Tagged Architecture.

I.I. Object protectionObject protection

A. General control of access to objects1. Methods:

a) File Directory – mechanism for protecting objects (files) from users (subjects);

Every file has a unique owner who possesses controls access and revocation rights, including who else has what access

Each user has a file directory, which lists all the files which that user has access

Implemented by using one list per user, naming all the objects the user is allowed to access

Figure 4-10  Directory Access.

I.I. Object protectionObject protectionb) Alternative is Access Control List (ACL) –

differs from directory list – one access control list per object i.e. no need for an entry for the object in the individual directory of each user

c) Capability – OS hands some protection burden to user; ticketing system; lead to Kerberos system

Figure 4-12  Access Control List.

I.I. Object protectionObject protection2. Kerberos: – implements authentication

and access authorisation by means of ticketing capabilities; MS OS NT+

Secure with symmetric cryptography Uses authentication server (AS) and ticket-

granting server (TGS), both part of KDC User presents authentication credentials (e.g.

password) to AS and receives authentication ticket showing that he/she has passed

To access a resource (e.g. Printer) user sends ticket to TGS; TGS returns authorised ticket and another ticket to present to Printer for access

II.II. AuthenticationAuthenticationA. Based on

1. something you know Passwords, PIN numbers, mother’s

name etc.

2. something you have Identify badges, physical keys,

driver’s licence etc

3. something you are Biometrics – physical characteristics of

users, such as fingerprint, pattern of person’s voice or face (picture

II.II. AuthenticationAuthentication

B. Attacks on Passwords1. Try all possible passwords

(exhaustive or brute force attack)2. Try frequently used passwords

(probable passwords? Think of a work!)

3. Try passwords likely for the user4. Search for the system list of

passwords (plaintext or encrypted?)5. Ask the user!

Figure 4-15  Users’ Password Choices.

II.II. AuthenticationAuthenticationC. Authentication techniques

(discussed later) 1. challenge-response (e.g. one-time

password)2. Impersonation of login – one sided

D. Biometrics (authentication without passwords)

1. Identification (“this is Captain Cook”) Vs Authentication (“ I am Captain Cook, present my hand to prove it”

2. Acceptance – people find them intrusive

II.II. AuthenticationAuthentication3. processing speed – speed at which

recognition is done impacts on accuracy

4. “false positive or “false accept” (a reading that is accepted when it should be rejected) Vs “false negatives” or “false reject” (one that rejects when it should accept)