asssd dd

1. Boundary Controls
• The boundary subsystem establishes the interface
between the would-be user of a computer system
and the computer system itself.
• Once boundary subsystem functions are complete,
the user can commence to use the resources of the
system.
• Boundary subsystem controls have one primary pu
rpose: to establish the identity and authenticity of
would-be users of a system.
• This chapter examines some of the major types of
security exercised in the boundary subsystem.

2. Boundary Controls
Access Controls
• An Access Control Mechanism is used in a compute
r installation for preventing unauthorized access to
and use of resources.

3. Boundary Controls
Functions of an Access Control Mechanism.
• An access control mechanism associates with ident
ified, authorized users the resources they are perm
itted to access and the action privileges they have
with respect to those resources.
• The mechanism processes user's requests in three
steps, identification, authentication, and authorizat
ion

4. Boundary Controls
Identification and Authentication
• Four classes of authentication are remembered inf
ormation, possessed information, personal charact
eristics and dialog.
• There is need to understand the weaknesses in ea
ch of the class of authentication.
• Passwords, possessed objects, and personal charac
teristics all have a further weakness that the authe
ntication information is reduced to a bit stream wh
ich can be wiretapped.

5. Boundary Controls
Object Resources
• In a generalized access control mechanism, all reso
urces must be named since the mechanism must c
ouple users with resources they are permitted to u
se.
• It is important that the access control mechanism h
ave a means of identifying the authenticity of the o
bject resources it provides to a user .e.g assigning
object resources a unique identity that can not be
copied.

6. Boundary Controls
Action Privileges
• If a user has the permission to use a commodity, th
e amount of the commodity that the user can cons
ume must be specified.
• The most complex action privileges relate to the us
e of data resources
• ADD, INSERT, APPEND,MODIFY
file1 File2 file3
John Own Read Write
Alice Read Own Read Write Read
bob Read Write Read Own Read Write

7. Boundary Controls
Action privileges
• Data independent – all the contents of a file ar
e subject to the action privilege assigned to th
e file
• Data dependent – can apply to data items wit
hin a file e.g salary < 15000

8. Boundary Controls
• One of the important restrictions on action privileg
es is a temporal restriction e.g no access to the ter
minal after working hours.
Implementing an Access Control Mechanism
• Full implementation of an ACM can be costly. In pr
actise, some trade offs must be made; overall contr
ol must be decreased to reduce costs and improve
performance.
• The tradeoffs made in the ACM must be reasonabl
e, given the overall control requirements of the ins
tallation

9. Boundary Controls
Problems in Implementing ACM
(1)Open Vs Closed Environment
• While an open environment allows for an easier im
plementation of an ACM, only in a closed environm
ent can effective control be exercised over users.
(2)Approaches to Authorization
• They are two approaches to implementing the aut
horization module in an access control mechanism:

10. Boundary Controls
• A ticket oriented approach?
• A list oriented approach?
(3)Dynamics of Authorization
• Some difficult implementation problems arise in th
e area of authorization dynamics.

11. Boundary Controls
Cryptographic Controls
• Cryptology is the science of secret codes
• Cryptography deals with systems for translating dat
a into codes that are meaningless to anyone who d
oes not possess the system for recovering the initia
l data.
• Cryptanalysis are the techniques for recovering enc
rypted data
• Cipher is an algorithm for performing encryption o
r decryption

12. Boundary Controls
Cryptographic Techniques
They are three classes of techniques for enciphering
plaintext:
• Substitution ciphers
• Transposition ciphers.
• Product ciphers

13. Boundary Controls
Choosing a Cipher System
• A cipher system has two components: algorithm an
d the key
• Five desirable properties of a cipher system:
• High Work Factor
• Small key
• Simplicity
• Low error propagation
• Little expansion of message size.

14. Boundary Controls
Public Key Cryptosystems
• Ensuring the secure distribution of a private key to
parties who need the key is a difficult task.
• The widely known public key cryptosystem is the R
SA scheme.
• The disadvantage of public key cryptosystems is th
at they are slow relative to the processing time req
uired for private-key cryptosystems.

15. Boundary Controls
Key Distribution
• Maintaining the secrecy of the cryptographic key is
of paramount importance.
• The method chosen to distribute a key must be reli
able; and maintain the secrecy of the keys thus mai
ntaining overall secrecy of the cryptosystem.
• Reminder: assignment diffie-hellman key exchange
protocol

16. Boundary Controls
Explain Diffie-Hellman key exchange, El-gamal a
nd RSA Algorithms.

17. Boundary Controls
Digital Signatures
• Establishing the authenticity of individuals and pre
venting the disavowal of messages and or contract
s are still critical requirements when data is exchan
ged in electronic form.
• A digital signature is simply a string of 0s and 1s rat
her than a line drawn on a page.
• Public key cryptosystems can be used to establish:
secret messages
signed messages
signed, secret messages.

18. Boundary Controls
Secret Message
• S Pur(m)
• R Prr[Pur(m)]
Signed Secret Message
• S Pur(m)
• S Prs[Pur(m)]
• R Pus (Prs[Pur(m)])
• R Prr[Pur(m)]