The document discusses boundary controls and access control mechanisms. It describes how boundary controls establish the interface between users and computer systems by authenticating users' identities. It then examines different types of access controls, including identification, authentication, authorization of users and resources. It discusses implementing access control mechanisms and challenges around open vs closed environments, authorization approaches, and dynamics of authorization. It also covers cryptographic controls like encryption techniques, choosing cipher systems, key distribution, and digital signatures.

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)]