Security models

Security Models
Copyright by Aakash Panchal
All Right reversed by LJ Projects

Terminology
3
Trusted Computing Base (TCB) – combination of protection
mechanisms within a computer system
Subjects / Objects
Subjects are active (e.g., users / programs)
Objects are passive (e.g., files)
Reference Monitor – abstract machine that mediates subject
access to objects
Security Kernel – core element of TCB that enforces the
reference monitor’s security policy

Types of Access Control
4
Discretionary Access Control (DAC) – data owners can
create and modify matrix of subject / object relationships
(e.g., ACLs)
Mandatory Access Control (MAC) – “insecure”
transactions prohibited regardless of DAC
Cannot enforce MAC rules with DAC security kernel
Someone with read access to a file can copy it and build a new
“insecure” DAC matrix because he will be an owner of the new
file.

Information Flow Models
5
In reality, there are state transitions
Key is to ensure transitions are secure
Models provide rules for how information flows from state to state.
Information flow models do not address covert channels
Trojan horses
Requesting system resources to learn about other users

State Machine Model
State is a snapshot of the system at one moment in time.
State transition is the change to the next state.
If all the state transitions in a system are secure and if the
initial state of the system is secure, then every subsequent
state will also be secure, no matter what input occurs.

Bell-LaPadula (BLP) Model
8
BLP is formal (mathematical) description of mandatory access control
First model that was created to control access to data.
Three properties:
ds-property (discretionary security)
ss-property (simple security – no “read up”)
*-property (star property – no “write down”)
A secure system satisfies all of these properties
BLP includes mathematical proof that if a system is secure and a
transition satisfies all of the properties, then the system will remain
secure.

Bell-LaPadula Model (Continued)
9
Honeywell Multics kernel was only true implementation of
BLP, but it never took hold
DOD information security requirements currently achieved
via discretionary access control and segregation of systems
rather than BLP-compliant computers
The problem with this model is that it does not deal with
integrity of the data.

Bell-LaPadula Model (Continued)
10
The star property makes it possible for a lower level subject
to write to a higher classified object.
A covert channel is an information flow that is not controlled
by a security mechanism.
A low level subject may see high level object name but are
denied access to the contents of the object.

Harrison-Ruzzo-Ullman Model
BLP model does not state policies for changing access rights
or for the creation or deletion of subjects and objects.
This model defines authorization system that address these
issues.
It operates on access matrices and verifies if there is any
sequence of instructions that cause an access right to leak
information.

Three Main Goals of Integrity
Preventing unauthorized users from making modifications to
data or programs.
Preventing authorized users from making improper or
unauthorized modifications.
Maintaining internal and external consistency of data and
programs.

Biba Model
13
Similar to BLP but focus is on integrity, not confidentiality
Implements the first goal of integrity.
Result is to turn the BLP model upside down
High integrity subjects cannot read lower integrity objects (no “read
down”)
Subjects cannot move low integrity data to high-integrity environment
(no “write up”)

Intuition Behind Models
Control of confidential information is important both in
military and commercial environment.
However in commercial environment the integrity of data is
also equally important to prevent errors and frauds.
The higher the level, the more confidence one has that a
program will execute correctly.
Data at higher level is more accurate, reliable and
trustworthy than data at the lower level.

Clark-Wilson Model
15
Reviews distinction between military and commercial policy
Military policy focus on confidentiality
Commercial policy focus on integrity
Mandatory commercial controls typically involve who gets to
do what type of transaction rather than who sees what
(Example: Handle a check above a certain amount)

Clark-Wilson Model (Continued)
16
Two types of objects:
Constrained Data Items (CDIs)
Unconstrained Data Items (UDIs)
Two types of transactions on CDIs in model
Integrity Verification Procedures (IVPs)
Transformation Procedures (TPs)
IVPs certify that TPs on CDIs result in valid state
All TPs must be certified to result in valid
transformation

17
System maintains list of valid relations of the form:
{UserID, TP, CDI/UDI}
Only permitted manipulation of CDI is via an authorized TP
If a TP takes a UDI as an input, then it must result in a
proper CDI or the TP will be rejected
Additional requirements
Auditing: TPs must write to an append-only CDI (log)
Separation of duties

18
Subjects have to identified and authenticated.
Objects can be manipulated only by a restricted set of
programs.
Subjects can execute only a restricted set of programs
A proper audit log has to be maintained.

Clark-Wilson versus Biba
19
In Biba’s model, UDI to CDI conversion is performed by
trusted subject only (e.g., a security officer), but this is
problematic for data entry function.
In Clark-Wilson, TPs are specified for particular users and
functions. Biba’s model does not offer this level of
granularity.

Chinese Wall
20
Focus is on conflicts of interest.
Principle: Users should not access the confidential
information of both a client organization and one or more of
its competitors.
How it works
Users have no “wall” initially.
Once any given file is accessed, files with competitor
information become inaccessible.
Unlike other models, access control rules change with
user behavior

Chinese Wall
21
Separation of Duty.
A given user may perform transaction A or Transaction B but
not both.
A simple security property
A subject has access to an object if and only if, all the objects that
subject can read are from non competing groups.
The *- Property
A subject can write to client only if the subject can not read any
object from a competing group.

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