Tuomas AuraT-110.4206 Information security technology

Access control models and policies

Aalto University, autumn 2011

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Outline1. Access control2. Discretionary AC3. Mandatory AC4. Other AC models

ACCESS CONTROL

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Access control (AC) Subjects request actions on objects– Alice wants to read a file– Bob wants to update account balance– Process wants to open a socket

AC = authentication + authorization– authentication = verifying the identity of the

subject– authorization = checking that the subject has the

right to perform the requested action on the subject

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Reference monitor

Reference monitor controls access by subjects to objects– Grants or denies access requests– Logs events to audit trail– Follows rules set by administrators (i.e. implements a policy)

Trusted computing base (TCB) = all system components that need to be trusted to implement access control– Security kernel = implementation of reference monitor in an OS

But more about the implementation later; now we are talking about policies

ObjectsSubjects Reference monitor

Audit trail

Access rules

Accessrequests

Access control matrix Access control matrix is the simplest, most general AC

modelM : Subjects × Objects → Actions

AC matrix represents the protection state of a system

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Alice Bob Process 4567

Process6789

file1.txt read, write read read, write append

file2.txt write read - -

Socket s - - - open, read, write, close

DISCRETIONARY ACCESS CONTROL

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Discretionary access control (DAC) Data owners, usually users, set access rights Subjects are trusted to make decisions about sharing access rights with

others– Users decide who is allowed to access their files– User who can read a secret file can also print and email it– Process that can read a secret file can also send it to the network

DAC is also called identity-based AC Typical in commercial and consumer systems There may be a policy against sharing and access may be audited, but

the policy is not enforced technically Examples of DAC outside computers:

– Person with a key can open the door to others; door keys can be shared and copied

– Tell your friend a secret on the condition that he does not tell it to anyone else

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Access control list (ACL) ACL = list of the access rights associated with an object ACLs are another way to represent the AC matrix: one

row of the matrix is stored with each object– file1.txt ACL:

Alice: { read, write }; Bob: { read }; Process 4567: { read, write }; Process 6789: { append }.

– file2.txt ACL:Alice: { write }; Bob: { read }.

– Socket s ACL:Process 6789: { open, read, write, close }.

ACL examples:– Key cards, table reservations, Windows file system

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Capabilities Capability = an access right associated with the subject Capabilities are another way to represent the AC matrix:

one column is stored with each subject– Alice’s capabilities:

file1.txt: { read, write }; file2.txt: { write }.– Bob’s capabilities:

file1.txt: { read }; file2.txt: { read }.– Process 4567 capabilities:

file1.txt: { read, write }.– Process 6789 capabilities:

file1.txt: { append }; Socket s: {open, read, write, close }. Examples of capabilities:

– metal keys, driver’s license, parking permit

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MANDATORY ACCESS CONTROL

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Mandatory access control (MAC) Access rights are based on rules (i.e. policy) set by

administration The AC policy is enforced and cannot be changed by users Subjects cannot leak access rights to others

– User can read a secret file but cannot copy, print or email; file viewer application prevents cut-and-paste and screen shots

– One process can access the Internet, another write files to the disk MAC is also called rule-based AC MAC originates from military policies

– Intelligence officer may not be allowed to read his own reports– Officer can read a secret plan but cannot take a copy out of the room– Officer who has contact with foreign agents may lose access to

classified information

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Mandatory access control (MAC) MAC has some uses in commercial systems

– DRM: Alice can play the music she has purchased, but cannot share it

– Malware isolation: Host firewall may block potential spyware from making outbound connections to prevent information leaks

Examples of MAC-like systems outside computers:– Biometric authentication cannot be shared, e.g. photo on

driver’s license or signature on credit card– Admit-one event tickets: UV stamps, shredding bracelets– In UK, jurors must not read newspapers or watch TV

about the case so that they are not influenced by them

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Clearance and classification Mandatory access control rules are

often based on security labels on subjects and objects– Subject clearance – Object classification

l : (Subjects Objects)→Labels∪ MAC based on clearance and

classification levels is also called multi-level security (MLS)

Simple security property: S can read O iff l(S) ≥ l(O)

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Top secret

Secret

Confidential

Unclassified

High

Low

Bell-LaPadula model Bell-LaPadula (BLP) is a MAC policy for protecting

secrets– Military security model for computers; military is mostly

concerned with protecting secrets– Observation: the simple security property is not sufficient

to prevent secrets from leaking Bell-LaPadula:– Simple security property: S can read O iff l(S) ≥ l(O)– *-property: S can write O iff l(O) ≥ l(S)

Also called: no read up, no write down

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Biba model In computer systems, integrity of data and the

system is often more important than confidentiality– Which is more important in a bank IT systems?

Biba is a MAC policy for protecting integrity of data

Biba is the dual of Bell-LaPadula:– S can write O iff l(S) ≥ l(O)– S can read O iff l(O) ≥ l(S)

Also called: no write up, no read down20

Information flow security BLP and Biba are information flow policies

– BLP prevents flow of information from high to low– Biba prevents flow if information from low to high

Information flow policies are the basis for many security proofs. Typical proofs show non-interference:– view of one subject is not affected by the data of the other– low output does not depend on high input, or

high output does not depend on low input

How to use BLP and Biba in the same system?22

System

high inputhigh output

low inputlow output

High water mark, low water mark What happens to classification when you

combine low and high information into a new object? High water mark policy for secrecy: always set the classification to the highest input Low water mark policy for integrity: always set the classification if to the lowest input

Problem:– Over time, all documents will become top secret

with the lowest integrity level23

Upgrading and downgrading Upgrading, downgrading:

– In practice, security levels need to be changed by humans– E.g. downgrading documents for publication– E.g. upgrading intelligence reports that aggregate a lot of low-level data

Documents may need to be sanitized (i.e. redacted) before downgrading– E.g. removing personal names from military documents before

publication Sanitization may be difficult

– High subjects can use covert channels to leak data intentionally, e.g. hide data in photos

– Mistakes easy to makee.g. US military painting black box over text in PDF e.g. AOL publishing anonymized web search data

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OTHER ACCESS CONTROL MODELS

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Clark-Wilson model Data integrity cannot always be expressed in terms of MLS, i.e. who

has access to what data– E.g. transfers between bank accounts must not change the total balance

Integrity in many commercial systems depends on following the correct procedures

Clark-Wilson model defines rules for commercial systems for how to maintain data integrity:– Transactions must transform data items from a consistent state to another

consistent state– Auditing and procedural controls to enforce this(The specific rules could be different in each system)

Clark-Wilson model has not really been implemented; it is important because of the idea of using accounting rules as a model for security policy

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Chinese Wall model Conflicts of interest are common in business:

– Consulting company, investment bank, or law office may be advising competing clients and must keep their information separate

– The clients are assigned to different employees who do not exchange information between them

To avoid conflicts of interest, the access control policy must take into account the information previously accessed by the subject

Chinese Wall model:– If subject S has previously accessed an object O1 and the objects O1

and O2 are in a conflict of interest, then S may not access O2 – Subject can fall to either side of the wall but cannot change sides

later

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Separation of duty Chinese Wall is an example of separation of duty Other separation of duty policies:– Expense claim requires two signatures: the claimant and

an authorized approver, e.g. department manager, but nobody is allowed to approve their own expenses

– Auditors are often required to be from outside the company

– Keys to a safe may be given to two different persons– Lecturers issue grades to students but only study office

staff can enter them into the study register Unlike BLP and Biba, separation of duty policies are

stateful28

Groups and roles Adding structure to policies Group = set of subjects

– E.g. Administrators, T-110.4206-students– Object ACL can list groups in addition to individual users– Both group membership and ACLs change over time

Role = set of permissions (i.e. permitted actions on objects)– E.g. Administrator, T-110.4206-teacher, SCI-professor– Roles are usually relatively static; their assignment to users changes

Both are forms of indirection

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Objects x ActionsSubjects Roles or

groups* * * *

Role-based access control (RBAC) NIST standard Modeling high-level roles in an organization– E.g. Doctor, Nurse, Student, Lecturer, Course-assistant– Roles defined once; changed infrequently

Roles may be parameterized – E.g. Treating doctor of Mr. Smith, Lecturer or Student of T-

110.4206 Roles may form a hierarchy with inheritance Roles are assigned to users for longer term but activated

on demand for each session Constraints on role assignment and activation can

implement separation of duty30

Other access control models Originator-controlled AC (ORCON)– Creator of data retains control over access to it

Attribute-based AC– Access control is based in subject attributes instead of

subject identity– AC = attribute verification + authorization– E.g. need to be 18 to buy tobacco; need to be an Aalto

student to access course material– Enables anonymous access

Double-blinded review for scientific journals Many other AC models have been proposed

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Reading material Dieter Gollmann: Computer Security, 2nd ed., chapters 4, 8, 9 Matt Bishop: Introduction to computer security, chapters 2-7 Edward Amoroso: Fundamentals of Computer Security

Technology, chapters 6-13 Ross Anderson: Security Engineering, 2nd ed., chapter 8

Online sources:– Trusted Computer System Evaluation Criteria. United States

Department of Defense. December 1985. DoD Standard 5200.28-STD. (This is the classic “Orange Book”. Just take a quick look.)

– Richard Smith, MLS Introduction, from Handbook of Information Security, Wiley 2006

– Ravi Sandhu, Role-based access control models, IEEE Computer, Feb 1996 (text also here)

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Exercises What are the subjects, object and actions in Noppa? What security labels and MAC policy would be suitable for Noppa? Give examples of systems that require confidentiality or integrity

but not both. Which AC model and what kind of security labels could be used to

describe VM isolation? What label would be hypervisor or VM monitor get?

Could you define different confidentiality labels and integrity labels and then use both Bell-LaPadula and Biba policies in the same system? Give an example.

Define RBAC roles that could be used in the implementations of Noppa.

To what extent can your RBAC policy (above) be implemented with groups?

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