Introduction to Computer SecurityAuthentication and Access Control

Pavel LaskovWilhelm Schickard Institute for Computer Science

Resource access: a big picture

1. Identification

2. Authentication

3. Authorization

4. Accountability

Which object O requests access to resource R?

Is O indeed what it claims to be?

Is O authorized to access R?

Log the transaction access(O,R)

Simple identification/authentication

Ask for an identity (e.g. user name)Check if an identity is knownAsk for a shared secret (e.g. apassword)Check if the secret matches the identityPassword is stored in an encrypted formusing a one-way hash function:root:!:14118:0:

laskov:$1$/et/grJh$xssVNwpdA35TwsSt7Yjvb/:14118:0:

Password encryption on UNIX

DESprepend password with 2-bit salttake 7 lowest bits from first 8 charactersencrypt a fixed 64-bit string with DES using 56 bits as a keyconvert the resulting 64 bits into 11 ASCII characters using 6bits for character (2 bits padded with zeros)

MD5originally written for FreeBSD to avoid export restrictionsno limit on password sizeis indicated by the starting $1$ in the shadow file

Identity management

A digital identity is a set of properties assigned to a givenobject (e.g. access rights)

Identity management:Creation and deletion of identitiesManagement of properties assigned to identitiesSecure storage of identitiesSecure handling of queries regarding identities and theirproperties.

Lightweight directory access protocol (LDAP)

A directory is a specialized database optimized for searchingand browsing.LDAP entries are collections of attributes identified by aunique distinguished name (dn).Entries are characterized by types that determine theirformat and syntax (e.g. ou = “Organisational Unit”).Entries are stored in a hierarchy. A relative distinguishedname defines a search path to an entry.

LDAP directory example

Authentication modes

User authentication Entity authenticationObject human being non-human beingApplications login

remote accessremote accesscommunication securityprogram securitykey distribution

Speed low highSecret only no yes

User authentication

Something you know: passwords, passphrases, sharedsecrets (e.g. mother’s maiden name), puzzlesSomething you have: smart cards, security tokensSomething you are: biometrics, signature dynamics,keyboard dynamics, voice print

What is a good password?

Long passwords are harder to break but tedious to type.Random passwords are hard to break but next to impossibleto memorize.Writing down a password on a sticky note doesn’t help.Password expiration reduces the damage but increases therisk of forgetting.Automatic password generation

easily memorizable wordspassphrases: sentences hashed into passwords

Passwords are only usable for services that are often used.

Password generation examples

Password generation examples

Smart cards

Physical tamper-proof credentialsMemory cards: simple storage of information(e.g. medical insurance)Microprocessor cards:

advanced functionality (e.g. transactions)cryptographic operations (e.g. key validationand key-pair generation)

Java cards: a mini-JVM on a chip (e.g. forcode update).Main problem: theft and loss risk

Biometric identification

“Unforgeable” features of a person:fingerprintshand geometryhand topographyiris scanretina scan

...or behavioral characteristics:signature dynamicskeyboard dynamicsvoice print

Disadvantages of biometric authentication

Non-zero probability of erroneousclassificationHigh cost and time consumptionImpossibility to revokePrivacy issues and social acceptanceCan be potentially falcified

Two-factor authentication

A combination of any two authenticationmodesExample: SecurID

PIN assigned to usertoken automatically generated inhardware every 30 seconds

Clock synchronization between a tokengenerator and an authentication serverrequired

Single sign-on

Motivation:Reduce the need to memorize passwordsReduce the time spent on typing passwordsReducing password maintenance effort

Solutions:Local password containers (e.g. KDE Wallet)Physical authentication devicesClient-server architectures (Kerberos, Active Directory)

Kerberos: a brief history

Development at MIT in the project Athena inthe 1980s, versions 1 – 3 for internal use,version 4 released in 1988.Version 5 released in 1993 fixed somesecurity flaws of the previous version;version 4 withdrawn in 2006.Banned for export by US government until2000 (due to the use of DES);re-implemented in Sweden at KTH.Currently, both implementations – MIT(Athena) and KTH (Heimdal) – support AESas the main encryption instrument.Supported by all major OS (Windows 2000up, Mac OS X, Solaris, Red Hat Enterprize)

Kerberos authentication protocol

Client

Application Server

Key Distribution Center (KDC)

Authentication Server(AS)

Ticket Granting Server(TGS)

Database

AS_REQ

AS_REP

TGS_REQ

TGS_REP

AP_REQ

AP_REP

Kerberos details: user login

User enters his user name U and password P at a clientworkstation.The hash value of his password Ku = h(P) becomes asecret key of the client/user.

Kerberos details: user authentication

The client sends his user ID U and the requested service Sto the authentication server AS:

C→ AS : (U, S)

If the user ID is found in a database, AS generates a TGSsession key kTGS and a ticket-generating ticket TGT:

TGT : (U, S, kTGS)

AS sends TGT encrypted to C:

C← AS : {S, kTGS}Ku{TGT}KTGS

Kerberos details: ticket request

The client generates an authenticator:

A : {U}kTGS

The client sends a ticket granting request to a ticket grantingserver:

C→ TGS : (S, A){TGT}KTGS

TGS verifies that U in A matches U in TGT, generates aservice session key kS and puts it into a service ticket:

TS : (U, S, kS)

TGS sends TS encrypted with the service key KS to C:

C← TGS : {S, kS}kTGS{TS}KS

Kerberos details: service authentication

The client generates an authenticator:

A : {U}kS

The client sends a request to a service:

C→ S : (A){TS}KS

The service checks if U in A matches U in TS and if soaccepts the request.

Kerberos resume

Provides secure authentication in an insecure network.A de-facto standard (at least open-source) in distributedauthentication services.Relatively complex in installation.Single point of failure.

Access control overview

Given a subject, which objects can it access and how?Given an object, which subjects can access it and how?

User process ? Resource

Subject Reference monitor Object

Policy

Access request

Main concepts of access control

Subject is an entity that initiates an access request.⇒ active entries, usually processes

Object is an entity an access to which is requested.⇒ passive entries, processes or resources

Principal is an identifiable entity, e.g., a user.⇒ each subject mapped to one principal⇒ multiple subjects per principal

Rights represent different types of access.Reference monitor makes authorization decisions.

Access control structures

Access control structures are mechanisms for implementingaccess policies:

access control matrixcapabilitiesaccess control listsintermediate controls (groups, negative permissions, roles,protection rings etc.)

Requirements for access control structures:an ability to express control policiesverifiability of correctness.scalability and manageability

Access control matrix

Access control matrix is a basic control structure.

bill.doc edit.exe fun.comAlice – {execute} {execute,read}Bob {read} {execute} {execute,read,write}

Advantages:clarity of definitioneasy to verify

Disadvantages:poor scalabilitypoor handling of changes

Capabilities

Capability is a subject-centered description of access rights:Alice: {edit.exe: execute}, {fun.com: execute, read}Bob: {bill.doc: read,write}, {edit.exe: execute}, {fun.com:execute, read,write}

Advantages:easy ownership transfereasy inheritance ofaccess rights

Disadvantages:poor overview of accessrights per objectdifficulty of revocationneed for extra integrityprotection

Access control lists (ACL)

ACL is an object-centered description of access rights:bill.doc: {Bob: read,write}exit.exe: {Alice: execute}, {Bob: execute}fun.com: {Alice: execute, read}, {Bob: execute, read,write}

Advantages:easy access to objectaccess rightsrelative easiness ofmanagement usingabstractions

Disadvantages:poor overview of accessrights per subjectdifficulty of revocationdifficulty of sharing

Access control abstractions

Group: an collection of related subjectseasy sharingeasy addition and removal of users

Negative permission: explicit revocation of access rightsPrivilege: a mapping of users to access rights

concise definition of access rights{admin: read,write,execute}, /etc/passwd: {Alice, admin}

Protection ring: a hierarchy of access right levels0 – operating system kernel1 – operating system2 – services3 – user processes

Mandatory access control (MAC)

Centralized access control by means of system-wide policy.Access control rights are fixed by an administrators.A limited number of implementations, e.g. SELinux,Systrace.

Advantages:strict control overinformation flowstrong exploitcontainment

Disadvantages:major usability problemscumbersomeadministration

Discretionary access control (DAC)

Access control is carried out by a resource owner.By associating ownership with principals, access rights areeasily transferred to other subjects.Deployed in a majority of common systems.

Advantages:simple and efficientaccess rightsmanagementscalability

Disadvantages:intentional abuse of accessrightsunintentional abuse ofaccess rightsno control over informationflow

Role based access control (RBAC)

RBAC attempt to handle the complexity of access control byextensive used of abstractions:

Data types are defined for all objects.Procedures are high level access control methods with amore complex semantics than elementary access controlrights. Procedures can be only applied to certain data types.Procedures are grouped into roles assigned to users. A usercan have more than one role and more than one user canhave the same role.Role hierarchies can be used to match natural relationsbetween roles.

Example: A Lecturer can create a role Student and give it aprivilege “read course material”.

Summary

Passwords and shared secrets are the most common toolsfor user authentication.⇒ limited security (cracking) and usability (forgetting,

management)⇒ alternatives: biometrics and smart cards

Single sign-on methods such as Kerberos provide means forautomating authentication in large distributed environments.⇒ single point of failure

Access control is based on the notion of the access matrixand its approximations (ACS, capabilities) and abstraction(groups, roles).