Introduction to Computer Security

Transcription

1 Introduction to Computer Security Authentication and Access Control Pavel Laskov Wilhelm Schickard Institute for Computer Science

2 Resource access: a big picture 1. Identification Which object O requests access to resource R? 2. Authentication Is O indeed what it claims to be? 3. Authorization Is O authorized to access R? 4. Accountability Log the transaction access(o, R)

3 Simple identification/authentication Ask for an identity (e.g. user name) Check if an identity is known Ask for a shared secret (e.g. a password) Check if the secret matches the identity Password is stored in an encrypted form using a one-way hash function: root:!:14118:0: laskov:$1$/et/grjh$xssvnwpda35twsst7yjvb/:14118:0:

4 Password encryption on UNIX DES prepend password with 2-bit salt take 7 lowest bits from first 8 characters encrypt a fixed 64-bit string with DES using 56 bits as a key convert the resulting 64 bits into 11 ASCII characters using 6 bits for character (2 bits padded with zeros) MD5 originally written for FreeBSD to avoid export restrictions no limit on password size is indicated by the starting $1$ in the shadow file

5 Identity management A digital identity is a set of properties assigned to a given object (e.g. access rights) Identity management: Creation and deletion of identities Management of properties assigned to identities Secure storage of identities Secure handling of queries regarding identities and their properties.

6 Lightweight directory access protocol (LDAP) A directory is a specialized database optimized for searching and browsing. LDAP entries are collections of attributes identified by a unique distinguished name (dn). Entries are characterized by types that determine their format and syntax (e.g. ou = Organisational Unit ). Entries are stored in a hierarchy. A relative distinguished name defines a search path to an entry.

7 LDAP directory example

8 Authentication modes User authentication Entity authentication Object human being non-human being Applications login remote access remote access communication security program security key distribution Speed low high Secret only no yes

9 User authentication Something you know: passwords, passphrases, shared secrets (e.g. mother s maiden name), puzzles Something you have: smart cards, security tokens Something you are: biometrics, signature dynamics, keyboard dynamics, voice print

10 What is a good password? Long passwords are harder to break but tedious to type. Random passwords are hard to break but next to impossible to memorize. Writing down a password on a sticky note doesn t help. Password expiration reduces the damage but increases the risk of forgetting. Automatic password generation easily memorizable words passphrases: sentences hashed into passwords Passwords are only usable for services that are often used.

11 Password generation examples

12 Password generation examples

13 Smart cards Physical tamper-proof credentials Memory cards: simple storage of information (e.g. medical insurance) Microprocessor cards: advanced functionality (e.g. transactions) cryptographic operations (e.g. key validation and key-pair generation) Java cards: a mini-jvm on a chip (e.g. for code update). Main problem: theft and loss risk

14 Biometric identification Unforgeable features of a person: fingerprints hand geometry hand topography iris scan retina scan...or behavioral characteristics: signature dynamics keyboard dynamics voice print

15 Disadvantages of biometric authentication Non-zero probability of erroneous classification High cost and time consumption Impossibility to revoke Privacy issues and social acceptance Can be potentially falcified

16 Two-factor authentication A combination of any two authentication modes Example: SecurID PIN assigned to user token automatically generated in hardware every 30 seconds Clock synchronization between a token generator and an authentication server required

17 Single sign-on Motivation: Reduce the need to memorize passwords Reduce the time spent on typing passwords Reducing password maintenance effort Solutions: Local password containers (e.g. KDE Wallet) Physical authentication devices Client-server architectures (Kerberos, Active Directory)

18 Kerberos: a brief history Development at MIT in the project Athena in the 1980s, versions 1 3 for internal use, version 4 released in Version 5 released in 1993 fixed some security flaws of the previous version; version 4 withdrawn in Banned for export by US government until 2000 (due to the use of DES); re-implemented in Sweden at KTH. Currently, both implementations MIT (Athena) and KTH (Heimdal) support AES as the main encryption instrument. Supported by all major OS (Windows 2000 up, Mac OS X, Solaris, Red Hat Enterprize)

19 Kerberos authentication protocol Key Distribution Center (KDC) Client AS_REQ AS_REP Authentication Server (AS) Database TGS_REQ Ticket Granting Server (TGS) TGS_REP AP_REQ Application Server AP_REP

20 Kerberos details: user login User enters his user name U and password P at a client workstation. The hash value of his password K u = h(p) becomes a secret key of the client/user.

21 Kerberos details: user authentication The client sends his user ID U and the requested service S to the authentication server AS: C AS : (U, S) If the user ID is found in a database, AS generates a TGS session key k TGS and a ticket-generating ticket TGT: TGT : (U, S, k TGS ) AS sends TGT encrypted to C: C AS : {S, k TGS } Ku {TGT} KTGS

22 Kerberos details: ticket request The client generates an authenticator: A : {U} ktgs The client sends a ticket granting request to a ticket granting server: C TGS : (S, A){TGT} KTGS TGS verifies that U in A matches U in TGT, generates a service session key k S and puts it into a service ticket: T S : (U, S, k S ) TGS sends T S encrypted with the service key K S to C: C TGS : {S, k S } ktgs {T S } KS

23 Kerberos details: service authentication The client generates an authenticator: A : {U} ks The client sends a request to a service: C S : (A){T S } KS The service checks if U in A matches U in T S and if so accepts the request.

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

25 Access control overview Given a subject, which objects can it access and how? Given an object, which subjects can access it and how? Subject Reference monitor Object User process? Resource Access request Policy

26 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.

27 Access control structures Access control structures are mechanisms for implementing access policies: access control matrix capabilities access control lists intermediate controls (groups, negative permissions, roles, protection rings etc.) Requirements for access control structures: an ability to express control policies verifiability of correctness. scalability and manageability

28 Access control matrix Access control matrix is a basic control structure. bill.doc edit.exe fun.com Alice {execute} {execute,read} Bob {read} {execute} {execute,read,write} Advantages: clarity of definition easy to verify Disadvantages: poor scalability poor handling of changes

29 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 transfer easy inheritance of access rights Disadvantages: poor overview of access rights per object difficulty of revocation need for extra integrity protection

30 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 object access rights relative easiness of management using abstractions Disadvantages: poor overview of access rights per subject difficulty of revocation difficulty of sharing

31 Access control abstractions Group: an collection of related subjects easy sharing easy addition and removal of users Negative permission: explicit revocation of access rights Privilege: 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 levels 0 operating system kernel 1 operating system 2 services 3 user processes

32 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 over information flow strong exploit containment Disadvantages: major usability problems cumbersome administration

33 Discretionary access control (DAC) Access control is carried out by a resource owner. By associating ownership with principals, access rights are easily transferred to other subjects. Deployed in a majority of common systems. Advantages: simple and efficient access rights management scalability Disadvantages: intentional abuse of access rights unintentional abuse of access rights no control over information flow

34 Role based access control (RBAC) RBAC attempt to handle the complexity of access control by extensive used of abstractions: Data types are defined for all objects. Procedures are high level access control methods with a more complex semantics than elementary access control rights. Procedures can be only applied to certain data types. Procedures are grouped into roles assigned to users. A user can have more than one role and more than one user can have the same role. Role hierarchies can be used to match natural relations between roles. Example: A Lecturer can create a role Student and give it a privilege read course material.

35 Summary Passwords and shared secrets are the most common tools for user authentication. limited security (cracking) and usability (forgetting, management) alternatives: biometrics and smart cards Single sign-on methods such as Kerberos provide means for automating authentication in large distributed environments. single point of failure Access control is based on the notion of the access matrix and its approximations (ACS, capabilities) and abstraction (groups, roles).