Understanding Role-Based Access Control (RBAC) and Trusted Computing Overview

1. CMSC 414Computer and Network SecurityLecture 12 Jonathan Katz

2. Role-based access control

3. RBAC • Access controls assigned based on roles • Can use an access matrix, where “subjects” are roles • Users assigned to different roles • Can be static or dynamic • A user can have multiple roles assigned • Can use “access matrix” with users as rows, and roles as columns • Will, in general, be more compact than a full-blown access control matrix

4. RBAC: basic idea Users Roles Resources research Server 1 Server 2 marketing Server 3 admin

5. Questions… • Where might RBAC make more sense than DAC? • Where might DAC make more sense than RBAC? • Advantages of RBAC • Users change more frequently than roles

6. RBAC reference models • RBAC0 – basic model • RBAC1 – Adds role hierarchies to RBAC0 • RBAC2 – Adds constraints to RBAC0 • RBAC3 – incorporates RBAC1 and RBAC2

7. RBAC0 • Users, roles, and permissions • A user may have multiple roles, a role may be granted multiple permissions • Session • Maps a user to a set of roles to which the user is currently assigned • Principle of least privilege • Role-to-permission mapping can be fine-grained • E.g., list/modify but not create; append but not modify

8. RBAC1 • Define a partial order (not total order) on the roles • Inheritance • If role r’ is subordinate to role r, then r has all the permissions that r’ does (and possibly more)

9. RBAC2 • Adds constraints to user-to-role mapping • Mutually exclusive roles • A set of roles R such that a user can be assigned to at most one role in this set, either overall (static) or per session (dynamic) • Enforces separation of duties • Cardinality • Max. users assigned to a role (e.g., superuser); max roles per user (or per session); or max roles having a certain permission

10. RBAC2 (continued) • Prerequisite • Mandates that a user can be assigned some role only if already assigned some other role • Can be used to enforce least privilege – a user can specify a weaker role when certain permissions are not needed

11. Comparing DAC/MAC/RBAC • What would an access control matrix look like in each case?

12. Trusted Computing

13. Overview • Secure hardware (“TPM”) installed in computer • Goals • Secure boot • Software verification • Attestation • Encrypted storage • This is already deployed

14. Disclaimer • The intent of the following is to give the high-level ideas, rather than completely correct low-level details • Full specification available on-line • TCG consortium

15. TPM chip Non Volatile Storage(> 1280 bytes) PCR Registers (16 registers) I/O Crypto Tools: RSA, SHA-1, …

16. Non-volatile storage • Endorsement keys (EK) [RSA] • Created at manufacturing time, bound to computer • Signing keys; used for attestation • Storage root key (SRK) [RSA] • Created by user; can be changed • Used to encrypt data

17. PCR • “Platform Configuration Registers” • 20 bytes; hold SHA-1 output • Can only be modified in two ways (enforced by the hardware): • TPM_Startup (initialize the contents of the PCR) • TPM_Extend(D): PCR = SHA-1 ( PCR || D ) • Used to obtain an “image” of the loaded software…

18. PCM usage Hardware BIOS boot block OS loader BIOS Application OS measuring TPM Extend PCR • Collision resistance of SHA1 ensures “uniqueness”

19. What next? • Compare computed value with reference value • Secure boot • Software validation • Verify signature • All this verifies is the source • Decrypt data • Decrypt only if in known configuration • Attestation • Prove to a third party that you are in a good configuration

20. Encrypted data • Encrypt AES key K with SRK; encrypt bulk data with K • Hybrid encryption! • When encrypting the AES key, embed current PCR value • E.g., SignEK(PCR, EncSRK(K)) • (This is not actually the way it is done) • When decrypting, check that the embedded value matches the current value • Refuse to decrypt if not the case! • Can also incorporate a user password, etc.

21. Attestation • Goal: prove to a remote party what software is running on my machine • Applications: • Prove to company network that no viruses are running on my machine • Prove to another player that I am running an unmodified version of Quake • Prove to Apple that I am running iTunes…

22. Basic idea • Sign PCR value with EK • Actually, sign with attestation identity key (AIK) validated with EK (ignore this for now) • Assume third party knows EK • There is actually a PKI • To prevent replay, use nonce provided by the third party • Third party verifies signature; verifies that PCR corresponds to “good” state

23. Controversy • Loss of anonymity • Signature using EK uniquely identified the machine it came from • Third parties can tell what software you are running • Loss of control • What if google says you need to have google desktop installed in order to use their search engine? • What if Sony says you must use their music player to download their music?