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PEI Models towards Scalable, Usable and High-assurance Information Sharing

Ram KrishnanLaboratory for Information

Security TechnologyGeorge Mason University

Kumar RanganathanIntel Systems

Research CenterBangalore, India

Ravi SandhuInstitute for Cyber-Security

ResearchUniv of Texas at San Antonio

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Presentation Outline

• Problem Description & Motivation• Background

– Trusted Computing– Information Sharing

• PEI Models for SIS• Future Work• Q&A

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Problem Description & Motivation• Secure Information Sharing (SIS)

– Share but protect• Distribute information and still retain control

– Long-standing and unsolved problem• Current approaches to SIS:

– Discretionary Access Control (DAC), Lampson 1971• Fundamentally limited• Controls access to the original but not to copies (or extracts)

– Mandatory Access Control (MAC), Bell-LaPadula 1971• Solves the problem for coarse-grained sharing• Does not scale to fine-grained sharing

– Explosion of security labels– Originator Control (ORCON), Graubart 1989

• Let copying happen but propagate ACLs to copies (or extracts)

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…continued

• SIS problem further amplified by the client-server model– No control once information leaves server– Current approaches are software-based

• Inherently weak

• Fundamental question:– How can I trust that policies will be enforced on clients in a trust-

worthy manner?• Trusted Computing (TC) Technology features root of

trust at hardware level– Potential to provide strong controls on client– Potential to solve SIS problem

• We need a family of models to guide TC-based solutions

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Background

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Trusted Computing

• An industry standard/alliance– Proposed by Trusted Computing Group

• Basic premise– Software alone cannot provide an adequate

foundation for trust• TCG proposes root of trust at the

hardware level using a Trusted Platform Module or TPM

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TPM: 3 novel features

• Trusted storage for keys– Encrypt user keys with a chain of keys– Root key is stored in TPM & never exposed

• Trusted Capabilities– Operations exposed by the TPM– Guaranteed to be trust-worthy

• Platform Configuration Registers (PCR)– Hardware registers used to store integrity of

software (e.g. boot-chain)

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TPM: An example functionality

• Seal– Trusted capability– Encrypts and binds data to a PCR value– Data can be unsealed iff PCR value at unseal

time matches with PCR value in sealed blob• Seal can be used to restrict data access

– E.g. A program can access sensitive information only if the platform is in trustworthy state

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What is Information Sharing?

• Share but protect– Distribute information and still retain control

• Requires strong controls on client– Server controls “information release” only

• Purpose is larger than retail DRM... →

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What is Information Sharing?Strength of Enforcement

Content type and value Weak Medium Strong

Sensitive and proprietary Password-protected documents Software-based client controls for documents

Hardware based trusted viewers, displays and inputs

Revenue driven IEEE, ACM digital libraries protected by server access controls

DRM-enabled media players such as for digital music and eBooks

Dongle-based copy protection, hardware based trusted viewers, displays and inputs

Sensitive and revenue Analyst and business reports protected by server access controls

Software-based client controls for documents

Hardware based trusted viewers, displays and inputs

Roshan Thomas and Ravi Sandhu, “Towards a Multi-Dimensional Characterization of Dissemination Control.” POLICY04.

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Functionality Strength of enforcement

Simple Complex Weak/Medium Strong

Legally enforceable versus system enforced rights.

Reliance on legal enforcement;Limited system enforced controls.

Strong system- enforceable rights, revocable rights.

Dissemination chains and flexibility.

Limited to one-step disseminations.

Flexible, multi-step, and multi-point. Mostly legal enforcement; System enforceable controls.

Object types supported.

Simple, read-only and single-version objects.

Support for complex, multi-version objects.Support for object sensitivity/confidentiality.

Reliance on legally enforceable rights.

System supported and enforceable rights and sanitization on multiple versions.

Persistence and modifiability of rights and licenses.

Immutable, persistent and viral on all disseminated copies.

Not viral and modifiable by recipient. Reliance on legally enforceable rights.

System enforceable.

Online versus offline access and persistent client-side copies

No offline access and no client-side copies.

Allows offline access to client-side copies.

Few unprotected copies are tolerated.

No unprotected copies are tolerated.

Usage controls Control of basic dissemination.

Flexible, rule-based usage controls on instances.

Some usage abuse allowed.

No potential for usage abuse.

Preservation of attribution.

Recipient has legal obligation to give attribution to disseminator.

System-enabled preservation and trace- back of the attribution chain back to original disseminator.

Attribution can only be legally enforced.

Attribution is system enforced.

Revocation Simple explicit revocations. Complex policy-based revocation. No timeliness guarantees. Guaranteed to take immediate effect.

Support for derived and value-added objects.

Not supported. Supported. Reliance on legally enforceable rights.

System enforceable rights for derived and valued-added objects.

Integrity protection for disseminated objects.

Out of band or non-crypto based validation.

Cryptographic schemes for integrity validation.

Off-line validation. High-assurance cryptographic validation.

Audit Audit support for basic dissemination operations.

Additional support for the audit of instance usage.

Offline audit analysis. Real-time audit analysis and alerts.

Payment Simple payment schemes (if any).

Multiple pricing models and payment schemes including resale.

Tolerance of some revenue loss.

No revenue loss; Objective is to maximize revenue.

With current state of knowledgethe information sharing spaceis too complex to characterizein a comprehensive manner

Look for areas thatare of practical interest

and where progress can be made

Roshan Thomas and Ravi Sandhu, “Towards a Multi-Dimensional Characterization of Dissemination Control.” POLICY04.

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PEI Models for SDS

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SDS Objectives• Super-distribution (encrypt once,

access wherever authorized)– Requires a notion of a “group”

• A subject is authorized to access a document if he is a group member

• Problem scope: group-based SDS

• Offline access• Document-level access control• Read-only document access

Security and system goals(requirements/objectives)

Target platform, e.g., TrustedComputing Technology

Enforcement models

Policy models

Implementation models

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SDS Policy Model

• Characterize policies applicable to a group-based SDS problem domain

Security and system goals(requirements/objectives)

Target platform, e.g., TrustedComputing Technology

Enforcement models

Policy models

Implementation models

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Various states of a group member

Initial state:Never been a

member

State I

Currently a member

State II

Past member

State III

enroll dis-enroll

enroll

1. Straight-forward. User has no access to any group documents.

1. Access to current documents only (or)2. Access to current documents and past

documents3. Access can be further restricted with rate

and/or usage limits4. Access can be further restricted on basis

of individual user credentials

1. Past member loses access to all documents (or)2. can access any document created during his membership (or)3. can access documents he accessed during membership (or)4. can access all documents created before he left the group (this

includes the ones created before his join time)5. all subject to possible additional rate, usage and user

credential restrictions

1. No rejoin of past members is allowed, rejoin with new ID (or)

2. Past members rejoin the group just like any other user who has never been a member

3. The same access policies defined during his prior membership should again be enforced (or)

4. access policies could vary between membership cycles

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Various states of a group document

Initial state:Never been a

group doc

State I

Currently a group doc

State II

Past group doc

State III

add remove

add

1. Straight-forward. No access to group members.

1. Access allowed only to current group members

2. Access allowed to current and past group members

1. No one can access2. Any one can access3. Past members can

access

1. Cannot be re-added.2. When a document is re-added, it

will be treated as a new document that is added into the group.

3. Only current members can access.4. Past members and current

members can access

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Policy model: member enroll/dis-enroll

Initial state:Never been a

member

State I

Currently a member

State II

Past member

State III

enroll dis-enroll

memberTS-joinTS-leave

nullnullnull

Truetime of joinnullenroll

Falsetime of jointime of leavedis-enroll

enroll

enroll, dis-enroll: authorized to Group-Admins

UCON elements:Pre-Authorization, attribute predicates, attribute mutability

enroll

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Policy model: document add/remove

Initial state:Never been a

group doc

State I

Currently a group doc

State II

Past group doc

State III

add remove

D-memberD-TS-joinD-TS-leave

nullnullnull

Truetime of joinnulladd

Falsetime of jointime of leaveremove

add, remove : authorized to Group-Admins

add

UCON elements:Pre-Authorization, attribute predicates, attribute mutability

add

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Enforcement Models

• Develop enforcement models for SDS

Security and system goals(requirements/objectives)

Target platform, e.g., TrustedComputing Technology

Enforcement models

Policy models

Implementation models

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SDS Enforcement Model

3

1

2 4 5

Group-Admin MemberJoining Member

Control Center (CC)

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Faithful Model: steps 3 and 4 are coupledApproximate Model: steps 3 and 4 are de-coupled

D-Member

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• Member enroll and dis-enroll (steps 1-2, 5)• Document add and remove (step 6, 7)• Read policy enforcement (step 3)• Attribute update (step 4)

Two sets of attributes• Authoritative: as known to the CC• Local: as known on a member’s computer

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Implementation Models

• Develop implementation models for SDS

Security and system goals(requirements/objectives)

Target platform, e.g., TrustedComputing Technology

Enforcement models

Policy models

Implementation models

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Implementation Model• Build a Trusted Reference Monitor (TRM)

– The reference monitor on the group members that enforces group policies in a trust-worthy manner

• Identify the Trusted Computing Base (TCB)– A minimal collection of entities that is absolutely essential to be

in integral state in order to preserve security of the entire system• Objective

– Build a TCB in order to enforce document read on group members using a TRM

• Two Key requirements– Only TRM can access the group key and read/write subject and

object attributes– TRM is provided with isolated environment to safely use the

group/document keys

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Implementation Model (continued)

TPM

VMM

Update Internal PCR

Linux Kernel + TPM Driver + MAC Policies

Internal PCRs

AppPCRs

TRM TVTSS

Indirect communication

Boot time measurement

Isolated executionVM0

VM1

• Use TC mechanisms to bind group key + attributes to TRM

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Future Work• Formal analysis of SIS

policy models– SDS: object level– SCS: attribute level

• Document/object Read and Write

• SIS across multiple groups– Information flow issues

• Document/object-level querying– Obtain sections of

document/object

Coffee Shop Book Store

Credit Credit

Alice

SCS in M-commerce Scenario

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Q&A

Thanks!

InformationSharing

Policy, Enforcement, Implementation (PEI) layers

Trusted Computing (TC)Usage CONtrol model (UCON)

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Backup

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Policy model: document read (S,O,read)

• Pre-authorization check– member(S) ≠ null AND D-member(O) ≠ null AND

TS-join(S) ≠ null AND D-TS-join(O) ≠ null AND• TS-leave(S) = null AND TS-join(S) ≤ D-TS-join(O) OR• TS-leave(S) ≠ null AND

TS-join(S) ≤ D-TS-join(O) ≤ TS-leave(O)

• Ongoing-authorization check: terminate if– D-TS-leave(O) ≠ null Details depend on details

of group-level policy

UCON elements:Pre-Authorization, attribute predicates, attribute mutabilityOngoing-authorization

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Enforcement Models• Design Principle

– Do not inject new policy– Focus on trade-offs for instant and pre-emptive revocation versus off-

line access• Faithful Enforcement w/o Off-line Access (Faithful Model):

– We need continuous online touch (at start of every access and during access)

– Continuous on-line touch can only be approximated• Usage-limited Off-line Access (Approximate Model):

– We need online touch periodically after some duration (at start of every access and during access)

• Duration between online touches can be based on time, but time is not practical for TPM-based TC

• Duration between online touches can be based on usage count, which is practical for TPM-based TC

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Faithful model highlights• enroll a member: two steps

– Step 1: Group-admin issues enrollment token to Joining Member– Step 2: Joining Member presents token to CC and receives group

membership credential• Group key (symmetric key)• Local attribute values

• dis-enroll a member– Updates authoritative attributes at CC– Takes effect on local attributes at next update

• add a document– Updates authoritative attributes at CC

• remove a document– Updates authoritative attributes at CC– Propagated to clients as DRLs (Document Revocation List)

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Faithful model highlights: (S,O,read)

• Pre-Obligation– Local attributes of S and O are updated based on authoritative values from CC– Local DRL updated from authoritative DRL at CC

• Pre-Condition– Requires connectivity to enable updates

• Pre-Authorization– Based on just updated local attributes of S and O and DRL

• Ongoing-Obligation– Local attributes of S and O continuously updated based on authoritative values

from CC– Local DRL continuously updated from authoritative DRL at CC

• Ongoing-Condition– Requires connectivity to enable updates

• Ongoing-Authorization– Based on continuously updated local attributes of S and O and DRL

UCON elements:Requires full power of UCON

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Approximate model highlights• enroll a member: two steps

– Step 1: Group-admin issues enrollment token to Joining Member– Step 2: Joining Member presents token to CC and receives group

membership credential• Group key (symmetric key)• Local attribute values

• dis-enroll a member– Updates authoritative attributes at CC– Takes effect on local attributes at next update

• add a document– Updates authoritative attributes at CC

• remove a document– Updates authoritative attributes at CC– Propagated to clients as DRLs (Document Revocation List)

Different from Faithful model

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Approximate model highlights: (S,O,read)

• Pre-Obligation– Local attributes of S and O are periodically updated based on

authoritative values from CC• Pre-Condition

– Requires connectivity to enable updates when required• Pre-Authorization

– Based on just updated local attributes of S and O• Ongoing-Obligation

– Local attributes of S and O are continuously periodically updated based on authoritative values from CC

• Ongoing-Condition– Requires connectivity to enable updates when required

• Ongoing-Authorization– Based on continuously periodically updated local attributes of S and O

UCON elements:Requires full power of UCON

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Contributions & Conclusions• SIS is a broad and complex problem

– PEI framework, UCON model and TC are highly useful for approaching the SIS problem space

• First steps towards developing a family of models for SIS problem domain– Classify SIS into two distinct levels:

• Object-level SIS• Attribute-level SIS

– Analyze object-level and attribute-level SIS• Demonstrate how TC can be used to address this long-

standing problem• Possibly extend the UCON model• Leave with more open questions!