applying trusted computing to a workflow system

Po-Wah Yau, Allan Tomlinson, Shane Balfe and Eimear Gallery

Information Security GroupRoyal Holloway, University of London

www.isg.rhul.ac.uk

Applying Trusted Computing to a Workflow System

Third e-Science Workshop

Trusted Services: Requirements and Prospects, 8-9 July 2008, Edinburgh

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Contents

• Introduction

• Grid workflow security

• Overview of Trusted Computing

• Securing workflows

• Summary

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Introduction

• Grid middleware used to achieve synergy from otherwise disparate resources:

– Hardware (CPU, storage, computationally steerable equipment),

– Applications,– Data, and– People.

• Security issues when running a Grid job at a resource provider:– See Andy’s talk!

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Introduction

• Grid workflows used to achieve automated synergy from multiple tasks:

– Logical ordering of tasks,

– Each task can either process results of another task or new set of data,

– Sequential, parallel, choice branching and loops.

• Variety of workflow systems:

– Low level, physical workflows, as opposed to

– High level (e.g. Pegasus, P-Grade, Taverna).

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Introduction

• Workflow Resource Broker (WRB):

– Typically maps abstract workflow of tasks to physical workflow of jobs (in a high level system),

– Selects resource providers to run jobs (according to static requirements), and

– Schedules jobs (taking into account dynamic requirements).

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Contents

• Introduction



• Securing Grid workflows

• Summary

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Grid Workflow Security

• Confidentiality, to protect:

– An individual job,

– A workflow of jobs,

– The workflow/sub-workflow, and

– The locations of where jobs are submitted.

• Integrity, to prevent:

– Error propagation,

– Wasted resources, and

– Loss of reputation.

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Grid Workflow Security

• WRB vulnerabilities:

– Delegated control of user credentials

– Resource provider selection

– Scheduling and location of workflow jobs

• Resource provider vulnerabilities:

– Complex Grid middleware

– Local user access

– Network vulnerabilities

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Contents

• Introduction



• Securing Grid workflows

• Summary

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Overview of Trusted Computing

• Defined by the Trusted Computing Group:– www.trustedcomputinggroup.org

• A ‘Trusted Platform’ consists of:

– Trusted Platform Module (TPM) embedded into the host platform,

– Protected capabilities, commands, that can access shielded locations (memory, registers), and

– Creating proxy ‘roots of trust’ in hardware.

http://www.trustedcomputinggroup.org/

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• Three types of key:

– Non-migratable keys never leave protection of the TPM in which they are created, and are certified by the TPM.

– Migratable keys can be released by a TPM, encrypted using the public key of the destination, but are not certified.

– Certifiable migratable keys are keys that are migrated under specific conditions, possibly under the control of a Migration Selection Authority (MSA).

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• Each TPM is shipped with a non-migratable Endorsement Key.

• A non-migratable Storage root key (SRK) is created when a TPM is initialised/reset:

– The SRK is used to encrypt other keys, which can then be stored outside of the TPM,

– If a non-migratable key is used to encrypt data, then that data is ‘bound’ to the TPM, and

– If use of that non-migratable key is only possible when the platform is in a specific state, then that data is ‘sealed’ to that platform state.

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• Integrity measurement:

– The ability to record events that modify platform state, which are

– Stored in Platform Configuration Registers (PCRs) via an ‘extend’ operation.

• Sealed storage:

– Binding data objects, including cryptographic keys, to a specific platform state.

• Attestation:

– The ability to prove platform state to an external entity, where

– The PCR values are signed using an Attestation Identity Key (AIK).


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Contents

• Introduction



• Securing Grid workflows:

– Assumptions

– Workflow preparation

– Workflow execution

• Summary

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Securing Grid Workflows

• The following proposal uses Trusted Computing to provide:

– Trusted resource provider selection

– Confidentiality of job information

– Integrity of job information

• Secondary properties:

– Confidentiality and integrity of workflow

– Information to possibly assist process provenance

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Assumptions

• Trusted Computing prevalence:

– WRB platform

– Subset of resource providers

• Means of verifying that WRB can be trusted

• User has a means of specifying high level security requirements:

– Translated by WRB into low-level platform state requirements

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Assumptions

• All resource providers have a certified copy of the WRB’s public signature verification key.

• The WRB has a copy of all resource providers’ public signature verification key.

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Workflow preparation (1)

• Consider a workflow of jobs a0, a1, … , an

• Each job ai is associated with a symmetric key Ki, which will be used to ‘protect’ the job.

• A private key SKi is also associated with each job ai:

– This will be stored in a TPM.

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• The resource provider RPi can obtain SKi using one of two methods:

– The WRB creates a certifiable migratable key pair

• Specifying the state i to which the private key is sealed

• The key is then migrated to TPM of RPi

– RPi creates a non-migratable key pair sealed to a specific platform state i:

• The public key and platform state are advertised as part of an attestation token [Lohr et al. 06]

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WRB RPi :

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WRB RPi : IDW

Identifiers of WRB and workflow

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WRB RPi : IDW || ri

Random nonce

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WRB RPi : IDW || ri || gKi (ai || r i)

Output of g is the ciphertext and message authentication code

for the job and nonce

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|| ePKi (Ki)

Ki encrypted using PKi corresponding to

SKi which is sealed to platform state i

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|| ePKi (Ki) || IDi+1 || PKi+1

Identifier of resource provider RPi+1 to send job results to, and

Public encryption key of RPi+1 corresponding to Ski+1

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|| IDRPi-1 ||VKRPi-1 || i-1

Identifier of preceding resource provider, Public verification key of RPi-1 (non-TPM key),

The platform state of RPi-1 required by WRB

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|| IDRPi-1 ||VKRPi-1 || i-1 ||W

The WRB’s digital signature on the whole message

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• In summary:

1. Each job is ‘protected’ using a symmetric key,

2. This key is sealed to the required platform state,

3. The platform states to which the keys are sealed are decided/known before workflow execution, and

4. Each resource provider knows the state that the previous resource provider should have been in, in order to execute their designated job.

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Workflow execution



|| IDRPi-1 ||VKRPi-1 || i-1 ||W

RPi-1 RPi : IDW || “ready” || RPi-1

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Workflow execution



|| IDRPi-1 ||VKRPi-1 || i-1 ||W


RPi : Verify W and RPi-1

RPi : Retrieve Ki using SKi (sealed to TPM)

RPi : Decrypt and retrieve ai , and verify integrity

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Workflow execution



|| IDRPi-1 ||VKRPi-1 || i-1 ||W


RPi-1 RPi :

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Workflow execution



|| IDRPi-1 ||VKRPi-1 || i-1 ||W


RPi-1 RPi : IDW || C(rRPi)

RPi : Generate random nonce

RPi : Send attestation challenge

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Workflow execution



|| IDRPi-1 ||VKRPi-1 || i-1 ||W



RPi-1 RPi :

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Workflow execution



|| IDRPi-1 ||VKRPi-1 || i-1 ||W



RPi-1 RPi : IDW || F(i-1, rRPi)

RPi-1 : Generates response to attestation challenge

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Workflow execution



|| IDRPi-1 ||VKRPi-1 || i-1 ||W




RPi-1 : Generates symmetric key Ki’ and…

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Workflow execution



|| IDRPi-1 ||VKRPi-1 || i-1 ||W




|| gKi’ (R(ai-1, rRPi) || rRPi)

RPi-1 : Protects job results using Ki’

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Workflow execution



|| IDRPi-1 ||VKRPi-1 || i-1 ||W





|| ePKi (Ki’)

RPi-1 : Encrypts Ki’ using public key PKi

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Workflow execution



|| IDRPi-1 ||VKRPi-1 || i-1 ||W





|| ePKi (Ki’)

RPi : Verifies F(i-1, rRPi)

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Workflow execution



|| IDRPi-1 ||VKRPi-1 || i-1 ||W





|| ePKi (Ki’)

RPi : Retrieves Ki’

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Workflow execution



|| IDRPi-1 ||VKRPi-1 || i-1 ||W





|| ePKi (Ki’)

RPi : Recovers ai-1, and processes ai

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Workflow execution

• In summary:

1. Job are ‘protected’ using a symmetric key,

2. This key is sealed to the required platform state of the next resource provider in the workflow,

3. A resource provider should challenge the previous one to attest to its platform state.

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Properties of the scheme

• Security is provided in both directions of a workflow:– Forward – trusted resource provider selection,– Backward – detection of compromised jobs.

• Efficient symmetric key cryptography to protect job data:– Symmetric key bound to trusted platform state,

via sealed private key.

• Each platform stores a “secure measurement log”:– Potentially useful (verifiable) information for

process provenance.

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Summary

• Securing Grid workflows is paramount because a user’s entire dataset is being exposed.

• Trusted computing can be used to improve trust establishment in Grids.

• Trust in the Workflow Resource Broker is critical.

• Proposed a scheme to ensure trusted workflow execution.

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Acknowledgements

• The first and second authors are being funded by the Engineering and Physical Sciences Research Council (EPSRC) UK e-Science programme of research (EP/D053269).

• The third author is sponsored by the U.S. Army Research Laboratory and the UK Ministry of Defence (Agreement no. W911NF-06-3-0001)

• The fourth author is sponsored by the Open Trusted Computing project of the European Commission Framework 6 Programme.

• Thanks to Professor Chris J. Mitchell.

• For more details of this project please refer to www.distributedtrust.org.

http://www.distributedtrust.org/

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Thank you for listening

• Any questions?

applying trusted computing to a workflow system

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