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Kari Kostiainen, N. Asokan Nokia Research Center

Alexandra Afanasyeva SUAI

ACNS 2011

Towards User-Friendly Credential Transfer on Open Credential Platforms

© 2011 Nokia Research Center

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Trusted execution environment (TrEE)

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Credential transfer

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Outline

1. Credential transfer problem

2. Credential transfer protocol

3. Analysis

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Credential transfer problem

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Closed provisioning

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Control point

Service provider Service provider Service provider

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Open provisioning

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User authentication

User authentication

User authentication

Copyable Non-transferable Non-transferable

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Late user binding

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Device certificate

Enc(credentials)

User binding User binding

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Temporal disconnection

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Requirements

• Usability −No additional user interaction

• Security −Credential secrecy

−“Credential fidelity”

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Credential transfer protocol

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Protocol overview

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HSM Provisioner (P)

Source device (S)

OS

TrEE

Target device (T)

OS

TrEE

PKTS/SKTS, CertTS

PKT/SKT, CertT

PKS/SKS, CertS

Trusted Server (TS)

OS

1. User identity installation

2. Provisioning and user binding

3. Credential backup and “identity verification delegation”

4. User identity installation

5. Identity verification, credential recovery and “provisioning delegation”

6. Automatic re-provisioning

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1. User identity installation

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PwdS

SealedPwdS

SealedPwdS Seal(PwdS)

PwdS

Store SealedPwdS

User Source OS Source TrEE

1. Trust on first use 2. Typical device login

ProvCred, SealedPwdS

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2. Credential provisioning

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ProvCred

CertS + user auth.

SealedCred

Verify CertS and extract PKS

Map Cred to PKS

ProvCred Enc(PKS, Cred || policy)

Cred || policy Dec(SK, ProvCred) PwdS Unseal(SealedPwdS) SealedCred Seal(Cred || policy || PwdS)

Store SealedCred

Provisioner Source OS Source TrEE

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3. Credential backup to server

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Source OS Source TrEE Trusted Server (TS)

CertS, EncCred / SDT

EncCred / SDT

CertTS, SealedCred

Verify CertTS and extract PKTS Cred || policy || PwdS Unseal(SealedCred)

Copyable: EncCred Enc(PKTS, Cred || PwdS) Non-transferable: SDT Sign(SKS, PKTS) || Enc(PKTS, PwdS)

CertTS

Store CertS and EncCred / SDT

CertS, EncCred / DT EncCred

SealedPwdT

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5. Credential recovery

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Trusted Server (TS) Target TrEE Target OS

PwdToken PwdToken, CertT

Verify CertT and extract PKT

PwdT Dec(SKTS, PwdToken) Copyable: Cred || PwdS Dec(SKTS, EncCred) Verify PwdT = PwdS

EncCred End(PKT, Cred || PwdT) Non-transferable: Extract PwdS from SDT Verify PwdT = PwdS

DT Sign(SKTS, PKT) || Sign(SKS, PKTS)

PwdT Unseal(SealedPwdT) PwdToken Enc(PKTS, PwdT)

Install EncCred

User identity installation

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6. Credential re-provisioning

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Target TrEE Target OS Provisioner

CertT, CertS, CertTS, DT

Verify Certs Verify signatures on DT

Find Cred for PKS

ProvCred Enc(PKT, Cred || policy)

ProvCred

ProvCred

Install ProvCred

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Analysis

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Analysis

• Usability −Reusing typical device login

• Credential secrecy −Common public key mechanism

−Trusted server (HSM)

• Credential fidelity −User identity password

−Brute force (throttling)

−Phishing (separate password)

−Password change (“trusted UI” or secure connection to server)

• Protocol validated with AVISPA tool 20 © 2011 Nokia Research Center

Summary

• Credential transfer challenging −Open provisioning

−Late user identity binding

−Temporal disconnection

• Server-assisted credential transfer protocol −Can be implemented using existing devices

21 © 2011 Nokia Research Center