Analysis of Remote Attestation
Lavina Jain, Jayesh Vyas
• Remote Attestation Protocol proposed by IBM
• Remote Attestation Protocol that we propose
• Protocol model on Murphi
• Analysis and Attacks
• Conclusion
Presentation Outline
Hardware
CRTM
BIOS
OS
Application
Reset PCR
Extend PCR with BIOS image
Extend PCR with OS image
Pre
-
boot
The Big Picture (Recap)
Integrity of kernel,applications,libraries, files
Dynamic/Run-timemeasurements
Pos
t -
boot
TPM
PCR
Hardware
Keys
Measurementlist (ML)
Kernel
Client/Attestator
Server/Challenger
RemoteAttestation
TrustedThird Party
AIK credential request
AIK Credential
AuthValidate
Remote Attestation Protocol (as proposed by IBM)
Client/Attestator
Ver (SigAIK
(PCR, N),
AIKpub) = true/false
TPM
PCR
Keys
Attestationservice Challenger
1. 160-bit Nonce, N2
. Quo
te R
eq
uest
3. Q
uote
Re
spo
nse
4. Integrity Response
5. Integrity Validation
{ SigAIK
(PCR, N), ML,
AIKcert
}
N
Sig
AIK
(PC
R, N
)
Assumption: A secure session is set up between the client and the server.
Authentication on top of SSL is not secure,
prone to MITM attack!!
Integrity Measurement Architecture (IMA)
Kernel Hooks(Measurement
Agents)
Kernel
Hardware
CRTM
BIOS
PCR_Extend(BIOS)
Measurementlist
Insmod Loader /bin/sh, /bin/perl
PCR
TPM
Reset PCR
Boot Up
PCR_Extend(OS)
PCR_Extend
Update
Remote Attestation Protocol (as we propose)
C
AIKpub/AIKpvt
S
PKs/
SKs
C, Nc
Ns, SIGCA(S, PKs)
EPKs(Secret, ML), SIGCA(C, AIKpub)
SIGAIKpvt(PCR, hash(Nc, Ns, Secret))
SIGSKs (Nc, AIKpub)
SIGAIKpvt (Ns, PKs)
TPM Quote Response
AIKcert obtained from TTP Three-step Protocol
Four-step
Five-step
Rational Construction using Murphi, with inputs from Prof. John and Prof. Dan.
• Entities modeled in Murphi:
– Client – Attestator, who wants to attest it’s platform to remote server.
– Server – Challenger, wants to determine it’s trust in client.
– Intruder – Malicious entity on network.
– Attacker – Malicious entity on client.
– Super Attacker – Capable of hardware attacks on TPM (Eg. PCR Reset).
• Assumptions:
– CRTM(BIOS boot block) that initiates chain of measurements is trusted.
– TPM is trusted.
– CPU is trusted.
– Memory cannot be hacked at runtime (e.g. via DMA).
• Flags that can be set or reset to study the behavior of protocol:
– FOUR_STEP: Run four-step protocol.
– FIVE_STEP: Run five step protocol.
– CERT_FORGED: Intruder can forge a certificate issued by CA.
– SUPER_ATTACKER: Enable hardware attacks when this flag is true.
– TPM_COMPROMISED: TPM on intruder is compromised (she knows AIKpvt).
– SERVER_NONCE_PREDICTABLE: To model predictable nonces.
Murphi Model
• Behavior of model:
– Both client and intruder are equipped with a TPM chip.
– Intruder is malicious, so TPM quote response always contains an invalid PCR value on intruder.
– If attacker is active on client, then TPM quote response contains an invalid PCR value.
– If super attacker is active on client, then TPM quote response contains a valid value.
– Client always sends a valid measurement list (ML), because an attacker/super attacker can always modify ML to reflect a valid value.
• Invariants:– Authentication invariants.
– Secrecy invariants:• Client and server should share the same session secret.
• Intruder should not learn any session secret.
– Remote attestation invariants:• If server attests a client, then attacker should not be active on that client.
• Server must not attest an intruder.
Murphi Model (contd.)
Three-step protocol
• Authentication and secrecy invariants fail.
• Remote attestation invariants fail if intruder is allowed to forge certificates, super attacker is active or TPM is compromised.
Four-step protocol
• Client authentication invariant fails, but server authentication invariant does not fail.
• Secrecy invariants still fail because of MITM attack.
• Remote attestation invariants fail as in 3-step protocol.
Five-step protocol
• Authentication invariants do not fail in any case.
• Secrecy invariants do not fail until intruder is allowed to forge certificates.
• Remote attestation invariant fail as in 4-step protocol.
Conclusion
• The proposed protocol refuses to attest a malicious client unless:– Certificates can be forged.
– Hardware attack on TPM is possible.
– TPM is compromised.
Attacks
• Intruder can forge certificates or can convince client/server to trust on forged certificates.
• Most severe attack!– Can generate fake signatures over PCR and Nonces, which look like a signatures
from a real TPM.
– Can impersonate as a server to a client and vice versa, and learn secrets.
– All attacks are possible: Authentication attacks, secrecy attacks and attestation attacks.
• Example: MITM Attack
Attacks (Certificate Forging)
ServerIntruderClient
Fake Certificate in step 2
Fake Signature in step 4
Step 3: Signature using fake AIKpvt and fake certificate in step 3
Fake Signature in step 5
• Attacker’s objective is to hide the presence of a malicious software running on client
• Possible ways:– Bug in Integrity Measurement Architecture (IMA): It did not measure the
malicious software and put it in PCRs.
– Reset PCRs without rebooting the system, and push known good values into it.
– Hardware attacks like changing a running process’s image (through DMA).
– Break the TPM and retrieve secret key used for signing PCR.
• Consequence: An attacker is able to start a malicious software without letting its fingerprints go into the PCRs.
• Modeled as SUPER_ATTACKER in Murphy.
• Attack: A client running a malicious software gets attested by server.
Hardware Attacks and Attacks on IMA
• Intruder breaks the TPM and retrieves AIKpvt.
• Intruder can attest itself to the server.
• Can it do more harm?
• No other attack found.
• Encouraging because if one TPM is compromised, it does not compromise the security of other systems.
TPM on Intruder Compromised
• Server uses a bad source of randomness, and so its nonce is predictable.
• Client can pre-compute signature over that nonce and PCRs before a malicious application starts (that is when PCRs have trustworthy value)
• But a client that does so is itself malicious!
• So two hosts need to collude to mount an attack.
Predictable Server Nonce
Server
Intruder
Client
1. Get sign over NS, NC, Secret (Normal Attestation)
3. Give the malicious Attestor, signature generated in step 1
2. Start a malicious Attesting service on client, which uses precomputed NS
and secret 4. Normal Attestation, but Server’s nonce (NS) is already known to client/attacker
• The proposed protocol for remote attestation is secure under the following assumptions:
– CRTM and CPU are trusted
– TPM adheres to TCG’s specifications
– IMA (Integrity Measurement Architecture) is bug free
– A running process behavior or image cannot be changed (e.g. via DMA)
• A malicious platform can be attested by:
– Forging certificates
– Hardware attacks (DMA, resetting PCRs)
– Breaking the IMA and retrieving keys
– Predicting server/challenger’s nonce
• Attacks were found and confirmed by modeling the protocol on Murphi.
Conclusions
References
1. Trusted Computing Group, http://www.trustedcomputing.org
2. Reiner Sailer, Trent Jaeger, Xiaolan Zhang, Leendert van Doorn, “Attestation-based Policy Enforcement for Remote Access”, In Proceedings of the 11th ACM conference on Computer and Communications Security, October 2004.
3. Reiner Sailer, Xiaolan Zhang, Trent Jaeger and Leendert van Doorn, “Design and Implementation of a TCG-Based Integrity Measurement Architecture”, In Thirteenth Usenix Security Symposium, pages 223-238, August 2004.
4. H. Maruyama, T. Nakamura, S. Munetoh, Y. Funaki, Y. Yamashita, “Linux with TCPA Integrity Measurement”, IBM Research Report, Tokyo Research Laboratory, Japan, Jan. 2003.
5. D. Safford, J. Kravitz, and L. van Doorn, “Take Control of TCPA”, Linux Journal, pages 50-55, August 2003.
6. Karim Faez, Ashkan H. Karimabad, “Open-Source Applications of TCPA Hardware”, In International Journal of Computer Science and Network Security, Vol. 7, No. 3, March 2007.
7. Murphi description language and verifier, http://verify.stanford.edu/dill/murphi.html