tpm.dev.tutorials/Enrollment

Device Enrollment

Device Enrollment is the act of registering a device -anything from an IoT to a server- and creating the state that will be referenced in future attestations from that device.

This can be as simple as sending the device's endorsement key certificate (EKcert) to a registration server (possibly authenticating to that server using some administrator user's credentials), to a more complex protocol similar to attestation.

Online Enrollment

Online enrollment means that the device to be enrolled interacts with an enrollment service over a network.

Off-line Enrollment

Off-line enrollment means that the device to be enrolled does not interact with an enrollment service.

For example, one might scan an endorsement key (EK) public key or certificate from a QR code on a shipment manifest and then enroll the device using only that information.

Safeboot.dev Enrollment Protocol

Safeboot.dev has an enrollment script, attest-enroll which can have a trivial HTTP API binding where an authenticated and authorized client principal POSTs an EKpub and a device name to the server. The server then creates enrollment state for the device that will be used during subsequence attestation.

The safeboot.dev enrollment process does not require any interaction with the enrollee device except to extract its EKpub. When the EKpub can be determined in an off-line manner, then the safeboot.dev enrollment process can be fully off-line.

Server-Side State to Create during Enrollment

• device name <-> EKpub binding
• enrolling user/admin
• that the device has a valid TPM (i.e., the EKcert validates to a trusted TPM vendor's trust anchor)
• initial root of trust measurement (RTM)
• backup, secret recovery keys
• encrypted secrets to send to the device

Client-side State to Create during Enrollment

• encrypted filesystems?
• device credentials? (e.g., TLS server certificates, Kerberos keys ["keytabs"], etc.)

Secrets Long-Term Storage and Transport

Every time an enrolled device reboots, or possibly more often, it may have to connect to an attestation server to obtain secrets from it that the device needs in order to proceed. For example, filesystem decryption keys, general network access, device authentication credentials, etc.

See attestation for details of how to store and transport secrets onto an enrolled device post-enrollment.

Encrypt-to-TPM Sample Scripts

A pair of scripts used in safeboot.dev are included here to demonstrate how to make long-term secrets encrypted to TPMs for use in attestation protocols. The method used is the one described in the attestation tutorial using either of two methods to encrypt to a TPM:

• The "EK" method of encryption to a TPM uses TPM2_MakeCredential() and TPM2_ActivateCredential() with a hard-coded, well-known activation key (WK) to implement encryption-to-EKpub with (optional) sender-asserted authorization policy.

• The "TK" method of encryption to a TPM uses a software implementation of TPM2_Duplicate() to wrap the private part of a "transport key" (TK) to the target TPM, then normal RSA encryption to the public part of the TK. The ciphertext consists of the outputs of TPM2_Duplicate() and the ciphertext produced by RSA encryption to the TK.

The "EK" method is the default. Both methods support sender-asserted policies.

The scripts:

• send-to-tpm
• tpm-receive

You can use these scripts like so:

• without policy:

  : ; # Make a secret
  : ; dd if=/dev/urandom of=secret.bin bs=16 count=1
  : ;
  : ; # Encrypt the secret to some TPM whose EKpub is in a file named
  : ; # ek.pub:
  : ; /safeboot/sbin/send-to-tpm ek.pub secret.bin cipher.bin
  : ; 
  

  : ; # Decrypt the secret:
  : ; tpm-receive cipher.bin plaintext.bin
  837197674484b3f81a90cc8d46a5d724fd52d76e06520b64f2a1da1b331469aa
  name: 000bc76d1462d32d5e6051d0aa121edfa5ed66b8e7f3632ce3c5a172b1ebd8aabc40
  : ;
  

• with policy

  Here we use a policy that PCR #11 has not been extended. The idea is to extend it immediately after decrypting the ciphertext, which means that the ciphertext cannot again be decrypted later (by, say, some other application with access to the same TPM) without rebooting.

  : ; # Make up a policy (here that PCR11 must be unextended):
  : ; dd if=/dev/zero of=pcr.dat bs=32 count=1
  : ; policy=(tpm2 policypcr -l sha256:11 -f pcr.dat)
  : ;
  : ; send-to-tpm ek.pub secret.bin cipher.bin "${policy[@]}"
  fd32fa22c52cfc8e1a0c29eb38519f87084cab0b04b0d8f020a4d38b2f4e223e
  7fdad037a921f7eec4f97c08722692028e96888f0b970dc7b3bb6a9c97e8f988
  7fdad037a921f7eec4f97c08722692028e96888f0b970dc7b3bb6a9c97e8f988
  policyDigest:
  7fdad037a921f7eec4f97c08722692028e96888f0b970dc7b3bb6a9c97e8f988
  : ; 
  

  : ; # We have to satisfy the same policy on the receive side:
  : ; policy=(tpm2 policypcr -l sha256:11 -f pcr.dat)
  : ;
  : ; tpm-receive cipher.bin plaintext.bin "${policy[@]}"
  fd32fa22c52cfc8e1a0c29eb38519f87084cab0b04b0d8f020a4d38b2f4e223e
  7fdad037a921f7eec4f97c08722692028e96888f0b970dc7b3bb6a9c97e8f988
  7fdad037a921f7eec4f97c08722692028e96888f0b970dc7b3bb6a9c97e8f988
  837197674484b3f81a90cc8d46a5d724fd52d76e06520b64f2a1da1b331469aa
  name: 000b20a6cc44c93ad206196c65028f9a8bf2590de0b8f89bca9e968f09f4e616dba6
  fd32fa22c52cfc8e1a0c29eb38519f87084cab0b04b0d8f020a4d38b2f4e223e
  7fdad037a921f7eec4f97c08722692028e96888f0b970dc7b3bb6a9c97e8f988
  7fdad037a921f7eec4f97c08722692028e96888f0b970dc7b3bb6a9c97e8f988
  : ;
  

  Multiple policy commands can be separated with a quoted semi-colon:

  send-to-tpm ... tpm2 policyblah ... \; policyfoo ...
  

  Multiple policy commands can be separated with a quoted semi-colon:

  send-to-tpm ... tpm2 policyblah ... \; policyfoo ...
  

  When a policy is specified, these scripts will automatically set the adminWithPolicy attribute of the activation object, and will add tpm2 policycommandcode TPM2_CC_ActivateCredential ("EK" method) or tpm2 policycommandcode TPM2_CC_RSA_Decrypt ("TK" method) to the policy.

Enrollment Semantics

• online vs. off-line

• client device trust semantics:

  • bind device name and EKpub on first use ("BOFU")?
  • enroll into inventory and then allow authorized users to bind a device name to an EKpub on a first-come-first-served basis?

• enrollment server trust semantics:

  • trust on first use (TOFU) (i.e., trust the first enrollment server found)
  • pre-install a trust anchor on the client device
  • use a user/admin credential on the device to establish trust on the server (e.g., intrinsically to how user authentication works, or having the user review and approve the server's credentials)

Threat Models

Threats:

• enrollment server impersonation
• enrollment of rogue devices
• eavesdroppers
• DoS

A typical enrollment protocol for servers in datacenters may well not bother protecting against all of the above.

A typical enrollment protocol for IoTs in a home network also may well not bother protecting against any of the above.

Enrollment protocols for personal devices must protect against all the listed threats except DoS attacks.

Enrollment Protocols

Trivial Enrollment Protocols

The simplest enrollment protocols just have the client device send its EKcert to the enrollment server. The enrollment server may have a user associate enrolled devices with device IDs (e.g., hostnames), and the device's enrollment is complete.

Enrollment Protocols with Proof of Possession and Attestation

A more complex enrollment protocol would have the device attest to possession of the EK whose EKpub is certified by its EKcert, and might as well also perform attestation of other things, such as RTM.

An enrollment protocol with proof of possession might look a lot like the two round trip attestation protocol, with the addition of enrollment_data in the last message from the client to the server (server authentication not shown):

  CS0:  [ID], EKpub, [EKcert], AKpub, PCRs, eventlog, timestamp,
        TPM2_Quote(AK, PCRs, extra_data)=Signed_AK({hash-of-PCRs, misc, extra_data})
  SC0:  {TPM2_MakeCredential(EKpub, AKpub, session_key), ticket}
  CS1:  {ticket, MAC_session_key(CS0), CS0, Encrypt_session_key(enrollment_data)}
                                            ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
                                            (new)
  SC1:  Encrypt_session_key({AKcert, filesystem_keys, etc.})

  <extra_data includes timestamp>

where

  enrollment_data = { Encrypt_TK(secrets), [TKpub], [HK_pub] }

  secrets = any secrets generated on the client side
  TKpub = public part of transport key for encrypting secrets to the
          client
  HKpub = public part of a host key for host authentication

Enrollment Protocols for Personal Devices

Enrollment of personal devices in their owners' personal device groups can be a lot like Bluetooth device pairing. Where such devices have TPMs then perhaps there is a role for the TPM to play in enrollment.

Security Considerations

TBD

The enrollment database, though it contains ciphertexts of secrets encrypted to enrolled devices' TPMs, is nonetheless to be kept confidential. This is necessary to avoid attacks where an attacker compromises an enrolled device then attempts to decrypt those ciphertexts with the enrolled device's TPM. These ciphertexts should only be furnished to the device as part of an attestation protocol.

For the same reason, these ciphertexts must be super-encrypted when delivering them to enrolled devices during attestation.

Ciphertexts in enrolled state should be made with suitable sender- asserted policies. For example, asserting that PCR #11 has not been extended so that immediately after decrypting such a ciphertext the client can extend PCR #11 to make decrypting that ciphertext again impossible without an intervening reboot.