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.

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 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 transport secrets onto an enrolled device post-enrollment.

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