tpm.dev.tutorials/Enrollment/README.md

# 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](/Attestation/README.md) 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](/Attestation/README.md).

## 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 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](/Attestation/README.md#Secret-Transport-Sub-Protocols)
for details of how to store and transport secrets onto an enrolled
device post-enrollment.

## Encrypt-to-TPM Sample Scripts

A pair of scripts are included here to demonstrate how to make long-term
secrets encrypted to TPMs for use in
[attestation](/Attestation/README.md) protocols.  The method used is the
one described in the [attestation
tutorial](/Attestation/README.md#Secret-Transport-Sub-Protocols) using
[`TPM2_MakeCredential()`](/TPM-Commands/TPM2_MakeCredential.md) and
[`TPM2_ActivateCredential()`](/TPM-Commands/TPM2_ActivateCredential.md)
with a hard-coded, _well-known_ activation key (`WK`) to implement
encryption-to-`EKpub` with (optional) sender-asserted authorization
policy:

 - [`send-to-tpm.sh`](send-to-tpm.sh)
 - [`tpm-receive.sh`](tpm-receive.sh)

You can use these scripts like so:

 - without policy:

   ```bash
   : ; # 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.sh ek.pub secret.bin cipher.bin
   fd32fa22c52cfc8e1a0c29eb38519f87084cab0b04b0d8f020a4d38b2f4e223e
   7fdad037a921f7eec4f97c08722692028e96888f0b970dc7b3bb6a9c97e8f988
   ```

   ```bash
   : ; # Decrypt the secret:
   : ; tpm-receive.sh cipher.bin secret.bin
   fd32fa22c52cfc8e1a0c29eb38519f87084cab0b04b0d8f020a4d38b2f4e223e
   7fdad037a921f7eec4f97c08722692028e96888f0b970dc7b3bb6a9c97e8f988
   name: 000be1fe1b777ead331f2da896ced2bf7a3949d732a0c6adf6f0a292567d587c4408
   837197674484b3f81a90cc8d46a5d724fd52d76e06520b64f2a1da1b331469aa
   fd32fa22c52cfc8e1a0c29eb38519f87084cab0b04b0d8f020a4d38b2f4e223e
   7fdad037a921f7eec4f97c08722692028e96888f0b970dc7b3bb6a9c97e8f988
   certinfodata:b7bd59980628c33a14377d53e165c229
   : ;
 - with policy

   ```bash
   : ; # 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.sh ek.pub secret.bin cipher.bin "${policy[@]}"
   fd32fa22c52cfc8e1a0c29eb38519f87084cab0b04b0d8f020a4d38b2f4e223e
   7fdad037a921f7eec4f97c08722692028e96888f0b970dc7b3bb6a9c97e8f988
   ```

   ```bash
   : ; # We have to satisfy the same policy on the receive side:
   : ; policy=(tpm2 policypcr -l sha256:11 -f pcr.dat)
   : ;
   : ; tpm-receive.sh -f cipher.bin "${policy[@]}"
   fd32fa22c52cfc8e1a0c29eb38519f87084cab0b04b0d8f020a4d38b2f4e223e
   7fdad037a921f7eec4f97c08722692028e96888f0b970dc7b3bb6a9c97e8f988
   name: 000be1fe1b777ead331f2da896ced2bf7a3949d732a0c6adf6f0a292567d587c4408
   837197674484b3f81a90cc8d46a5d724fd52d76e06520b64f2a1da1b331469aa
   fd32fa22c52cfc8e1a0c29eb38519f87084cab0b04b0d8f020a4d38b2f4e223e
   7fdad037a921f7eec4f97c08722692028e96888f0b970dc7b3bb6a9c97e8f988
   certinfodata:b7bd59980628c33a14377d53e165c229
   : ;
   ```

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

   ```bash
   send-to-tpm.sh ... tpm2 policyblah ... \; policyfoo ...
   ```

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

   ```bash
   send-to-tpm.sh ... 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` to the policy, as
   that is required for activation objects with `adminWithPolicy` set.

# 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](/Attestation/README.md#two-round-trip-stateless-attestation-protocol-patterns),
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