tpm.dev.tutorials/Attestation

What Attestation is

A computer can use a TPM to demonstrate:

• possession of a valid TPM

• it being in a trusted state by dint of having executed (possibly only) trusted code to get to that state

• possession of objects such as asymmetric keypairs being resident on the TPM (objects that might be used in the attestation protocol)

Possible results of succesful attestation:

• encrypted filesystems getting unlocked with the help of an attestation server

• issuance of X.509 certificate(s) for TPM-resident public keys

• other secrets (e.g., credentials for various authentication systems)

Attestation Protocols

Attestation is done by a computer with a TPM interacting with an attestation service over a network. This requires an attestation protocol.

Notation

• Encrypt_<name> == encryption with the named private or secret key (if symmetric, then this primitive is expected to provide authenticated encryption).
• Sign_<name> == digital signature with the named private key.
• MAC_<name> == message authentication code keyed with the named secret key.
• CSn == client-to-server message number n
• SCn == server-to-client message number n
• {stuff, more_stuff} == a sequence of data, a "struct"

Proof of Possession of TPM

Proof of possession of a valid TPM is performed by the attestation client sending its TPM's Endorsement Key (EK) certificate (if one is available, else the attestation service must recognize the EK public key) and then exchanging additional messages by which the client can prove its possession of the EK.

Proof of possession of an EK is complicated by the fact that EKs are generally decrypt-only (some TPMs also support signing EKs, but the TCG specifications only require decrypt-only EKs). The protocol has to have the attestation service send a challenge (or key) encrypted to the EKpub and then the attestation client demonstrate that it was able to decrypt that with the EK. However, this is not quite how attestation protocols work! Instead of plain asymmetric encryption the server will use TPM2_MakeCredential(), while the attestation client will use TPM2_ActivateCredential() instead of plain asymmetric decryption.

Trusted State Attestation

Trusted state is attested by sending a quote of Platform Configuration Registers (PCRs) and the eventlog describing the evolution of the system's state from power-up to the current state. The attestation service vallidates the digests used to extend the various PCRs, and perhaps the sequence in which they appear in the eventlog, typically by checking a list of known-trusted digests (these are, for example, checksums of firmware images).

Typically the attestation protocol will have the client generate a signing-only asymmetric public key pair known as the attestation key (AK) with which to sign the PCR quote and eventlog. Binding of the EKpub and AKpub will happen via TPM2_MakeCredential() / TPM2_ActivateCredential().

Binding of Other Keys to EKpub

The semantics of TPM2_MakeCredential() / TPM2_ActivateCredential() make it possible to bind a TPM-resident object to the TPM's EKpub.

TPM2_MakeCredential() encrypts to the EKpub a small secret datum and the name (digest of public part) of the TPM-resident object being bound. The counter-part to this, TPM2_ActivateCredential(), will decrypt that and return the secret to the application IFF (if and only if) the caller has access to the named object.

Typically attestation protocols have the client send its EKpub, EKcert (if it has one), AKpub (the public key of an "attestation key"), and other things (e.g., PCR quote and eventlog signed with the AK), and the server will then send the output of TPM2_MakeCredential() that the client can recover a secret from using TPM2_ActivateCredential().

The implication is that if the client can extract the cleartext payload of TPM2_MakeCredential(), then it must possess a) the EK private key corresponding to the EKpub, b) the AK private key corresponding to the object named by the server.

Proof of possession can be completed immediately by demonstrating knowledge of the secret sent by the server. Proof of possession can also be delayed to an eventual use of that secret, allowing for single round trip attestation.

Attestation Protocol Patterns

Single Round Trip Attestation Protocols

An attestation protocol need not complete proof-of-possession immediately if the successful outcome of the protocol has the client demonstrate possession to other services/peers.

In the following example the client obtains a certificate (AKcert) for its AK, filesystem decryption keys, and possibly other things, and eventually it will use those items in ways that -by virtue of having thus been used- demonstrate that it possesses the EK used in the protocol:

  CS0:  Signed_AK({timestamp, [ID], EKpub, [EKcert],
                   AKpub, PCR_quote, eventlog})
  SC0:  {TPM2_MakeCredential(EKpub, AKpub, session_key),
         Encrypt_session_key({AKcert, filesystem_keys, etc.})}

(ID might be, e.g., a hostname.)

The server will validate that the timestamp is near the current time, the EKcert (if provided, else the EKpub), the signature using the asserted (but not yet bound to the EKpub) AKpub, then it will validate the PCR quote and eventlog, and, if everything checks out, will issue a certificate for the AKpub and return various secrets that the client may need.

The client obtains those items IFF (if and only if) the AK is resident in the same TPM as the EK, courtesy of TPM2_ActivateCredential()'s semantics.

NOTE well that in this example it is essential that the AKcert not be logged in any public place since otherwise an attacker can make and send CS0 using a non-TPM-resident AK and any TPM's EKpub/EKcert known to the attacker, and then it may recover the AK certificate from the log in spite of being unable to recover the AK certificate from SC1!

Two Round Trip Attestation Protocols

We can add a round trip to the protocol in the previous section to make the client prove possession of the EK and binding of the AK to the EK before it can get the items it needs. This avoids the security consideration of having to not log the AKcert.

Below is a sketch of a stateless, two round trip attestation protocol.

Actual protocols tend to use a secret challenge that the client echoes back to the server rather than a secret key possesion of which is proven with symmetriclly-keyed cryptographic algorithms.

  CS0:  Signed_AK({timestamp, [ID], EKpub, [EKcert],
                   AKpub, PCR_quote, eventlog})
  SC0:  {TPM2_MakeCredential(EKpub, AKpub, session_key), ticket}
  CS1:  {ticket, MAC_session_key(CS0), CS0}
  SC1:  Encrypt_session_key({AKcert, filesystem_keys, etc.})

where session_key is an ephemeral secret symmetric authenticated encryption key, and ticket is an authenticated encrypted state cookie:

  ticket = {vno, Encrypt_server_secret_key({session_key, timestamp, MAC_session_key(CS0)})}

where server_secret_key is a key known only to the attestation service and vno identifies that key (in order to support key rotation without having to try authenticated decryption twice near key rotation events).

The attestation server could validate that the timestamp is recent upon receipt of CS0. But the attestation server can delay validation of EKcert, signatures, and PCR quote and eventlog until receipt of CS1. In order to produce SC0 the server need only digest the AKpub to produce the name input of TPM2_MakeCredential(). Upon receipt of CS1 (which repeats CS0), the server can decrypt the ticket, validate the MAC of CS0, validate CS0, and produce SC1 if everything checks out.

In this protocol the client must successfully call TPM2_ActivateCredential() to obtain the session_key that it then proves possession of in CS1, and only then does the server send the AKcert and/or various secret values to the client, this time saving the cost of asymmetric encryption by using the session_key to key a symmetric authenticated cipher.

Actual Protocols: ibmacs

(TBD)

Actual Protocols: safeboot.dev

(TBD)

Actual Protocols: ...

(TBD)

Long-Term State Kept by Attestation Services

Attestation servers need to keep some long-term state:

• binding of EKpub and ID
• PCR validation profile for each identified client

The PCR validation profile for a client consists of a set of required and/or acceptable digests that must appear in each PCR's extension log. These required and/or acceptable digests may be digests of firmware images, boot loaders, boot loader configurations (e.g., menu.lst, for Grub), operating system kernels, initrd images, filesystem root hashes (think ZFS), etc.

Some of these are obtained by administrators on a trust-on-first-use (TOFU) basis.

Long-Term State Created by Attestation Services

An attestation service might support creation of host<->EKpub bindings on a first-come-first-served basis.

An attestation service might support deletion of host PCR validation profiles that represent past states upon validation of PCR quotes using newer profiles. This could be used to permit firmware and/or operating system upgrades and then disallow downgrades after evidence of successful upgrade.