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 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 outputs of succesful attestation:

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

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

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

  For servers these certificates would have dNSName subject alternative names (SANs).

  For a user device such a certificate might have a subject name and/or SANs identifying the user.

Possible outputs of unsuccessful attestation:

• alerting

• diagnostics (e.g., which PCR extensions in the PCR quote and eventlog are not recognized)

Attestation Protocols

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

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"
• {"key":<value>,...} == JSON text
• TPM2_MakeCredential(<args>) == outputs of calling TPM2_MakeCredential() with args arguments
• TPM2_Certify(<args>) == outputs of calling TPM2_Certify() with args arguments

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 validates 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.

Binding hosts to TPMs

(TBD. Talk about IDevID or similar certificates binding hosts to their factory-installed TPMs, and how to obtain those from vendors.)

Attestation Protocol Patterns and Actual Protocols (decrypt-only EKs)

Note: all the protocols described below are based on decrypt-only TPM endorsement keys.

Let's start with few observations and security considerations:

• Clients need to know which PCRs to quote. E.g., the Safe Boot project and the IBM sample attestation client and server have the client ask for a list of PCRs and then the client quotes just those.

  But clients could just quote all PCRs. It's more data to send, but probably not a big deal, and it saves a round trip if there's no need to ask what PCRs to send.

• Some replay protection or freshness indication for client requests is needed. A stateful method of doing this is to use a server-generated nonce. A stateless method is to use a timestamp.

• Replay protection of server to client responses is mostly either not needed or implicitly provided by TPM2_MakeCredential() because TPM2_MakeCredential() generates a secret seed that randomizes its outputs even when all the inputs are the same across multiple calls to it.

• Ultimately the protocol must make use of TPM2_MakeCredential() and TPM2_ActivateCredential() in order to authenticate a TPM-running host via its TPM's EKpub.

• Privacy protection of client identifiers may be needed, in which case TLS may be desired.

• Even if a single round trip attestation protocol is adequate, a return routability check may be needed to avoid denial of service attacks. I.e., do not run a single round trip attestation protocol over UDP without first requiring the client to echo a nonce/cookie.

• Statelessness on the server side is highly desirable, as that should permit having multiple servers and each of a client's messages can go to different servers. Conversely, keeping state on the server across multiple round trips can cause resource exhaustion / denial of service attack considerations.

• Statelessness maps well onto HTTP / REST. Indeed, attestation protocol messages could all be idempotent and therefore map well onto HTTP GET requests but for the fact that all the things that may be have to be sent may not fit on a URI local part or URI query parameters, therefore HTTP POST is the better option.

Single Round Trip Attestation Protocol Patterns

An attestation protocol need not complete proof-of-possession immediately if the successful outcome of the protocol has the client subsequently demonstrate possession to other services/peers. This is a matter of taste and policy. However, one may want to have cryptographically secure "client attested successfully" state on the server without delay, in which case two round trips are the minimum for an attestation protocol.

In the following example the client obtains a certificate (AKcert) for its AKpub, 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:

  <client knows a priori what PCRs to quote, possibly all, saving a round trip>

  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.})}

  <subsequent client use of AK w/ AKcert, or of credentials made
   available by dint of being able to access filesystems unlocked by
   SC0, demonstrate that the client has attested successfully>

(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!

Alternatively, a single round trip attestation protocol can be implemented as an optimization to a two round trip protocol when the AK is persisted both, in the client TPM and in the attestation service's database:

  <having previously successfully enrolled AKpub and bound it to EKpub...>

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

Two Round Trip Stateless Attestation Protocol Patterns

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).

[Note: ticket here is not in the sense used by TPM specifications, but in the sense of "TLS session resumption ticket" or "Kerberos ticket", and, really, it's just an encrypted state cookie so that the server can be stateless.]

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.

(The server_secret_key, ticket, session_key, and proof of possession used in CS1 could even conform to Kerberos or encrypted JWT and be used for authentication, possibly with an off-the-shelf HTTP stack.)

An HTTP API binding for this protocol could look like:

  POST /get-attestation-ticket
      Body: CS0
      Response: SC0

  POST /attest
      Body: CS1
      Response: SC1

Actual Protocols: ibmacs

The IBM TPM Attestation Client Server (ibmacs) open source project has sample code for a "TCG attestation application".

It implements a stateful (state is kept in a database) attestation and enrollment protocol over TCP sockets that consists of JSON texts of the following form, sent prefixed with a 32-bit message length in host byte order:

  CS0: {"command":"nonce","hostname":"somehostname",
        "userid":"someusername","boottime":"2021-04-29 16:37:06"}
  SC0: {"response":"nonce","nonce":"<hex>", "pcrselect":"<hex>", ...}

  <nonce is used in production of signed PCR quote>

  CS1: {"command":"quote","hostname":"somehostname",
        "quoted":"<hex>","signature":"<hex>",
        "event1":"<hex>","imaevent0":"<hex>"}
  SC1: {"response":"quote"}

  CS2: {"command":"enrollrequest","hostname":"somehost",
        "tpmvendor":"...","ekcert":"<PEM>","akpub":"<hex(DER)>"}
  SC2: {"response":"enrollrequest",
        "credentialblob":"<hex of credentialBlob output of TPM2_MakeCredential()>",
        "secret":"<hex of secret output of TPM2_MakeCredential()>"}

  CS3: {"command":"enrollcert","hostname":"somecert","challenge":"<hex>"}
  SC3: {"response":"enrollcert","akcert":"<hex>"}

The server keeps state across round trips.

Note that this protocol has up to four (4) round trips. Because the ibmacs server keeps state in a database, it should be possible to elide some of these round trips in attestations subsequent to enrollment.

The messages of the second and third round trips could be combined since there should be no need to wait for PCR quote validation before sending the EKcert and AKpub. The messages of the first round trip too could be combined with the messages of the second and third round trip by using a timestamp as a nonce -- with those changes this protocol would get down to two round trips.

Actual Protocols: safeboot.dev

(TBD)

Actual Protocols: ...

(TBD)

Attestation Protocol Patterns and Actual Protocols (signing-only EKs)

Some TPMs come provisioned with signing-only endorsement keys in addition to decrypt-only EKs. For example, vTPMs in Google cloud provides both, decrypt-only and signing-only EKs.

Signing-only EKs can be used for attestation as well.

[Ideally signing-only EKs can be restricted to force the use of TPM2_Certify()? Restricted signing keys can only sign payloads that start with a magic value, whereas unrestricted signing keys can sign any payload.]

Signing-only EKs make single round trip attestation protocols possible that also provide immediate attestation status because signing provides proof of possession non-interactively, whereas asymmetric encryption requires interaction to prove possession:

  CS0:  Signed_AK({timestamp, [ID], EKpub, [EKcert],
                   AKpub, TPM2_Certify(EKpub, AKpub),
                   PCR_quote, eventlog})
  SC0:  AKcert

If secrets need to be sent back, then a decrypt-only EK also neds to be used:

  CS0:  Signed_AK({timestamp, [ID],
                   EKpub_signing, EKpub_encrypt,
                   [EKcert_signing], [EKcert_encrypt],
                   AKpub, TPM2_Certify(EKpub, AKpub),
                   PCR_quote, eventlog})
  SC0:  {TPM2_MakeCredential(EKpub_encrypt, AKpub, session_key),
         Encrypt_session_key({AKcert, filesystem_keys, etc.})}

Long-Term State Kept by Attestation Services

Attestation servers need to keep some long-term state:

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

Log-like attestation state:

• client attestation status (last time successfully attested, last time unsuccessfully attested)

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.

Things to log:

• client attestation attempts and outcomes
• AK certificates issued (WARNING: see note about single round trip attestation protocols above -- do not log AKcerts in public places when using single round trip attestation protocols!)

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.

References

• TCG TPM Library part 1: Architecture, sections 23 and 24
• https://sourceforge.net/projects/ibmtpm20acs/
• https://safeboot.dev/
• https://github.com/osresearch/safeboot/