Attestation: Add swimlane diagrams

Attestation/Protocol-Four-Messages.puml

@@ -0,0 +1,20 @@
+@startuml
+participant TPM as T
+participant Client as C
+participant Server as S
+participant ServerSimTPM as ST
+title Four-message (two round trips) attestation protocol
+C   ->  C: timestamp = gettimeofday();
+C  -->  T: TPM2_Quote(AK, set-of-all-PCRs, timestamp)
+T  -->  C: quote=Signed_AK({hash-of-PCRs, misc, timestamp})
+C   ->  S: [ID], EKpub, [EKcert], AKpub,\nPCRs, eventlog, timestamp, quote
+S   ->  S: check that timestamp is recent;\ndata = Lookup(EKpub, [EKcert], [ID]);\n[Validate(EKcert)];\ncompute PCRs hash from eventlog and PCRs;\nvalidate quote;\nsession_key = genkey();\nAKcert = CA_Certify(AKpub, data.ID, AKtbscert);\nticket = {vno, Encrypt_server_secret_key({timestamp,\n\t\t\tgettimeofday(),\n\t\t\tsession_key})}
+S  --> ST: TPM2_MakeCredential(EKpub, AKpub, session_key)
+ST -->  S: credentialBlob, secret
+S   ->  C: credentialBlob, secret, ticket
+C  -->  T: TPM2_ActivateCredential(AKhandle, EKhandle,\n\t\t\t\t\tcredentialBlob, secret)
+T  -->  C: certInfo = session_key
+C   ->  C: PoP = HMAC_session_key(ticket)
+S   ->  C: Encrypt_session_key(stuff =\n\t\t\t\t\t\t{AKcert, data.for_client})
+C   ->  C: {AKcert, secrets} =\n\t\tDecrypt_session_key(stuff)
+@enduml

Attestation/Protocol-Three-Messages.puml

@@ -0,0 +1,19 @@
+@startuml
+participant TPM as T
+participant Client as C
+participant Server as S
+participant ServerSimTPM as ST
+title Three-message (1.5 round trips) attestation protocol w/ Proof-of-Possession
+C   ->  C: timestamp = gettimeofday();
+C  -->  T: TPM2_Quote(AK, set-of-all-PCRs, timestamp)
+T  -->  C: quote=Signed_AK({hash-of-PCRs, misc, timestamp})
+C   ->  S: [ID], EKpub, [EKcert], AKpub,\nPCRs, eventlog, timestamp, quote
+S   ->  S: check that timestamp is recent;\ndata = Lookup(EKpub, [EKcert], [ID]);\n[Validate(EKcert)];\ncompute PCRs hash from eventlog and PCRs;\nvalidate quote;\nsession_key = genkey();\nAKcert = CA_Certify(AKpub, data.ID, AKtbscert);\nstuff = Encrypt_session_key({AKcert,\n\t\t\t\t\tdata.for_client})
+S  --> ST: TPM2_MakeCredential(EKpub, AKpub, session_key)
+ST -->  S: credentialBlob, secret
+S   ->  C: credentialBlob, secret, stuff
+C  -->  T: TPM2_ActivateCredential(AKhandle, EKhandle,\n\t\t\t\t\tcredentialBlob, secret)
+T  -->  C: certInfo = session_key
+C   ->  C: {AKcert, secrets} =\n\t\tDecrypt_session_key(stuff);
+C   ->  S: {AKcert, PoP = Digest(AKcert)}
+@enduml

Attestation/Protocol-Two-Messages.puml

@@ -0,0 +1,18 @@
+@startuml
+participant TPM as T
+participant Client as C
+participant Server as S
+participant ServerSimTPM as ST
+title Two-message (one round trip) attestation protocol
+C   ->  C: timestamp = gettimeofday();
+C  -->  T: TPM2_Quote(AK, set-of-all-PCRs, timestamp)
+T  -->  C: quote=Signed_AK({hash-of-PCRs, misc, timestamp})
+C   ->  S: [ID], EKpub, [EKcert], AKpub,\nPCRs, eventlog, timestamp, quote
+S   ->  S: check that timestamp is recent;\ndata = Lookup(EKpub, [EKcert], [ID]);\n[Validate(EKcert)];\ncompute PCRs hash from eventlog and PCRs;\nvalidate quote;\nsession_key = genkey();\nAKcert = CA_Certify(AKpub, data.ID, AKtbscert);\nstuff = Encrypt_session_key({AKcert,\n\t\t\t\t\tdata.for_client})
+S  --> ST: TPM2_MakeCredential(EKpub, AKpub, session_key)
+ST -->  S: credentialBlob, secret
+S   ->  C: credentialBlob, secret, stuff
+C  -->  T: TPM2_ActivateCredential(AKhandle, EKhandle,\n\t\t\t\t\tcredentialBlob, secret)
+T  -->  C: certInfo = session_key
+C   ->  C: {AKcert, secrets} =\n\t\tDecrypt_session_key(stuff);
+@enduml

Attestation/README.md

@@ -12,29 +12,55 @@ A computer can use a TPM to demonstrate:
 
 Possible outputs of succesful attestation:
 
- - encrypted filesystems getting unlocked with the help of an
-   attestation server
+ - authorize client to join its network
 
- - other secrets (e.g., credentials for various authentication systems)
+ - delivery of configuration metadata to the client
 
- - issuance of X.509 certificate(s) for TPM-resident public keys
+ - unlocking of storage / filesystems on the client
+
+ - delivery of various secrets, such credentials for various authentication systems:
+
+    - issuance of X.509 certificate(s) for TPM-resident attestaion
+      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.
+      For a user device such a certificate might have a subject name
+      and/or SANs identifying the user or device.
+
+    - issuance of non-PKIX certificates (e.g., OpenSSH-style certificates)
+
+    - issuance of Kerberos host-based service principal long-term keys
+      ("keytabs")
+
+    - service account tokens
+
+    - etc.
+
+ - client state tracking
+
+ - etc.
 
 Possible outputs of unsuccessful attestation:
 
  - alerting
 
  - diagnostics (e.g., which PCR extensions in the PCR quote and eventlog
-   are not recognized)
+   are not recognized, which then might be used to determine what
+   firmware / OS updates a client has installed, or that it has been
+   compromised)
+
+In this tutorial we'll focus on attestion of servers in an enterprise
+environment.  However, the concepts described here are applicable to
+other environments, such as IoTs and personal devices, where the
+attestation database could be hosted on a user's personal devices for
+use in joining new devices to the user's set of devices, or for joining
+new IoTs to the user's SOHO network.
 
 # Attestation Protocols
 
-Attestation is done by a computer with a TPM interacting with an
+Attestation is done by a client computer with a TPM interacting with an
 attestation service over a network.  This requires a network protocol
 for attestation.
 
@@ -91,6 +117,27 @@ EKpub and AKpub will happen via
 [`TPM2_MakeCredential()`](TPM2_MakeCredential.md) /
 [`TPM2_ActivateCredential()`](TPM2_ActivateCredential.md).
 
+Note that the [`TPM2_Quote()`](TPM2_Quote.md) function produces a signed
+message -- signed with a TPM-resident AK named by the caller (and to
+which they have access), which would be the AK used in the attestation
+protocol.
+
+The output of [`TPM2_Quote()`](TPM2_Quote.md) might be the only part of
+a client's messages to the attestation service that include a signature
+made with the AK, but integrity protection of everything else can be
+implied (e.g., the eventlog and PCR values are used to reconstruct the
+PCR digest signed in the quote).  `TPM2_Quote()` signs more than just a
+digest of the selected PCRs.  `TPM2_Quote()` signs all of:
+
+ - digest of selected PCRs
+ - caller-provided extra data (e.g., a cookie/nonce/timestamp/...),
+ - the TPM's firmware version number,
+ - `clock` (the TPM's time since startup),
+ - `resetCount` (an indirect indicator of reboots),
+ - `restartCount` (an indirect indicator of suspend/resume events)
+ - and `safe` (a boolean indicating whether the `clock` might have ever
+   gone backwards).
+
 ## Binding of Other Keys to EKpub
 
 The semantics of [`TPM2_MakeCredential()`](TPM2_MakeCredential.md) /
@@ -143,7 +190,9 @@ Let's start with few observations and security considerations:
 
  - 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.
+   nonce (as an encrypted state cookie embedding a timestamp).  A
+   stateless method is to use a timestamp and reject requests with old
+   timestamps.
 
  - Replay protection of server to client responses is mostly either not
    needed or implicitly provided by [`TPM2_MakeCredential()`](TMP2_MakeCredential.md)
@@ -176,6 +225,33 @@ Let's start with few observations and security considerations:
    have to be sent may not fit on a URI local part or URI query
    parameters, therefore HTTP `POST` is the better option.
 
+### Error Cases Not Shown
+
+Note that error cases are not shown in the protocols described below.
+
+Naturally, in case of error the attestation server will send a suitable
+error message back to the client.
+
+### Databases, Log Sinks, and Dashboarding / Alerting Systems Not Shown
+
+In order to simplify the protocol diagrams below, interactions with
+databases, log sinks, and alerting systems are not shown.
+
+A typical attestation service will, however, have interactions with
+those components, some or all of which might even be remote:
+
+ - attestation database
+ - log sinks
+ - dashboarding / alerting
+
+If an attestation service must be on the critical path for booting an
+entire datacenter, it may be desirable for the attestation service to be
+able to run with no remote dependencies, at least for some time.  This
+means, for example, that the attestation database should be locally
+available and replicated/synchronized only during normal operation.  It
+also means that there should be a local log sink that can be sent to
+upstream collectors during normal operation.
+
 ### Single Round Trip Attestation Protocol Patterns
 
 An attestation protocol need not complete proof-of-possession
@@ -195,11 +271,13 @@ 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})
+  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),
          Encrypt_session_key({AKcert, filesystem_keys, etc.})}
 
+  <extra_data includes timestamp>
+
   <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>
@@ -207,6 +285,11 @@ protocol:
 
 (`ID` might be, e.g., a hostname.)
 
+![Protocol Diagram](Protocol-Two-Messages.png)
+
+(In this diagram we show the use of a TPM simulator on the server side
+for implementing [`TPM2_MakeCredential()`](TPM2_MakeCredential.md).)
+
 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
@@ -218,26 +301,67 @@ 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`!
+NOTE well that in single round trip attestation protocols using only
+decrypt-only EKs 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})
+  CS0:  timestamp, AKpub, PCRs, eventlog,
+        TPM2_Quote(AK, PCRs, extra_data)=Signed_AK({hash-of-PCRs, misc, extra_data})
   SC0:  {TPM2_MakeCredential(EKpub, AKpub, session_key),
          Encrypt_session_key({AKcert, filesystem_keys, etc.})}
 ```
 
+### Three-Message Attestation Protocol Patterns
+
+A single round trip protocol using encrypt-only EKpub will not
+demonstrate proof of possession immediately, but later on when the
+certified AK is used elsewhere.  A proof-of-possession (PoP) may be
+desirable anyways for monitoring and alerting purposes.
+
+```
+  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),
+         Encrypt_session_key({AKcert, filesystem_keys, etc.})}
+  CS1:  AKcert, Signed_AK(AKcert)
+```
+
+![Protocol Diagram](Protocol-Three-Messages.png)
+
+(In this diagram we show the use of a TPM simulator on the server side
+for implementing [`TPM2_MakeCredential()`](TPM2_MakeCredential.md).)
+
+NOTE well that in this protocol, like single round trip attestation
+protocols using only decrypt-only EKs, 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`!
+
+If such a protocol is instantiated over HTTP or TCP, it will really be
+more like a two round trip protocol:
+
+```
+  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),
+         Encrypt_session_key({AKcert, filesystem_keys, etc.})}
+  CS1:  AKcert, Signed_AK(AKcert)
+  SC1:  <empty>
+```
+
 ### Two Round Trip Stateless Attestation Protocol Patterns
 
 We can add a round trip to the protocol in the previous section to make
@@ -252,11 +376,13 @@ 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})
+  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}
   SC1:  Encrypt_session_key({AKcert, filesystem_keys, etc.})
+
+  <extra_data includes timestamp>
 ```
 
 where `session_key` is an ephemeral secret symmetric authenticated
@@ -267,6 +393,8 @@ encryption key, and `ticket` is an authenticated encrypted state cookie:
                                             MAC_session_key(CS0)})}
 ```
 
+![Protocol Diagram](Protocol-Four-Messages.png)
+
 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).
@@ -309,6 +437,10 @@ An HTTP API binding for this protocol could look like:
       Response: SC1
 ```
 
+Here the attestation happens in the first round trip, but the proof of
+possession is completed in the second, and the delivery of secrets and
+AKcert also happens in the second round trip.
+
 ### Actual Protocols: ibmacs
 
 The [`IBM TPM Attestation Client Server`](https://sourceforge.net/projects/ibmtpm20acs/)
@@ -358,7 +490,20 @@ to two round trips.
 
 ### Actual Protocols: safeboot.dev
 
-(TBD)
+```
+  CS0:  <empty>
+  SC0:  nonce, PCR_list
+  CS1:  [ID], EKpub, [EKcert], AKpub, PCRs, eventlog, nonce,
+        TPM2_Quote(AK, PCRs, extra_data)=Signed_AK({hash-of-PCRs, misc, extra_data})
+  SC1:  {TPM2_MakeCredential(EKpub, AKpub, session_key),
+         Encrypt_session_key({filesystem_keys})}
+```
+
+Nonce validation is currently not well-developed in Safeboot.
+If a timestamp is used instead of a nonce, and if the client assumes all
+PCRs are desired, then this becomes a one round trip protocol.
+
+An AKcert will be added to the Safeboot protocol soon.
 
 ### Actual Protocols: ...
 
@@ -383,9 +528,8 @@ 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})
+  CS0:  timestamp, [ID], EKpub, [EKcert], AKpub, PCRs, eventlog,
+        TPM2_Certify(EKpub, AKpub), TPM2_Quote()
   SC0:  AKcert
 ```
 
@@ -393,11 +537,9 @@ 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})
+  CS0:  timestamp, [ID], EKpub_signing, EKpub_encrypt,
+        [EKcert_signing], [EKcert_encrypt], AKpub, PCRs, eventlog,
+        TPM2_Certify(EKpub, AKpub), TPM2_Quote()
   SC0:  {TPM2_MakeCredential(EKpub_encrypt, AKpub, session_key),
          Encrypt_session_key({AKcert, filesystem_keys, etc.})}
 ```
@@ -408,6 +550,7 @@ Attestation servers need to keep some long-term state:
 
  - binding of `EKpub` and `ID`
  - PCR validation profile(s) for each identified client
+ - resetCount (for reboot detection)
 
 Log-like attestation state:
 
@@ -431,16 +574,138 @@ Things to log:
    attestation protocols above -- do not log AKcerts in public places
    when using single round trip attestation protocols!)
 
-## Long-Term State Created by Attestation Services
+## Long-Term State Created or Updated by Attestation Services
 
-An attestation service might support creation of host&lt;-&gt;EKpub
-bindings on a first-come-first-served basis.
+ - An attestation service might support creation of host&lt;-&gt;EKpub
+   bindings on a first-come-first-served basis.  In this mode the
+   attestation server might validate an EKcert and that the desired
+   hostname has not been bound to an EK, then create the binding.
 
-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.
+ - 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.
+
+ - An attestation service might keep track of client reboots so as to:
+    - revoke old AKcerts when the client reboots (but note that this is
+      really not necessary if we trust the client's TPM, since then the
+      previous AKs will never be usable again)
+    - alert if the reboot count ever goes backwards
+
+## Schema for Attestation Server Database
+
+A schema for the attestation server's database entries might look like:
+
+```JSON
+{
+  "EKpub": "<EKpub>",
+  "hostname": "<hostname>",
+  "EKcert": "<EKcert in PEM, if available>",
+  "previous_firmware_profile": "FWProfile0",
+  "current_firmware_profiles": ["FWProfile1", "FWProfile2", "..."],
+  "previous_operating_system_profiles": "OSProfile0",
+  "current_operating_system_profiles": ["OSProfile1", "OSProfile2", "..."],
+  "previous_PCRs": "<...>",
+  "proposed_PCRs": "<...>",
+  "ak_cert_template": "<AKCertTemplate>",
+  "secrets": "<secrets>",
+  "resetCount": "<resetCount value from last quote>"
+}
+```
+
+The attestation server's database should have two lookup keys:
+
+ - EKpub
+ - hostname
+
+The attestation server's database's entry for any client should provide,
+de minimis:
+
+ - a way to validate the root of trust measurements in the client's
+   quoted PCRs, for which two methods are possible:
+    - save the PCRs quoted last as the ones expected next time
+    - or, name profiles for validating firmware RTM PCRs and profiles
+      for validating operating system RTM PCRs
+
+A profile for validating PCRs should contain a set of expected extension
+values for each of a set of PCRs.  The attestation server can then check
+that the eventlog submitted by the client lists exactly those extension
+values and no others.  PCR extension order in the eventlog probably
+doesn't matter here.  If multiple profiles are named, then one of those
+must match -- this allows for upgrades and downgrades.
+
+```JSON
+{
+  "profile_name":"SomeProfile",
+  "values":[
+    {
+      "PCR":0,
+      "values":["aaaaaaa","bbbbbb","..."]
+    },
+    {
+      "PCR":1,
+      "values":["ccccccc","dddddd","..."]
+    }
+  ]
+}
+```
+
+Using the PCR values from the previous attestation makes upgrades
+tricky, probably requiring an authenticated and authorized administrator
+to bless new PCR values after an upgrade.  A client that presents a PCR
+quote that does not match the previous one would cause the
+`proposed_PCRs` field to be updated but otherwise could not continue,
+then an administrator would confirm that the client just did a
+firmware/OS upgrade and if so replace the `previous_PCRs` with the
+`proposed_PCRs`, then the client could attempt attestation again.
+
+## Dealing with Secrets
+
+An attestation server might want to return storage/filesystem decryption
+key-encryption-keys to a client.  But one might not want to store those
+keys in the clear on the attestation server.  As well, one might want a
+break-glass way to recover those secrets.
+
+For break-glass recover, the simplest thing to do is to store
+`Encrypt_backupKey({EKpub, hostname, secrets})`, where `backupKey` is an
+asymmetric key whose private key is stored offline (e.g., in a safe, or
+in an offline HSM).  To break the glass and recover the key, just bring
+the ciphertext to the offline system where the private backup key is
+kept, decrypt it, and then use the secrets manually to recover the
+affected system.
+
+Here are some ideas for how to make an attestation client depend on the
+attestation server giving it keys needed to continue booting after
+successful attestation:
+
+ - Store `TPM2_MakeCredential(EKpub, someObjectName, key0), Encrypt_key0(secrets)`.
+
+   In this mode the server sends the client the stored data, then client
+   gets to recreate `someObject` (possibly by loading a saved object) on
+   its TPM so that the corresponding call to `TPM2_ActivateCredential()`
+   can succeed, then the client recovers `key0` and decrypts the
+   encrypted secrets.  Here `someObject` can be trivial and need only
+   exist to make the `{Make,Activate}Credential` machinery work.
+
+   TPM replacement and/or migration of a host from one physical system
+   to another can be implemented by learning the new system's TPM's
+   EKpub and using the offline `backupKey` to compute
+   `TPM2_MakeCredential(EKpub_new, someObjectName, key0)` and update the
+   host's entry.
+
+ - Store a secret value that will be extended into an application PCR
+   that is used as a policy PCR for unsealing a persistent object stored
+   on the client's TPM.
+
+   In this mode the server sends the client the secret PCR extension
+   value, and the client uses it to extend a PCR such that it can then
+   unseal the real storage / filesystem decryption keys.
+
+ - A hybrid of the previous two options, where the server stores a
+   secret PCR extension value wrapped with `TPM2_MakeCredential()`.
+
+Other ideas?
 
 # References
 

Attestation/TPM2_Quote.md

@@ -0,0 +1,45 @@
+# `TPM2_Quote()`
+
+`TPM2_Quote()` computes a hash of the PCRs selected by the caller, and
+signs that hash, some additional metadata, and any extra data provided
+by the caller, with a signing key named by the caller.  The caller must
+have access to that key, naturally.
+
+The PCRs' values are NOT included in the quote produced by
+`TPM2_Quote()`.  Instead, an attestation service can review an unsigned
+eventlog to ensure it leads to the same values as unsigned PCR values
+also provided by the attestation client, and then the attestation
+service can verify that the hash of the PCR values is indeed signed by
+the quote supplied by the client.
+
+## Inputs
+
+ - `TPMI_DH_OBJECT sigHandle` (handle for an AK)
+ - `TPM2B_DATA qualifyingData` (extra data)
+ - `TPMT_SIG_SCHEME inScheme` ("signing scheme to use if the schemefor signHandleis `TPM_ALG_NULL`")
+ - `TPML_PCR_SELECTION PCRselect` (set of PCRs to quote)
+
+## Outputs (success case)
+
+ - `TPM2B_ATTEST quoted`
+ - `TPMT_SIGNATURE signature`
+
+Where `TPM2B_ATTEST` is basically a `TPMS_ATTEST`, which contains the
+following fields:
+
+ - `TPM_GENERATED magic`
+ - `TPMI_ST_ATTEST type`
+ - `TPM2B_NAME signer` (name of AK)
+ - `TPM2B_DATA extraData` ("external information supplied by caller")
+ - `TPMS_CLOCK_INFO clockInfo` ("Clock, resetCount, restartCount, and Safe")
+ - `UINT64 firmwareVersion`
+ - `TPMU_ATTEST attested`, a discriminated union with the
+   `TPMS_QUOTE_INFO` arm (indicated by the `TPM_ST_ATTEST_QUOTE`
+   discriminant value), which contains:
+    - `TPML_PCR_SELECTION pcrSelect` (the set of PCRs digested by `pcrDigest`)
+    - `TPM2B_DIGEST pcrDigest` (the digest of the PCRs indicated by `pcrSelect`)
+
+## References
+
+ - [TCG TPM Library part 3: Commands, section 18.4](https://trustedcomputinggroup.org/wp-content/uploads/TCG_TPM2_r1p59_Part3_Commands_pub.pdf)
+