HTTP Transport for Trusted Execution Environment Provisioning: Agent-to- TAM Communication
draft-ietf-teep-otrp-over-http-02

TEEP WG                                                        D. Thaler
Internet-Draft                                                 Microsoft
Intended status: Informational                          October 22, 2019
Expires: April 24, 2020

HTTP Transport for Trusted Execution Environment Provisioning: Agent-to-
                           TAM Communication
                   draft-ietf-teep-otrp-over-http-02

Abstract

   The Open Trust Protocol (OTrP) is used to manage code and
   configuration data in a Trusted Execution Environment (TEE).  This
   document specifies the HTTP transport for OTrP communication where a
   Trusted Application Manager (TAM) service is used to manage TEEs in
   devices that can initiate communication to the TAM.  An
   implementation of this document can (if desired) run outside of any
   TEE, but interacts with an OTrP implementation that runs inside a
   TEE.

Status of This Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
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   This Internet-Draft will expire on April 24, 2020.

Copyright Notice

   Copyright (c) 2019 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (http://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect

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   to this document.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   4
   3.  TEEP Broker Models  . . . . . . . . . . . . . . . . . . . . .   4
     3.1.  Use of Abstract APIs  . . . . . . . . . . . . . . . . . .   5
   4.  Use of HTTP as a Transport  . . . . . . . . . . . . . . . . .   6
   5.  OTrP/HTTP Client Behavior . . . . . . . . . . . . . . . . . .   7
     5.1.  Receiving a request to install a new Trusted Application    7
       5.1.1.  Session Creation  . . . . . . . . . . . . . . . . . .   7
     5.2.  Getting a message buffer back from an OTrP implementation   8
     5.3.  Receiving an HTTP response  . . . . . . . . . . . . . . .   8
     5.4.  Handling checks for policy changes  . . . . . . . . . . .   9
     5.5.  Error handling  . . . . . . . . . . . . . . . . . . . . .   9
   6.  OTrP/HTTP Server Behavior . . . . . . . . . . . . . . . . . .  10
     6.1.  Receiving an HTTP POST request  . . . . . . . . . . . . .  10
     6.2.  Getting an empty buffer back from the OTrP implementation  10
     6.3.  Getting a message buffer from the OTrP implementation . .  10
     6.4.  Error handling  . . . . . . . . . . . . . . . . . . . . .  10
   7.  Sample message flow . . . . . . . . . . . . . . . . . . . . .  10
   8.  Security Considerations . . . . . . . . . . . . . . . . . . .  12
   9.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  12
   10. References  . . . . . . . . . . . . . . . . . . . . . . . . .  13
     10.1.  Normative References . . . . . . . . . . . . . . . . . .  13
     10.2.  Informative References . . . . . . . . . . . . . . . . .  13
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .  14

1.  Introduction

   Trusted Execution Environments (TEEs), including environments based
   on Intel SGX, ARM TrustZone, Secure Elements, and others, enforce
   that only authorized code can execute within the TEE, and any memory
   used by such code is protected against tampering or disclosure
   outside the TEE.  The Open Trust Protocol (OTrP) is designed to
   provision authorized code and configuration into TEEs.

   To be secure against malware, an OTrP implementation (referred to as
   a TEEP "Agent" on the client side, and a "Trusted Application Manager
   (TAM)" on the server side) must themselves run inside a TEE.
   However, the transport for OTrP, along with the underlying TCP/IP
   stack, does not necessarily run inside a TEE.  This split allows the
   set of highly trusted code to be kept as small as possible, including

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   allowing code (e.g., TCP/IP) that only sees encrypted messages to be
   kept out of the TEE.

   The OTrP specification ([I-D.ietf-teep-opentrustprotocol] or
   [I-D.tschofenig-teep-otrp-v2]) describes the behavior of TEEP Agents
   and TAMs, but does not specify the details of the transport.  The
   purpose of this document is to provide such details.  That is, an
   OTrP over HTTP (OTrP/HTTP) implementation delivers messages up to an
   OTrP implementation, and accepts messages from the OTrP
   implementation to be sent over a network.  The OTrP over HTTP
   implementation can be implemented either outside a TEE (i.e., in a
   TEEP "Broker") or inside a TEE.

   There are two topological scenarios in which OTrP could be deployed:

   1.  TAMs are reachable on the Internet, and Agents are on networks
       that might be behind a firewall, so that communication must be
       initiated by an Agent.  Thus, the Agent has an HTTP Client and
       the TAM has an HTTP Server.

   2.  Agents are reachable on the Internet, and TAMs are on networks
       that might be behind a firewall, so that communication must be
       initiated by a TAM.  Thus, the Agent has an HTTP Server and the
       TAM has an HTTP Client.

   The remainder of this document focuses primarily on the first
   scenario as depicted in Figure 1, but some sections (Section 4 and
   Section 8) may apply to the second scenario as well.  A fuller
   discussion of the second scenario may be handled by a separate
   document.

      +------------------+           OTrP           +------------------+
      |    TEEP Agent    | <----------------------> |        TAM       |
      +------------------+                          +------------------+
               |                                              |
      +------------------+      OTrP over HTTP      +------------------+
      | OTrP/HTTP Client | <----------------------> | OTrP/HTTP Server |
      +------------------+                          +------------------+
               |                                              |
      +------------------+           HTTP           +------------------+
      |    HTTP Client   | <----------------------> |    HTTP Server   |
      +------------------+                          +------------------+

                   Figure 1: Agent-to-TAM Communication

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2.  Terminology

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
   "OPTIONAL" in this document are to be interpreted as described in BCP
   14 [RFC2119] [RFC8174] when, and only when, they appear in all
   capitals, as shown here.

   This document also uses various terms defined in
   [I-D.ietf-teep-architecture], including Trusted Execution Environment
   (TEE), Trusted Application (TA), Trusted Application Manager (TAM),
   TEEP Agent, TEEP Broker, and Rich Execution Environment (REE).

3.  TEEP Broker Models

   Section 6 of the TEEP architecture [I-D.ietf-teep-architecture]
   defines a TEEP "Broker" as being a component on the device, but
   outside the TEE, that facilitates communication with a TAM.  As
   depicted in Figure 2, there are multiple ways in which this can be
   implemented, with more or fewer layers being inside the TEE.  For
   example, in model A, the model with the smallest TEE footprint, only
   the OTrP implementation is inside the TEE, whereas the OTrP/HTTP
   implementation is in the TEEP Broker outside the TEE.

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                           Model:    A      B      C     ...

                                    TEE    TEE    TEE
        +----------------+           |      |      |
        |      OTrP      |     Agent |      |      | Agent
        | implementation |           |      |      |
        +----------------+           v      |      |
                 |                          |      |
        +----------------+           ^      |      |
        |    OTrP/HTTP   |    Broker |      |      |
        | implementation |           |      |      |
        +----------------+           |      v      |
                 |                   |             |
        +----------------+           |      ^      |
        |      HTTP      |           |      |      |
        | implementation |           |      |      |
        +----------------+           |      |      v
                 |                   |      |
        +----------------+           |      |      ^
        |   TCP or QUIC  |           |      |      | Broker
        | implementation |           |      |      |
        +----------------+           |      |      |
                                    REE    REE    REE

                       Figure 2: TEEP Broker Models

   In other models, additional layers are moved into the TEE, increasing
   the TEE footprint, with the Broker either containing or calling the
   topmost protocol layer outside of the TEE.  An implementation is free
   to choose any of these models, although model A is the one we will
   use in our examples.

   Passing information from an REE component to a TEE component is
   typically spoken of as being passed "in" to the TEE, and informaton
   passed in the opposite direction is spoken of as being passed "out".
   In the protocol layering sense, information is typically spoken of as
   being passed "up" or "down" the stack.  Since the layer at which
   information is passed in/out may vary by implementation, we will
   generally use "up" and "down" in this document.

3.1.  Use of Abstract APIs

   This document refers to various APIs between an OTrP implementation
   and an OTrP/HTTP implementation in the abstract, meaning the literal
   syntax and programming language are not specified, so that various
   concrete APIs can be designed (outside of the IETF) that are
   compliant.

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   Some TEE architectures (e.g., SGX) may support API calls both into
   and out of a TEE.  In other TEE architectures, there may be no calls
   out from a TEE, but merely data returned from calls into a TEE.  This
   document attempts to be agnostic as to the concrete API architecture
   for Broker/Agent communication.  Since in model A, the Broker/Agent
   communication is done at the layer between the OTrP and OTrP/HTTP
   implementations, and there may be some architectures that do not
   support calls out of the TEE (which would be downcalls from OTrP in
   model A), we will refer to passing information up to the OTrP
   implementation as API calls, but will simply refer to "passing data"
   back down from an OTrP implementation.  A concrete API might pass
   data back via an API downcall or via data returned from an API
   upcall.

   This document will also refer to passing "no" data back out of an
   OTrP implementation.  In a concrete API, this might be implemented by
   not making any downcall, or by returning 0 bytes from an upcall, for
   example.

4.  Use of HTTP as a Transport

   This document uses HTTP [I-D.ietf-httpbis-semantics] as a transport.
   When not called out explicitly in this document, all implementation
   recommendations in [I-D.ietf-httpbis-bcp56bis] apply to use of HTTP
   by OTrP.

   Redirects MAY be automatically followed, and no additional request
   headers beyond those specified by HTTP need be modified or removed
   upon a following such a redirect.

   Content is not intended to be treated as active by browsers and so
   HTTP responses with content SHOULD have the following headers as
   explained in Section 4.12 of [I-D.ietf-httpbis-bcp56bis] (replacing
   the content type with the relevant OTrP content type per the OTrP
   specification):

       Content-Type: <content type>
       Cache-Control: no-store
       X-Content-Type-Options: nosniff
       Content-Security-Policy: default-src 'none'
       Referrer-Policy: no-referrer

   Only the POST method is specified for TAM resources exposed over
   HTTP.  A URI of such a resource is referred to as a "TAM URI".  A TAM
   URI can be any HTTP(S) URI.  The URI to use is configured in a TEEP
   Agent via an out-of-band mechanism, as discussed in the next section.

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   When HTTPS is used, TLS certificates MUST be checked according to
   [RFC2818].

5.  OTrP/HTTP Client Behavior

5.1.  Receiving a request to install a new Trusted Application

   In some environments, an application installer can determine (e.g.,
   from an app manifest) that the application being installed or updated
   has a dependency on a given Trusted Application (TA) being available
   in a given type of TEE.  In such a case, it will notify a TEEP
   Broker, where the notification will contain the following:

   -  A unique identifier of the TA

   -  Optionally, any metadata to provide to the OTrP implementation.
      This might include a TAM URI provided in the application manifest,
      for example.

   -  Optionally, any requirements that may affect the choice of TEE, if
      multiple are available to the TEEP Broker.

   When a TEEP Broker receives such a notification, it first identifies
   in an implementation-dependent way which TEE (if any) is most
   appropriate based on the constraints expressed.  If there is only one
   TEE, the choice is obvious.  Otherwise, the choice might be based on
   factors such as capabilities of available TEE(s) compared with TEE
   requirements in the notification.  Once the TEEP Broker picks a TEE,
   it passes the notification to the OTrP/HTTP Cient for that TEE.

   The OTrP/HTTP Client then informs the OTrP implementation in that TEE
   by invoking an appropriate "RequestTA" API that identifies the TA
   needed and any other associated metadata.  The OTrP/HTTP Client need
   not know whether the TEE already has such a TA installed or whether
   it is up to date.

   The OTrP implementation will either (a) pass no data back, (b) pass
   back a TAM URI to connect to, or (c) pass back a message buffer and
   TAM URI to send it to.  The TAM URI passed back may or may not be the
   same as the TAM URI, if any, provided by the OTrP/HTTP Client,
   depending on the OTrP implementation's configuration.  If they
   differ, the OTrP/HTTP Client MUST use the TAM URI passed back.

5.1.1.  Session Creation

   If no data is passed back, the OTrP/HTTP Client simply informs its
   caller (e.g., the application installer) of success.

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   If the OTrP implementation passes back a TAM URI with no message
   buffer, the OTrP/HTTP Client attempts to create session state, then
   sends an HTTP(S) POST to the TAM URI with an Accept header and an
   empty body.  The HTTP request is then associated with the OTrP/HTTP
   Client's session state.

   If the OTrP implementation instead passes back a TAM URI with a
   message buffer, the OTrP/HTTP Client attempts to create session state
   and handles the message buffer as specified in Section 5.2.

   Session state consists of:

   -  Any context (e.g., a handle) that identifies the API session with
      the OTrP implementation.

   -  Any context that identifies an HTTP request, if one is
      outstanding.  Initially, none exists.

5.2.  Getting a message buffer back from an OTrP implementation

   When an OTrP implementation passes a message buffer (and TAM URI) to
   an OTrP/HTTP Client, the OTrP/HTTP Client MUST do the following,
   using the OTrP/HTTP Client's session state associated with its API
   call to the OTrP implementation.

   The OTrP/HTTP Client sends an HTTP POST request to the TAM URI with
   Accept and Content-Type headers with the OTrP media type in use, and
   a body containing the OTrP message buffer provided by the OTrP
   implementation.  The HTTP request is then associated with the OTrP/
   HTTP Client's session state.

5.3.  Receiving an HTTP response

   When an HTTP response is received in response to a request associated
   with a given session state, the OTrP/HTTP Client MUST do the
   following.

   If the HTTP response body is empty, the OTrP/HTTP Client's task is
   complete, and it can delete its session state, and its task is done.

   If instead the HTTP response body is not empty, the OTrP/HTTP Client
   calls a "ProcessOTrPMessage" API (Section 6.2 of
   [I-D.ietf-teep-opentrustprotocol]) to pass the response body up to
   the OTrP implementation associated with the session.  The OTrP
   implementation will then either pass no data back, or pass back a
   message buffer.

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   If no data is passed back, the OTrP/HTTP Client's task is complete,
   and it can delete its session state, and inform its caller (e.g., the
   application installer) of success.

   If instead the OTrP implementation passes back a message buffer, the
   OTrP/HTTP Client handles the message buffer as specified in
   Section 5.2.

5.4.  Handling checks for policy changes

   An implementation MUST provide a way to periodically check for OTrP
   policy changes.  This can be done in any implementation-specific
   manner, such as:

   A) The OTrP/HTTP Client might call up to the OTrP implementation at
   an interval previously specified by the OTrP implementation.  This
   approach requires that the OTrP/HTTP Client be capable of running a
   periodic timer.

   B) The OTrP/HTTP Client might be informed when an existing TA is
   invoked, and call up to the OTrP implementation if more time has
   passed than was previously specified by the OTrP implementation.
   This approach allows the device to go to sleep for a potentially long
   period of time.

   C) The OTrP/HTTP Client might be informed when any attestation
   attempt determines that the device is out of compliance, and call up
   to the OTrP implementation to remediate.

   The OTrP/HTTP Client informs the OTrP implementation by invoking an
   appropriate "RequestPolicyCheck" API.  The OTrP implementation will
   either (a) pass no data back, (b) pass back a TAM URI to connect to,
   or (c) pass back a message buffer and TAM URI to send it to.
   Processing then continues as specified in Section 5.1.1.

5.5.  Error handling

   If any local error occurs where the OTrP/HTTP Client cannot get a
   message buffer (empty or not) back from the OTrP implementation, the
   OTrP/HTTP Client deletes its session state, and informs its caller
   (e.g., the application installer) of a failure.

   If any HTTP request results in an HTTP error response or a lower
   layer error (e.g., network unreachable), the OTrP/HTTP Client calls
   the OTrP implementation's "ProcessError" API, and then deletes its
   session state and informs its caller of a failure.

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6.  OTrP/HTTP Server Behavior

6.1.  Receiving an HTTP POST request

   When an HTTP POST request is received with an empty body, the OTrP/
   HTTP Server invokes the TAM's "ProcessConnect" API.  The TAM will
   then pass back a (possibly empty) message buffer.

   When an HTTP POST request is received with a non-empty body, the
   OTrP/HTTP Server calls the TAM's "ProcessOTrPMessage" API to pass it
   the request body.  The TAM will then pass back a (possibly empty)
   message buffer.

6.2.  Getting an empty buffer back from the OTrP implementation

   If the OTrP implementation passes back an empty buffer, the OTrP/HTTP
   Server sends a successful (2xx) response with no body.

6.3.  Getting a message buffer from the OTrP implementation

   If the OTrP implementation passes back a non-empty buffer, the OTrP/
   HTTP Server generates a successful (2xx) response with a Content-Type
   header with the OTrP media type in use, and with the message buffer
   as the body.

6.4.  Error handling

   If any error occurs where the OTrP/HTTP Server cannot get a message
   buffer (empty or not) back from the OTrP implementation, the OTrP/
   HTTP Server generates an appropriate HTTP error response.

7.  Sample message flow

   The following shows a sample OTrP message flow that uses application/
   otrp+json as the Content-Type.

   1.   An application installer determines (e.g., from an app manifest)
        that the application has a dependency on TA "X", and passes this
        notification to the TEEP Broker.  The TEEP Broker picks a TEE
        (e.g., the only one available) based on this notification, and
        passes the information to the OTrP/HTTP Cient for that TEE.

   2.   The OTrP/HTTP Client calls the OTrP implementation's "RequestTA"
        API, passing TA Needed = X.

   3.   The OTrP implementation finds that no such TA is already
        installed, but that it can be obtained from a given TAM.  The
        TEEP Agent passes the TAM URI (e.g., "https://example.com/tam")

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        to the OTrP/HTTP Client.  (If the OTrP implementation already
        had a cached TAM certificate that it trusts, it could skip to
        step 9 instead and generate a GetDeviceStateResponse.)

   4.   The OTrP/HTTP Client sends an HTTP POST request to the TAM URI:

           POST /tam HTTP/1.1
           Host: example.com
           Accept: application/otrp+json
           Content-Length: 0
           User-Agent: Foo/1.0

   5.   On the TAM side, the OTrP/HTTP Server receives the HTTP POST
        request, and calls the OTrP implementation's "ProcessConnect"
        API.

   6.   The OTrP implementation generates an OTrP message (where
        typically GetDeviceStateRequest is the first message) and passes
        it to the OTrP/HTTP Server.

   7.   The OTrP/HTTP Server sends an HTTP successful response with the
        OTrP message in the body:

           HTTP/1.1 200 OK
           Content-Type: application/otrp+json
           Content-Length: [length of OTrP message here]
           Server: Bar/2.2
           Cache-Control: no-store
           X-Content-Type-Options: nosniff
           Content-Security-Policy: default-src 'none'
           Referrer-Policy: no-referrer

           [OTrP message here]

   8.   Back on the TEEP Agent side, the OTrP/HTTP Client gets the HTTP
        response, extracts the OTrP message and calls the OTrP
        implementation's "ProcessOTrPMessage" API to pass it the
        message.

   9.   The OTrP implementation processes the OTrP message, and
        generates an OTrP response (e.g., GetDeviceStateResponse) which
        it passes back to the OTrP/HTTP Client.

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   10.  The OTrP/HTTP Client gets the OTrP message buffer and sends an
        HTTP POST request to the TAM URI, with the OTrP message in the
        body:

          POST /tam HTTP/1.1
          Host: example.com
          Accept: application/otrp+json
          Content-Type: application/otrp+json
          Content-Length: [length of OTrP message here]
          User-Agent: Foo/1.0

          [OTrP message here]

   11.  The OTrP/HTTP Server receives the HTTP POST request, and calls
        the OTrP implementation's "ProcessOTrPMessage" API.

   12.  Steps 6-11 are then repeated until the OTrP implementation
        passes no data back to the OTrP/HTTP Server in step 6.

   13.  The OTrP/HTTP Server sends an HTTP successful response with no
        body:

          HTTP/1.1 204 No Content
          Server: Bar/2.2

   14.  The OTrP/HTTP Client deletes its session state.

8.  Security Considerations

   Although OTrP is protected end-to-end inside of HTTP, there is still
   value in using HTTPS for transport, since HTTPS can provide
   additional protections as discussed in Section 6 of
   [I-D.ietf-httpbis-bcp56bis].  As such, OTrP/HTTP implementations MUST
   support HTTPS.  The choice of HTTP vs HTTPS at runtime is up to
   policy, where an administrator configures the TAM URI to be used, but
   it is expected that real deployments will always use HTTPS TAM URIs.

9.  IANA Considerations

   This document has no actions for IANA.

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10.  References

10.1.  Normative References

   [I-D.ietf-httpbis-semantics]
              Fielding, R., Nottingham, M., and J. Reschke, "HTTP
              Semantics", draft-ietf-httpbis-semantics-05 (work in
              progress), July 2019.

   [I-D.ietf-teep-opentrustprotocol]
              Pei, M., Atyeo, A., Cook, N., Yoo, M., and H. Tschofenig,
              "The Open Trust Protocol (OTrP)", draft-ietf-teep-
              opentrustprotocol-03 (work in progress), May 2019.

   [I-D.tschofenig-teep-otrp-v2]
              Pei, M., Tschofenig, H., and D. Wheeler, "The Open Trust
              Protocol (OTrP) v2", draft-tschofenig-teep-otrp-v2-00
              (work in progress), July 2019.

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997, <https://www.rfc-
              editor.org/info/rfc2119>.

   [RFC2818]  Rescorla, E., "HTTP Over TLS", RFC 2818,
              DOI 10.17487/RFC2818, May 2000, <https://www.rfc-
              editor.org/info/rfc2818>.

   [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
              2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
              May 2017, <https://www.rfc-editor.org/info/rfc8174>.

10.2.  Informative References

   [I-D.ietf-httpbis-bcp56bis]
              Nottingham, M., "Building Protocols with HTTP", draft-
              ietf-httpbis-bcp56bis-08 (work in progress), November
              2018.

   [I-D.ietf-teep-architecture]
              Pei, M., Tschofenig, H., Wheeler, D., Atyeo, A., and D.
              Liu, "Trusted Execution Environment Provisioning (TEEP)
              Architecture", draft-ietf-teep-architecture-03 (work in
              progress), July 2019.

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Author's Address

   Dave Thaler
   Microsoft

   EMail: dthaler@microsoft.com

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