TEEP WG D. Thaler Internet-Draft Microsoft Intended status: Informational June 21, 2019 Expires: December 23, 2019 HTTP Transport for the Open Trust Protocol (OTrP) draft-ietf-teep-otrp-over-http-00 Abstract This document specifies the HTTP transport for the Open Trust Protocol (OTrP), which is used to manage code and configuration data in a Trusted Execution Environment (TEE). An implementation of this document can 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 Task Force (IETF). Note that other groups may also distribute working documents as Internet-Drafts. The list of current Internet- Drafts is at http://datatracker.ietf.org/drafts/current/. Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as "work in progress." This Internet-Draft will expire on December 23, 2019. 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 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. Thaler Expires December 23, 2019 [Page 1] Internet-Draft OTrP HTTP Transport June 2019 Table of Contents 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2 2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 3 3. Use of Abstract APIs . . . . . . . . . . . . . . . . . . . . 3 4. Use of HTTP as a Transport . . . . . . . . . . . . . . . . . 3 5. Client Broker Behavior . . . . . . . . . . . . . . . . . . . 4 5.1. Receiving a request to install a new Trusted Application 4 5.1.1. Session Creation . . . . . . . . . . . . . . . . . . 5 5.2. Getting a message buffer back from an OTrP Agent . . . . 5 5.3. Receiving an HTTP response . . . . . . . . . . . . . . . 6 5.4. Handling checks for policy changes . . . . . . . . . . . 6 5.5. Error handling . . . . . . . . . . . . . . . . . . . . . 7 6. Server Broker Behavior . . . . . . . . . . . . . . . . . . . 7 6.1. Receiving an HTTP POST request . . . . . . . . . . . . . 7 6.2. Getting an empty buffer back from the TAM . . . . . . . . 7 6.3. Getting a message buffer from the TAM . . . . . . . . . . 7 6.4. Error handling . . . . . . . . . . . . . . . . . . . . . 7 7. Sample message flow . . . . . . . . . . . . . . . . . . . . . 7 8. Security Considerations . . . . . . . . . . . . . . . . . . . 9 9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 10 10. References . . . . . . . . . . . . . . . . . . . . . . . . . 11 10.1. Normative References . . . . . . . . . . . . . . . . . . 11 10.2. Informative References . . . . . . . . . . . . . . . . . 11 Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 12 1. Introduction Trusted Execution Environments (TEEs), including 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 an OTrP "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 typical networking stacks, need not run inside a TEE. This split allows the set of highly trusted code to be kept as small as possible, including 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] describes the behavior of OTrP Agents and TAMs, but does not specify the details of the transport, an implementation of which is referred to as a "Broker". The purpose of this document is to provide such Thaler Expires December 23, 2019 [Page 2] Internet-Draft OTrP HTTP Transport June 2019 details. That is, the HTTP transport for OTrP is implemented in a Broker (typically outside a TEE) that delivers messages up to an OTrP implementation, and accepts messages from the OTrP implementation to be sent over a network. 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), Agent, and Broker. 3. Use of Abstract APIs This document refers to various APIs between a Broker and an OTrP 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. It is common in some TEE architectures (e.g., SGX) to refer to calls into a Trusted Application (TA) as "ECALLs" (or enclave-calls), and calls out from a Trusted Application (TA) as "OCALLs" (or out-calls). In other TEE architectures, there may be no OCALLs, but merely data returned from calls into a TA. This document attempts to be agnostic as to the concrete API architecture. As such, abstract APIs used in this document will refer to calls into a TA as API calls, and will simply refer to "passing data" back out of the TA. A concrete API might pass data back via an OCALL or via data returned from an API call. This document will also refer to passing "no" data back out of a TA. In an OCALL-based architecture, this might be implemented by not making any such call. In a return-based architecture, this might be implemented by returning 0 bytes. 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. Thaler Expires December 23, 2019 [Page 3] Internet-Draft OTrP HTTP Transport June 2019 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]: Content-Type: application/otrp+json 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 an OTrP Agent via an out-of-band mechanism, as discussed in the next section. When HTTPS is used, TLS certificates MUST be checked according to [RFC2818]. 5. Client Broker Behavior 5.1. Receiving a request to install a new Trusted Application When the Broker receives a notification (e.g., from an application installer) that an application has a dependency on a given Trusted Application (TA) being available in a given type of TEE, the notification will contain the following: - A unique identifier of the TA - Optionally, any metadata to pass to the OTrP Agent. 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 Broker. When such a notification is received, the Broker 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. Thaler Expires December 23, 2019 [Page 4] Internet-Draft OTrP HTTP Transport June 2019 The Broker then informs the OTrP Agent in that TEE by invoking an appropriate "RequestTA" API that identifies the TA needed and any other associated metadata. The Broker need not know whether the TEE already has such a TA installed or whether it is up to date. The OTrP Agent 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 broker, depending on the OTrP Agent's configuration. If they differ, the Broker MUST use the TAM URI passed back. 5.1.1. Session Creation If no data is passed back, the Broker simply informs its client (e.g., the application installer) of success. If the OTrP Agent passes back a TAM URI with no message buffer, the TEEP Broker attempts to create session state, then sends an HTTP(S) POST to the TAM URI with an "Accept: application/otrp+json" header and an empty body. The HTTP request is then associated with the Broker's session state. If the OTrP Agent instead passes back a TAM URI with a message buffer, the TEEP Broker 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 Agent. - Any context that identifies an HTTP request, if one is outstanding. Initially, none exists. 5.2. Getting a message buffer back from an OTrP Agent When a message buffer (and TAM URI) is passed to a Broker from an OTrP Agent, the Broker MUST do the following, using the Broker's session state associated with its API call to the OTrP Agent. The Broker sends an HTTP POST request to the TAM URI with "Accept: application/otrp+json" and "Content-Type: application/otrp+json" headers, and a body containing the OTrP message buffer provided by the OTrP Agent. The HTTP request is then associated with the Broker's session state. Thaler Expires December 23, 2019 [Page 5] Internet-Draft OTrP HTTP Transport June 2019 5.3. Receiving an HTTP response When an HTTP response is received in response to a request associated with a given session state, the Broker MUST do the following. If the HTTP response body is empty, the Broker'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 Broker calls a "ProcessOTrPMessage" API (Section 6.2 of [I-D.ietf-teep-opentrustprotocol]) to pass the response body to the OTrP Agent associated with the session. The OTrP Agent will then pass no data back, or pass pack a message buffer. If no data is passed back, the Broker's task is complete, and it can delete its session state, and inform its client (e.g., the application installer) of success. If instead the OTrP Agent passes back a message buffer, the TEEP Broker 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 Broker might call into the OTrP Agent at an interval previously specified by the OTrP Agent. This approach requires that the Broker be capable of running a periodic timer. B) The Broker might be informed when an existing TA is invoked, and call into the OTrP Agent if more time has passed than was previously specified by the OTrP Agent. This approach allows the device to go to sleep for a potentially long period of time. C) The Broker might be informed when any attestation attempt determines that the device is out of compliance, and call into the OTrP Agent to remediate. The Broker informs the OTrP Agent by invoking an appropriate "RequestPolicyCheck" API. The OTrP Agent 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. Thaler Expires December 23, 2019 [Page 6] Internet-Draft OTrP HTTP Transport June 2019 5.5. Error handling If any local error occurs where the Broker cannot get a message buffer (empty or not) back from the OTrP Agent, the Broker deletes its session state, and informs its client (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 Broker calls the OTrP Agent's "ProcessError" API, and then deletes its session state and informs its client of a failure. 6. Server Broker Behavior 6.1. Receiving an HTTP POST request When an HTTP POST request is received with an empty body, the Broker 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 Broker 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 TAM If the TAM passes back an empty buffer, the Broker sends a successful (2xx) response with no body. 6.3. Getting a message buffer from the TAM If the TAM passes back a non-empty buffer, the Broker generates a successful (2xx) response with a "Content-Type: application/ otrp+json" header, and with the message buffer as the body. 6.4. Error handling If any error occurs where the Broker cannot get a message buffer (empty or not) back from the TAM, the Broker generates an appropriate HTTP error response. 7. Sample message flow 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 Client Broker. The Client Broker picks an Thaler Expires December 23, 2019 [Page 7] Internet-Draft OTrP HTTP Transport June 2019 OTrP Agent (e.g., the only one available) based on this notification. 2. The Client Broker calls the OTrP Agent's "RequestTA" API, passing TA Needed = X. 3. The OTrP Agent finds that no such TA is already installed, but that it can be obtained from a given TAM. The OTrP Agent passes the TAM URI (e.g., "https://example.com/tam") to the Client Broker. (If the OTrP Agent already had a cached TAM certificate that it trusts, it could skip to step 9 instead and generate a GetDeviceStateResponse.) 4. The Client Broker 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. The Server Broker receives the HTTP POST request, and calls the TAM's "ProcessConnect" API. 6. The TAM generates an OTrP message (typically GetDeviceStateRequest is the first message) and passes it to the Server Broker. 7. The Server Broker 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] Thaler Expires December 23, 2019 [Page 8] Internet-Draft OTrP HTTP Transport June 2019 8. The Client Broker gets the HTTP response, extracts the OTrP message and calls the OTrP Agent's "ProcessOTrPMessage" API to pass it the message. 9. The OTrP Agent processes the OTrP message, and generates an OTrP response (e.g., GetDeviceStateResponse) which it passes back to the Client Broker. 10. The Client Broker 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 Server Broker receives the HTTP POST request, and calls the TAM's "ProcessOTrPMessage" API. 12. Steps 6-11 are then repeated until the TAM passes no data back to the Server Broker in step 6. 13. The Server Broker sends an HTTP successful response with no body: HTTP/1.1 204 No Content Server: Bar/2.2 14. The Client Broker 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, Broker 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. Thaler Expires December 23, 2019 [Page 9] Internet-Draft OTrP HTTP Transport June 2019 9. IANA Considerations [[NOTE: This section should probably be moved to the OTrP spec.]] This section requests that IANA assign the "application/otrp+json" media type. Type name: application Subtype name: otrp+json Required parameters: none Optional parameters: none Encoding considerations: Same as encoding considerations of application/json as specified in Section 11 of [RFC7159]. Security considerations: See Section 12 of [RFC7159] and Section 8 of this document. Interoperability considerations: Same as interoperability considerations of application/json as specified in [RFC7159]. Published specification: [I-D.ietf-teep-opentrustprotocol] Applications that use this media type: OTrP implementations. Fragment identifier considerations: N/A Additional information: Deprecated alias names for this type: N/A Magic number(s): N/A File extension(s): N/A Macintosh file type code(s): N/A Person & email address to contact for further information: teep@ietf.org Intended usage: COMMON Restrictions on usage: none Author: See the "Authors' Addresses" section of this document. Change controller: IETF Thaler Expires December 23, 2019 [Page 10] Internet-Draft OTrP HTTP Transport June 2019 10. References 10.1. Normative References [I-D.ietf-httpbis-semantics] Fielding, R., Nottingham, M., and J. Reschke, "HTTP Semantics", draft-ietf-httpbis-semantics-04 (work in progress), March 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. [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/RFC2119, March 1997, . [RFC2818] Rescorla, E., "HTTP Over TLS", RFC 2818, DOI 10.17487/RFC2818, May 2000, . [RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174, May 2017, . 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-02 (work in progress), March 2019. [RFC7159] Bray, T., Ed., "The JavaScript Object Notation (JSON) Data Interchange Format", RFC 7159, DOI 10.17487/RFC7159, March 2014, . Thaler Expires December 23, 2019 [Page 11] Internet-Draft OTrP HTTP Transport June 2019 Author's Address David Thaler Microsoft EMail: dthaler@microsoft.com Thaler Expires December 23, 2019 [Page 12]