Internet DRAFT - draft-schinazi-masque-obfuscation

draft-schinazi-masque-obfuscation







Network Working Group                                        D. Schinazi
Internet-Draft                                                Google LLC
Intended status: Experimental                         September 10, 2020
Expires: March 14, 2021


                           MASQUE Obfuscation
                  draft-schinazi-masque-obfuscation-03

Abstract

   This document describes MASQUE Obfuscation.  MASQUE Obfuscation is a
   mechanism that allows co-locating and obfuscating networking
   applications behind an HTTPS web server.  The currently prevalent
   use-case is to allow running a proxy or VPN server that is
   indistinguishable from an HTTPS server to any unauthenticated
   observer.  We do not expect major providers and CDNs to deploy this
   behind their main TLS certificate, as they are not willing to take
   the risk of getting blocked, as shown when domain fronting was
   blocked.  An expected use would be for individuals to enable this
   behind their personal websites via easy to configure open-source
   software.

   This document is a straw-man proposal.  It does not contain enough
   details to implement the protocol, and is currently intended to spark
   discussions on the approach it is taking.  Discussion of this work is
   encouraged to happen on the MASQUE IETF mailing list masque@ietf.org
   or on the GitHub repository which contains the draft:
   https://github.com/DavidSchinazi/masque-drafts.

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 https://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 March 14, 2021.





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Copyright Notice

   Copyright (c) 2020 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 (https://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.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
     1.1.  Conventions and Definitions . . . . . . . . . . . . . . .   3
   2.  Usage Scenarios . . . . . . . . . . . . . . . . . . . . . . .   3
     2.1.  Protection from Network Providers . . . . . . . . . . . .   3
     2.2.  Protection from Web Servers . . . . . . . . . . . . . . .   4
     2.3.  Onion Routing . . . . . . . . . . . . . . . . . . . . . .   4
   3.  Requirements  . . . . . . . . . . . . . . . . . . . . . . . .   4
     3.1.  Invisibility of Usage . . . . . . . . . . . . . . . . . .   4
     3.2.  Invisibility of the Server  . . . . . . . . . . . . . . .   4
     3.3.  Fallback to HTTP/2 over TLS over TCP  . . . . . . . . . .   5
   4.  Overview of the Mechanism . . . . . . . . . . . . . . . . . .   5
   5.  Connection Resumption . . . . . . . . . . . . . . . . . . . .   5
   6.  Path MTU Discovery  . . . . . . . . . . . . . . . . . . . . .   5
   7.  Operation over HTTP/2 . . . . . . . . . . . . . . . . . . . .   6
   8.  Security Considerations . . . . . . . . . . . . . . . . . . .   6
     8.1.  Traffic Analysis  . . . . . . . . . . . . . . . . . . . .   6
     8.2.  Untrusted Servers . . . . . . . . . . . . . . . . . . . .   7
   9.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   7
   10. References  . . . . . . . . . . . . . . . . . . . . . . . . .   7
     10.1.  Normative References . . . . . . . . . . . . . . . . . .   7
     10.2.  Informative References . . . . . . . . . . . . . . . . .   8
   Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . .   9
   Design Justifications . . . . . . . . . . . . . . . . . . . . . .  10
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .  11











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1.  Introduction

   This document describes MASQUE Obfuscation.  MASQUE Obfuscation is a
   mechanism that allows co-locating and obfuscating networking
   applications behind an HTTPS web server.  The currently prevalent
   use-case is to allow running a proxy or VPN server that is
   indistinguishable from an HTTPS server to any unauthenticated
   observer.  We do not expect major providers and CDNs to deploy this
   behind their main TLS certificate, as they are not willing to take
   the risk of getting blocked, as shown when domain fronting was
   blocked.  An expected use would be for individuals to enable this
   behind their personal websites via easy to configure open-source
   software.

   This document is a straw-man proposal.  It does not contain enough
   details to implement the protocol, and is currently intended to spark
   discussions on the approach it is taking.  Discussion of this work is
   encouraged to happen on the MASQUE IETF mailing list masque@ietf.org
   or on the GitHub repository which contains the draft:
   https://github.com/DavidSchinazi/masque-drafts.

   MASQUE Obfuscation is built upon the MASQUE protocol [MASQUE].
   MASQUE Obfuscation leverages the efficient head-of-line blocking
   prevention features of the QUIC transport protocol [QUIC] when MASQUE
   Obfuscation is used in an HTTP/3 [HTTP3] server.  MASQUE Obfuscation
   can also run in an HTTP/2 server [HTTP2] but at a performance cost.

1.1.  Conventions and Definitions

   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.

2.  Usage Scenarios

   There are currently multiple usage scenarios that can benefit from
   MASQUE Obfuscation.

2.1.  Protection from Network Providers

   Some users may wish to obfuscate the destination of their network
   traffic from their network provider.  This prevents network providers
   from using data harvested from this network traffic in ways the user
   did not intend.





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2.2.  Protection from Web Servers

   There are many clients who would rather not establish a direct
   connection to web servers, for example to avoid location tracking.
   The clients can do that by running their traffic through a MASQUE
   Obfuscation server.  The web server will only see the IP address of
   the MASQUE Obfuscation server, not that of the client.

2.3.  Onion Routing

   Routing traffic through a MASQUE Obfuscation server only provides
   partial protection against tracking, because the MASQUE Obfuscation
   server knows the address of the client.  Onion routing as it exists
   today mitigates this issue for TCP/TLS.  A MASQUE Obfuscation server
   could allow onion routing over QUIC.

   In this scenario, the client establishes a connection to the MASQUE
   Obfuscation server, then through that to another MASQUE Obfuscation
   server, etc.  This creates a tree of MASQUE servers rooted at the
   client.  QUIC connections are mapped to a specific branch of the
   tree.  The first MASQUE Obfuscation server knows the actual address
   of the client, but the other MASQUE Obfuscation servers only know the
   address of the previous server.  To assure reasonable privacy, the
   path should include at least 3 MASQUE Obfuscation servers.

3.  Requirements

   This section describes the goals and requirements chosen for MASQUE
   Obfuscation.

3.1.  Invisibility of Usage

   An authenticated client using MASQUE Obfuscation appears to observers
   as a regular HTTPS client.  Observers only see that HTTP/3 or HTTP/2
   is being used over an encrypted channel.  No part of the exchanges
   between client and server may stick out.  Note that traffic analysis
   is discussed in Section 8.1.

3.2.  Invisibility of the Server

   To anyone without private keys, the server is indistinguishable from
   a regular web server.  It is impossible to send an unauthenticated
   probe that the server would reply to differently than if it were a
   normal web server.







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3.3.  Fallback to HTTP/2 over TLS over TCP

   When QUIC is blocked, MASQUE Obfuscation can run over TCP and still
   satisfy previous requirements.  Note that in this scenario
   performance may be negatively impacted.

4.  Overview of the Mechanism

   The server runs an HTTPS server on port 443, and has a valid TLS
   certificate for its domain.  The client has a public/private key
   pair, and the server maintains a list of authorized MASQUE
   Obfuscation clients, and their public key.  (Alternatively, clients
   can also be authenticated using a shared secret.)  The client starts
   by establishing a regular HTTPS connection to the server (HTTP/3 over
   QUIC or HTTP/2 over TLS 1.3 [TLS13] over TCP), and validates the
   server's TLS certificate as it normally would for HTTPS.  If
   validation fails, the connection is aborted.  At this point the
   client can send regular unauthenticated HTTP requests to the server.
   When it wishes to start MASQUE Obfuscation, the client uses HTTP
   Transport Authentication [TRANSPORT-AUTH] to prove its possession of
   its associated key.  The client sends the Transport-Authentication
   header alongside its MASQUE Negotiation request.

   When the server receives the MASQUE Negotiation request, it
   authenticates the client and if that fails responds with code "404
   Not Found", making sure its response is the same as what it would
   return for any unexpected POST request.  If authentication succeeds,
   the server sends its list of supported MASQUE applications and the
   client can start using them.

5.  Connection Resumption

   Clients MUST NOT attempt to "resume" MASQUE Obfuscation state
   similarly to how TLS sessions can be resumed.  Every new QUIC or TLS
   connection requires fully authenticating the client and server.  QUIC
   0-RTT and TLS early data MUST NOT be used with MASQUE Obfuscation as
   they are not forward secure.

6.  Path MTU Discovery

   In the main deployment of this mechanism, QUIC will be used between
   client and server, and that will most likely be the smallest MTU link
   in the path due to QUIC header and authentication tag overhead.  The
   client is responsible for not sending overly large UDP packets and
   notifying the server of the low MTU.  Therefore PMTUD is currently
   seen as out of scope of this document.





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7.  Operation over HTTP/2

   We will need to define the details of how to run MASQUE over HTTP/2.
   When running over HTTP/2, MASQUE uses the Extended CONNECT method to
   negotiate the use of datagrams over an HTTP/2 stream
   [HTTP2-TRANSPORT].

   MASQUE Obfuscation implementations SHOULD discover that HTTP/3 is
   available (as opposed to only HTTP/2) using the same mechanism as
   regular HTTP traffic.  This current standardized mechanism for this
   is HTTP Alternative Services [ALT-SVC], but future mechanisms such as
   [HTTPSSVC] can be used if they become widespread.

   MASQUE Obfuscation implementations using HTTP/3 MUST support the
   fallback to HTTP/2 to avoid incentivizing censors to block HTTP/3 or
   QUIC.

   When the client wishes to use the "UDP Proxying" MASQUE application
   over HTTP/2, the client opens a new stream with a CONNECT request to
   the "masque-udp-proxy" protocol and then sends datagrams encapsulated
   inside the stream with a two-byte length prefix in network byte
   order.  The target IP and port are sent as part of the URL query.
   Resetting that stream instructs the server to release any associated
   resources.

   When the client wishes to use the "IP Proxying" MASQUE application
   over HTTP/2, the client opens a new stream with a CONNECT request to
   the "masque-ip-proxy" protocol and then sends IP datagrams with a two
   byte length prefix.  The server can inspect the IP datagram to look
   for the destination address in the IP header.

8.  Security Considerations

   Here be dragons.  TODO: slay the dragons.

8.1.  Traffic Analysis

   While MASQUE Obfuscation ensures that proxied traffic appears similar
   to regular HTTP traffic, it doesn't inherently defeat traffic
   analysis.  However, the fact that MASQUE leverages QUIC allows it to
   segment STREAM frames over multiple packets and add PADDING frames to
   change the observable characteristics of its encrypted traffic.  The
   exact details of how to change traffic patterns to defeat traffic
   analysis is considered an open research question and is out of scope
   for this document.






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   When multiple MASQUE Obfuscation servers are available, a client can
   leverage QUIC connection migration to seamlessly transition its end-
   to-end QUIC connections by treating separate MASQUE Obfuscation
   servers as different paths.  This could afford an additional level of
   obfuscation in hopes of rendering traffic analysis less effective.

8.2.  Untrusted Servers

   As with any proxy or VPN technology, MASQUE Obfuscation hides some of
   the client's private information (such as who they are communicating
   with) from their network provider by transferring that information to
   the MASQUE server.  It is paramount that clients only use MASQUE
   Obfuscation servers that they trust, as a malicious actor could
   easily setup a MASQUE Obfuscation server and advertise it as a
   privacy solution in hopes of attracting users to send it their
   traffic.

9.  IANA Considerations

   We will need to register the "masque-udp-proxy" and "masque-ip-proxy"
   extended HTTP CONNECT protocols.

10.  References

10.1.  Normative References

   [ALT-SVC]  Nottingham, M., McManus, P., and J. Reschke, "HTTP
              Alternative Services", RFC 7838, DOI 10.17487/RFC7838,
              April 2016, <https://www.rfc-editor.org/info/rfc7838>.

   [HTTP2]    Belshe, M., Peon, R., and M. Thomson, Ed., "Hypertext
              Transfer Protocol Version 2 (HTTP/2)", RFC 7540,
              DOI 10.17487/RFC7540, May 2015,
              <https://www.rfc-editor.org/info/rfc7540>.

   [HTTP2-TRANSPORT]
              Kinnear, E. and T. Pauly, "Using HTTP/2 as a Transport for
              Arbitrary Bytestreams", Work in Progress, Internet-Draft,
              draft-kinnear-httpbis-http2-transport-02, November 4,
              2019, <http://www.ietf.org/internet-drafts/draft-kinnear-
              httpbis-http2-transport-02.txt>.

   [HTTP3]    Bishop, M., "Hypertext Transfer Protocol Version 3
              (HTTP/3)", Work in Progress, Internet-Draft, draft-ietf-
              quic-http-29, June 9, 2020, <http://www.ietf.org/internet-
              drafts/draft-ietf-quic-http-29.txt>.





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   [MASQUE]   Schinazi, D., "The MASQUE Protocol", Work in Progress,
              Internet-Draft, draft-schinazi-masque-protocol-01, March
              12, 2020, <http://www.ietf.org/internet-drafts/draft-
              schinazi-masque-protocol-01.txt>.

   [QUIC]     Iyengar, J. and M. Thomson, "QUIC: A UDP-Based Multiplexed
              and Secure Transport", Work in Progress, Internet-Draft,
              draft-ietf-quic-transport-29, June 9, 2020,
              <http://www.ietf.org/internet-drafts/draft-ietf-quic-
              transport-29.txt>.

   [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>.

   [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>.

   [TLS13]    Rescorla, E., "The Transport Layer Security (TLS) Protocol
              Version 1.3", RFC 8446, DOI 10.17487/RFC8446, August 2018,
              <https://www.rfc-editor.org/info/rfc8446>.

   [TRANSPORT-AUTH]
              Schinazi, D., "HTTP Transport Authentication", Work in
              Progress, Internet-Draft, draft-schinazi-httpbis-
              transport-auth-03, March 13, 2020, <http://www.ietf.org/
              internet-drafts/draft-schinazi-httpbis-transport-auth-
              03.txt>.

10.2.  Informative References

   [HTTPSSVC] Schwartz, B., Bishop, M., and E. Nygren, "Service binding
              and parameter specification via the DNS (DNS SVCB and
              HTTPSSVC)", Work in Progress, Internet-Draft, draft-ietf-
              dnsop-svcb-httpssvc-03, June 11, 2020,
              <http://www.ietf.org/internet-drafts/draft-ietf-dnsop-
              svcb-httpssvc-03.txt>.

   [I-D.ietf-httpbis-http2-secondary-certs]
              Bishop, M., Sullivan, N., and M. Thomson, "Secondary
              Certificate Authentication in HTTP/2", Work in Progress,
              Internet-Draft, draft-ietf-httpbis-http2-secondary-certs-
              06, May 14, 2020, <http://www.ietf.org/internet-drafts/
              draft-ietf-httpbis-http2-secondary-certs-06.txt>.





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   [I-D.pardue-httpbis-http-network-tunnelling]
              Pardue, L., "HTTP-initiated Network Tunnelling (HiNT)",
              Work in Progress, Internet-Draft, draft-pardue-httpbis-
              http-network-tunnelling-01, October 18, 2018,
              <http://www.ietf.org/internet-drafts/draft-pardue-httpbis-
              http-network-tunnelling-01.txt>.

   [I-D.schwartz-httpbis-helium]
              Schwartz, B., "Hybrid Encapsulation Layer for IP and UDP
              Messages (HELIUM)", Work in Progress, Internet-Draft,
              draft-schwartz-httpbis-helium-00, June 25, 2018,
              <http://www.ietf.org/internet-drafts/draft-schwartz-
              httpbis-helium-00.txt>.

   [I-D.sullivan-tls-post-handshake-auth]
              Sullivan, N., Thomson, M., and M. Bishop, "Post-Handshake
              Authentication in TLS", Work in Progress, Internet-Draft,
              draft-sullivan-tls-post-handshake-auth-00, August 5, 2016,
              <http://www.ietf.org/internet-drafts/draft-sullivan-tls-
              post-handshake-auth-00.txt>.

   [RFC8441]  McManus, P., "Bootstrapping WebSockets with HTTP/2",
              RFC 8441, DOI 10.17487/RFC8441, September 2018,
              <https://www.rfc-editor.org/info/rfc8441>.

   [RFC8471]  Popov, A., Ed., Nystroem, M., Balfanz, D., and J. Hodges,
              "The Token Binding Protocol Version 1.0", RFC 8471,
              DOI 10.17487/RFC8471, October 2018,
              <https://www.rfc-editor.org/info/rfc8471>.

Acknowledgments

   This proposal was inspired directly or indirectly by prior work from
   many people.  In particular, this work is related to
   [I-D.schwartz-httpbis-helium] and
   [I-D.pardue-httpbis-http-network-tunnelling].  The mechanism used to
   run the MASQUE protocol over HTTP/2 streams was inspired by
   [RFC8441].  Brendan Moran is to thank for the idea of leveraging
   connection migration across MASQUE servers.  The author would also
   like to thank Nick Harper, Christian Huitema, Marcus Ihlar, Eric
   Kinnear, Mirja Kuehlewind, Lucas Pardue, Tommy Pauly, Zaheduzzaman
   Sarker, Ben Schwartz, and Christopher A.  Wood for their input.

   The author would like to express immense gratitude to Christophe A.,
   an inspiration and true leader of VPNs.






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Design Justifications

   Using an exported key as a nonce allows us to prevent replay attacks
   (since it depends on randomness from both endpoints of the TLS
   connection) without requiring the server to send an explicit nonce
   before it has authenticated the client.  Adding an explicit nonce
   mechanism would expose the server as it would need to send these
   nonces to clients that have not been authenticated yet.

   The rationale for a separate MASQUE protocol stream is to allow
   server-initiated messages.  If we were to use HTTP semantics, we
   would only be able to support the client-initiated request-response
   model.  We could have used WebSocket for this purpose but that would
   have added wire overhead and dependencies without providing useful
   features.

   There are many other ways to authenticate HTTP, however the
   authentication used here needs to work in a single client-initiated
   message to meet the requirement of not exposing the server.

   The current proposal would also work with TLS 1.2, but in that case
   TLS false start and renegotiation must be disabled, and the extended
   master secret and renegotiation indication TLS extensions must be
   enabled.

   If the server or client want to hide that HTTP/2 is used, the client
   can set its ALPN to an older version of HTTP and then use the Upgrade
   header to upgrade to HTTP/2 inside the TLS encryption.

   The client authentication used here is similar to how Token Binding
   [RFC8471] operates, but it has very different goals.  MASQUE does not
   use token binding directly because using token binding requires
   sending the token_binding TLS extension in the TLS ClientHello, and
   that would stick out compared to a regular TLS connection.

   TLS post-handshake authentication
   [I-D.sullivan-tls-post-handshake-auth] is not used by this proposal
   because that requires sending the "post_handshake_auth" extension in
   the TLS ClientHello, and that would stick out from a regular HTTPS
   connection.

   Client authentication could have benefited from Secondary Certificate
   Authentication in HTTP/2 [I-D.ietf-httpbis-http2-secondary-certs],
   however that has two downsides: it requires the server advertising
   that it supports it in its SETTINGS, and it cannot be sent unprompted
   by the client, so the server would have to request authentication.
   Both of these would make the server stick out from regular HTTP/2
   servers.



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   MASQUE proposes a new client authentication method (as opposed to
   reusing something like HTTP basic authentication) because HTTP
   authentication methods are conceptually per-request (they need to be
   repeated on each request) whereas the new method is bound to the
   underlying connection (be it QUIC or TLS).  In particular, this
   allows sending QUIC DATAGRAM frames without authenticating every
   frame individually.  Additionally, HMAC and asymmetric keying are
   preferred to sending a password for client authentication since they
   have a tighter security bound.  Going into the design rationale,
   HMACs (and signatures) need some data to sign, and to avoid replay
   attacks that should be a fresh nonce provided by the remote peer.
   Having the server provide an explicit nonce would leak the existence
   of the server so we use TLS keying material exporters as they provide
   us with a nonce that contains entropy from the server without
   requiring explicit communication.

Author's Address

   David Schinazi
   Google LLC
   1600 Amphitheatre Parkway
   Mountain View, California 94043,
   United States of America

   Email: dschinazi.ietf@gmail.com


























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