Internet DRAFT - draft-schinazi-masque-obfuscation
draft-schinazi-masque-obfuscation
Network Working Group D. Schinazi
Internet-Draft Google LLC
Intended status: Experimental 12 March 2021
Expires: 13 September 2021
MASQUE Obfuscation
draft-schinazi-masque-obfuscation-04
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 13 September 2021.
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Copyright Notice
Copyright (c) 2021 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/rfc/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/rfc/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, 4 November 2019,
<https://tools.ietf.org/html/draft-kinnear-httpbis-http2-
transport-02>.
[HTTP3] Bishop, M., "Hypertext Transfer Protocol Version 3
(HTTP/3)", Work in Progress, Internet-Draft, draft-ietf-
quic-http-34, 2 February 2021,
<https://tools.ietf.org/html/draft-ietf-quic-http-34>.
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[MASQUE] Schinazi, D., "The MASQUE Protocol", Work in Progress,
Internet-Draft, draft-schinazi-masque-protocol-02, 9
September 2020, <https://tools.ietf.org/html/draft-
schinazi-masque-protocol-02>.
[QUIC] Iyengar, J. and M. Thomson, "QUIC: A UDP-Based Multiplexed
and Secure Transport", Work in Progress, Internet-Draft,
draft-ietf-quic-transport-34, 14 January 2021,
<https://tools.ietf.org/html/draft-ietf-quic-transport-
34>.
[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/rfc/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/rfc/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/rfc/rfc8446>.
[TRANSPORT-AUTH]
Schinazi, D., "HTTP Transport Authentication", Work in
Progress, Internet-Draft, draft-schinazi-httpbis-
transport-auth-05, 12 March 2021,
<https://tools.ietf.org/html/draft-schinazi-httpbis-
transport-auth-05>.
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, 11 June 2020,
<https://tools.ietf.org/html/draft-ietf-dnsop-svcb-
httpssvc-03>.
[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, 14 May 2020, <https://tools.ietf.org/html/draft-ietf-
httpbis-http2-secondary-certs-06>.
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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, 18 October 2018,
<https://tools.ietf.org/html/draft-pardue-httpbis-http-
network-tunnelling-01>.
[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, 25 June 2018,
<https://tools.ietf.org/html/draft-schwartz-httpbis-
helium-00>.
[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, 5 August 2016,
<https://tools.ietf.org/html/draft-sullivan-tls-post-
handshake-auth-00>.
[RFC8441] McManus, P., "Bootstrapping WebSockets with HTTP/2",
RFC 8441, DOI 10.17487/RFC8441, September 2018,
<https://www.rfc-editor.org/rfc/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/rfc/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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