Network Working Group D. Schinazi
Internet-Draft Google LLC
Intended status: Experimental February 28, 2019
Expires: September 1, 2019
The MASQUE Protocol
draft-schinazi-masque-00
Abstract
This document describes MASQUE (Multiplexed Application Substrate
over QUIC Encryption). MASQUE 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 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. As we have not yet found a
home for this work, discussion is encouraged to happen on the GitHub
repository which contains the draft:
https://github.com/DavidSchinazi/masque-drafts [1].
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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Internet-Drafts are draft documents valid for a maximum of six months
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material or to cite them other than as "work in progress."
This Internet-Draft will expire on September 1, 2019.
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Copyright Notice
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document authors. All rights reserved.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
1.1. Conventions and Definitions . . . . . . . . . . . . . . . 3
2. Requirements . . . . . . . . . . . . . . . . . . . . . . . . 3
2.1. Invisibility of VPN Usage . . . . . . . . . . . . . . . . 3
2.2. Invisibility of the Server . . . . . . . . . . . . . . . 3
2.3. Fallback to HTTP/2 over TLS over TCP . . . . . . . . . . 4
3. Overview of the Mechanism . . . . . . . . . . . . . . . . . . 4
4. Mechanisms the Server Can Advertise to Authenticated Clients 5
4.1. HTTP Proxy . . . . . . . . . . . . . . . . . . . . . . . 5
4.2. DNS over HTTPS . . . . . . . . . . . . . . . . . . . . . 5
4.3. UDP Proxying . . . . . . . . . . . . . . . . . . . . . . 5
4.4. IP Proxying . . . . . . . . . . . . . . . . . . . . . . . 6
4.5. Path MTU Discovery . . . . . . . . . . . . . . . . . . . 6
5. Security Considerations . . . . . . . . . . . . . . . . . . . 6
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 6
7. References . . . . . . . . . . . . . . . . . . . . . . . . . 7
7.1. Normative References . . . . . . . . . . . . . . . . . . 7
7.2. Informative References . . . . . . . . . . . . . . . . . 8
7.3. URIs . . . . . . . . . . . . . . . . . . . . . . . . . . 8
Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . 9
Design Justifications . . . . . . . . . . . . . . . . . . . . . . 9
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 10
1. Introduction
This document describes MASQUE (Multiplexed Application Substrate
over QUIC Encryption). MASQUE 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 VPN server
that is indistinguishable from an HTTPS server to any unauthenticated
observer. We do not expect major providers and CDNs to deploy this
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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. As we have not yet found a
home for this work, discussion is encouraged to happen on the GitHub
repository which contains the draft:
https://github.com/DavidSchinazi/masque-drafts [2].
MASQUE leverages the efficient head-of-line blocking prevention
features of the QUIC transport protocol [I-D.ietf-quic-transport]
when MASQUE is used in an HTTP/3 [I-D.ietf-quic-http] server. MASQUE
can also run in an HTTP/2 server [RFC7540] 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. Requirements
This section describes the goals and requirements chosen for the
MASQUE protocol.
2.1. Invisibility of VPN Usage
An authenticated client using the VPN 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 currently considered out of scope.
2.2. 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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2.3. Fallback to HTTP/2 over TLS over TCP
When QUIC is blocked, MASQUE can run over TCP and still satisfy
previous requirements. Note that in this scenario performance may be
negatively impacted.
3. 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 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 [RFC8446] over TCP), and validates the
server's TLS certificate as it normally would for HTTPS. If
validation fails, the connection is aborted. The client then uses a
TLS keying material exporter [RFC5705] with label "EXPORTER-masque"
and no context to generate a 32-byte key. This key is then used as a
nonce to prevent replay attacks. The client then sends an HTTP
CONNECT request for "/.well-known/masque/initial" with the :protocol
pseudo-header field set to "masque", and a "Masque-Authentication:"
header. The MASQUE authentication header differs from the HTTP
"Authorization" header in that it applies to the underlying
connection instead of being per-request. It can use either a shared
secret or asymmetric authentication. The asymmetric variant uses
authentication method "PublicKey", and it transmits a signature of
the nonce with the client's public key encoded in base64 format,
followed by other information such as the client username and
signature algorithm OID. The symmetric variant uses authentication
method "HMAC" and transmits an HMAC of the nonce with the shared
secret instead of a signature. For example this header could look
like:
Masque-Authentication: PublicKey u="am9obi5kb2U=";a=1.3.101.112;
s="SW5zZXJ0IHNpZ25hdHVyZSBvZiBub25jZSBoZXJlIHdo
aWNoIHRha2VzIDUxMiBiaXRzIGZvciBFZDI1NTE5IQ=="
Masque-Authentication: HMAC u="am9obi5kb2U=";a=2.16.840.1.101.3.4.2.3;
s="SW5zZXJ0IHNpZ25hdHVyZSBvZiBub25jZSBoZXJlIHdo
aWNoIHRha2VzIDUxMiBiaXRzIGZvciBFZDI1NTE5IQ=="
Figure 1: MASQUE Authentication Format Example
When the server receives this CONNECT request, it verifies the
signature and if that fails responds with code "405 Method Not
Allowed", making sure its response is the same as what it would
return for any unexpected CONNECT request. If the signature
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verifies, the server responds with code "101 Switching Protocols",
and from then on this HTTP stream is now dedicated to the MASQUE
protocol. That protocol provides a reliable bidirectional message
exchange mechanism, which is used by the client and server to
negotiate what protocol options are supported and enabled by policy,
and client VPN configuration such as IP addresses. When using QUIC,
this protocol also allows endpoints to negotiate the use of QUIC
extensions, such as support for the DATAGRAM extension
[I-D.pauly-quic-datagram].
4. Mechanisms the Server Can Advertise to Authenticated Clients
Once a server has authenticated the client's MASQUE CONNECT request,
it advertises services that the client may use. These services allow
for example varying degrees of proxying services to help a client
obfuscate the ultimate destination of their traffic.
4.1. HTTP Proxy
The client can make proxied HTTP requests through the server to other
servers. In practice this will mean using the CONNECT method to
establish a stream over which to run TLS to a different remote
destination.
4.2. DNS over HTTPS
The client can send DNS queries using DNS over HTTPS (DoH) [RFC8484]
to the MASQUE server.
4.3. UDP Proxying
In order to support WebRTC or QUIC to further servers, clients need a
way to relay UDP onwards to a remote server. In practice for most
widely deployed protocols other than DNS, this involves many
datagrams over the same ports. Therefore this mechanism implements
that efficiently: clients can use the MASQUE protocol stream to
request an UDP association to an IP address and UDP port pair. In
QUIC, the server would reply with a DATAGRAM_ID that the client can
then use to have UDP datagrams sent to this remote server. Datagrams
are then simply transferred between the DATAGRAMs with this ID and
the outer server. There will also be a message on the MASQUE
protocol stream to request shutdown of a UDP association to save
resources when it is no longer needed. When running over TCP, 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 associates resources.
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4.4. IP Proxying
For the rare cases where the previous mechanisms are not sufficient,
proxying can be performed at the IP layer. This would use a
different DATAGRAM_ID and IP datagrams would be encoded inside it
without framing. Over TCP, a dedicated stream with two byte length
prefix would be used. The server can inspect the IP datagram to look
for the destination address in the IP header.
4.5. 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.
5. Security Considerations
Here be dragons. TODO: slay the dragons.
6. IANA Considerations
We will need to register:
o the TLS keying material exporter label "EXPORTER-masque" (spec
required)
https://www.iana.org/assignments/tls-parameters/tls-
parameters.xhtml#exporter-labels [3]
o the new HTTP header "Masque-Authentication"
https://www.iana.org/assignments/message-headers/message-
headers.xhtml [4]
o the "/.well-known/masque/" URI (expert review)
https://www.iana.org/assignments/well-known-uris/well-known-
uris.xhtml [5]
o The "masque" and "masque-udp-proxy" extended HTTP CONNECT
protocols
We will also need to define the MASQUE control protocol and that will
be likely to define new registries of its own.
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7. References
7.1. Normative References
[I-D.ietf-quic-http]
Bishop, M., "Hypertext Transfer Protocol Version 3
(HTTP/3)", draft-ietf-quic-http-18 (work in progress),
January 2019.
[I-D.ietf-quic-transport]
Iyengar, J. and M. Thomson, "QUIC: A UDP-Based Multiplexed
and Secure Transport", draft-ietf-quic-transport-18 (work
in progress), January 2019.
[I-D.pauly-quic-datagram]
Pauly, T., Kinnear, E., and D. Schinazi, "An Unreliable
Datagram Extension to QUIC", draft-pauly-quic-datagram-02
(work in progress), February 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>.
[RFC5705] Rescorla, E., "Keying Material Exporters for Transport
Layer Security (TLS)", RFC 5705, DOI 10.17487/RFC5705,
March 2010, <https://www.rfc-editor.org/info/rfc5705>.
[RFC7540] 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>.
[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>.
[RFC8446] 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>.
[RFC8484] Hoffman, P. and P. McManus, "DNS Queries over HTTPS
(DoH)", RFC 8484, DOI 10.17487/RFC8484, October 2018,
<https://www.rfc-editor.org/info/rfc8484>.
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7.2. Informative References
[I-D.ietf-httpbis-http2-secondary-certs]
Bishop, M., Sullivan, N., and M. Thomson, "Secondary
Certificate Authentication in HTTP/2", draft-ietf-httpbis-
http2-secondary-certs-03 (work in progress), October 2018.
[I-D.pardue-httpbis-http-network-tunnelling]
Pardue, L., "HTTP-initiated Network Tunnelling (HiNT)",
draft-pardue-httpbis-http-network-tunnelling-01 (work in
progress), October 2018.
[I-D.schwartz-httpbis-helium]
Schwartz, B., "Hybrid Encapsulation Layer for IP and UDP
Messages (HELIUM)", draft-schwartz-httpbis-helium-00 (work
in progress), June 2018.
[I-D.sullivan-tls-post-handshake-auth]
Sullivan, N., Thomson, M., and M. Bishop, "Post-Handshake
Authentication in TLS", draft-sullivan-tls-post-handshake-
auth-00 (work in progress), August 2016.
[RFC7427] Kivinen, T. and J. Snyder, "Signature Authentication in
the Internet Key Exchange Version 2 (IKEv2)", RFC 7427,
DOI 10.17487/RFC7427, January 2015,
<https://www.rfc-editor.org/info/rfc7427>.
[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>.
7.3. URIs
[1] https://github.com/DavidSchinazi/masque-drafts
[2] https://github.com/DavidSchinazi/masque-drafts
[3] https://www.iana.org/assignments/tls-parameters/tls-
parameters.xhtml#exporter-labels
[4] https://www.iana.org/assignments/message-headers/message-
headers.xhtml
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[5] https://www.iana.org/assignments/well-known-uris/well-known-
uris.xhtml
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]. Using the OID for the signature algorithm was inspired by
Signature Authentication in IKEv2 [RFC7427].
The author would like to thank Christophe A., an inspiration and true
leader of VPNs.
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
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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.
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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