SNI Encryption in TLS Through Tunneling
draft-ietf-tls-sni-encryption-02
Abstract
This draft describes the general problem of encryption of the Server
Name Identification (SNI) parameter. The proposed solutions hide a
Hidden Service behind a Fronting Service, only disclosing the SNI of
the Fronting Service to external observers. The draft starts by
listing known attacks against SNI encryption, discusses the current
"co-tenancy fronting" solution, and then presents two potential TLS
layer solutions that might mitigate these attacks.
The first solution is based on TLS in TLS "quasi tunneling", and the
second solution is based on "combined tickets". These solutions only
require minimal extensions to the TLS protocol.
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
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and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
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This Internet-Draft will expire on September 2, 2018.
Copyright Notice
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document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
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Table of Contents
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Historically, adversaries have been able to monitor the use of web
services through three channels: looking at DNS requests, looking at
IP addresses in packet headers, and looking at the data stream
between user and services. These channels are getting progressively
closed. A growing fraction of Internet communication is encrypted,
mostly using Transport Layer Security (TLS) [RFC5246]. Progressive
deployment of solutions like DNS in TLS [RFC7858] mitigates the
disclosure of DNS information. More and more services are colocated
on multiplexed servers, loosening the relation between IP address and
web service. However, multiplexed servers rely on the Service Name
Information (SNI) to direct TLS connections to the appropriate
service implementation. This protocol element is transmitted in
clear text. As the other methods of monitoring get blocked,
monitoring focuses on the clear text SNI. The purpose of SNI
encryption is to prevent that.
In the past, there have been multiple attempts at defining SNI
encryption. These attempts have generally floundered, because the
simple designs fail to mitigate several of the attacks listed in
Section 2. In the absence of a TLS level solution, the most popular
approach to SNI privacy is HTTP level fronting, which we discuss in
Section 3.
The current draft proposes two designs for SNI Encryption in TLS.
Both designs hide a "Hidden Service" behind a "Fronting Service". To
an external observer, the TLS connections will appear to be directed
towards the Fronting Service. The cleartext SNI parameter will
document the Fronting Service. A second SNI parameter will be
transmitted in an encrypted form to the Fronting Service, and will
allow that service to redirect the connection towards the Hidden
Service.
The first design relies on tunneling TLS in TLS, as explained in
Section 4. It does not require TLS extensions, but relies on
conventions in the implementation of TLS 1.3 [I-D.ietf-tls-tls13] by
the Client and the Fronting Server.
The second design, presented in Section 5 removes the requirement for
tunneling, on simply relies on Combined Tickets. It uses the
extension process for session tickets already defined in
[I-D.ietf-tls-tls13].
This draft is presented as is to trigger discussions. It is expected
that as the draft progresses, only one of the two proposed solutions
will be retained.
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1.1. Key Words
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [RFC2119].
Over the past years, there have been multiple proposals to add an SNI
encryption option in TLS. Many of these proposals appeared
promising, but were rejected after security reviews pointed plausible
attacks. In this section, we collect a list of these known attacks.
2.1. Mitigate Replay Attacks
The simplest SNI encryption designs replace in the initial TLS
exchange the clear text SNI with an encrypted value, using a key
known to the multiplexed server. Regardless of the encryption used,
these designs can be broken by a simple replay attack, which works as
follow:
1- The user starts a TLS connection to the multiplexed server,
including an encrypted SNI value.
2- The adversary observes the exchange and copies the encrypted SNI
parameter.
3- The adversary starts its own connection to the multiplexed server,
including in its connection parameters the encrypted SNI copied from
the observed exchange.
4- The multiplexed server establishes the connection to the protected
service, thus revealing the identity of the service.
One of the goals of SNI encryption is to prevent adversaries from
knowing which Hidden Service the client is using. Successful replay
attacks breaks that goal by allowing adversaries to discover that
service.
2.2. Avoid Widely Shared Secrets
It is easy to think of simple schemes in which the SNI is encrypted
or hashed using a shared secret. This symmetric key must be known by
the multiplexed server, and by every users of the protected services.
Such schemes are thus very fragile, since the compromise of a single
user would compromise the entire set of users and protected services.
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2.3. Prevent SNI-based Denial of Service Attacks
Encrypting the SNI may create extra load for the multiplexed server.
Adversaries may mount denial of service attacks by generating random
encrypted SNI values and forcing the multiplexed server to spend
resources in useless decryption attempts.
It may be argued that this is not an important DOS avenue, as regular
TLS connection attempts also require the server to perform a number
of cryptographic operations. However, in many cases, the SNI
decryption will have to be performed by a front end component with
limited resources, while the TLS operations are performed by the
component dedicated to their respective services. SNI based DOS
attacks could target the front end component.
2.4. Do not stick out
In some designs, handshakes using SNI encryption can be easily
differentiated from "regular" handshakes. For example, some designs
require specific extensions in the Client Hello packets, or specific
values of the clear text SNI parameter. If adversaries can easily
detect the use of SNI encryption, they could block it, or they could
flag the users of SNI encryption for special treatment.
In the future, it might be possible to assume that a large fraction
of TLS handshakes use SNI encryption. If that was the case, the
detection of SNI encryption would be a lesser concern. However, we
have to assume that in the near future, only a small fraction of TLS
connections will use SNI encryption.
2.5. Forward Secrecy
The general concerns about forward secrecy apply to SNI encryption
just as well as to regular TLS sessions. For example, some proposed
designs rely on a public key of the multiplexed server to define the
SNI encryption key. If the corresponding private key was
compromised, the adversaries would be able to process archival
records of past connections, and retrieve the protected SNI used in
these connections. These designs failed to maintain forward secrecy
of SNI encryption.
2.6. Proper Security Context
We can design solutions in which the multiplexed server or a fronting
service act as a relay to reach the protected service. Some of those
solutions involve just one TLS handshake between the client and the
multiplexed server, or between the client and the fronting service.
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The master secret is verified by verifying a certificate provided by
either of these entities, but not by the protected service.
These solutions expose the client to a Man-In-The-Middle attack by
the multiplexed server or by the fronting service. Even if the
client has some reasonable trust in these services, the possibility
of MITM attack is troubling.
The multiplexed server or the fronting services could be pressured by
adversaries. By design, they could be forced to deny access to the
protected service, or to divulge which client accessed it. But if
MITM is possible, the adversaries would also be able to pressure them
into intercepting or spoofing the communications between client and
protected service.
2.7. Fronting Server Spoofing
Adversaries could mount an attack by spoofing the Fronting Service.
A spoofed Fronting Service could act as a "honeypot" for users of
hidden services. At a minimum, the fake server could record the IP
addresses of these users. If the SNI encryption solution places too
much trust on the fronting server, the fake server could also serve
fake content of its own choosing, including various forms of malware.
There are two main channels by which adversaries can conduct this
attack. Adversaries can simply try to mislead users into believing
that the honeypot is a valid Fronting Server, especially if that
information is carried by word of mouth or in unprotected DNS
records. Adversaries can also attempt to hijack the traffic to the
regular Fronting Server, using for example spoofed DNS responses or
spoofed IP level routing, combined with a spoofed certificate.
2.8. Supporting multiple protocols
The SNI encryption requirement do not stop with HTTP over TLS.
Multiple other applications currently use TLS, including for example
SMTP [RFC5246], DNS [RFC7858], or XMPP [RFC7590]. These applications
too will benefit of SNI encryption. HTTP only methods like those
described in Section 3.1 would not apply there. In fact, even for
the HTTPS case, the HTTPS tunneling service described in Section 3.1
is compatible with HTTP 1.0 and HTTP 1.1, but interacts awkwardly
with the multiple streams feature of HTTP 2.0 [RFC7540]. This points
to the need of an application agnostic solution, that would be
implemented fully in the TLS layer.
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2.8.1. Hiding the Application Layer Protocol Negotiation
The Application Layer Protocol Negotiation (ALPN) parameters of TLS
allow implementations to negotiate the application layer protocol
used on a given connection. TLS provides the ALPN values in clear
text during the initial handshake. While exposing the ALPN does not
create the same privacy issues as exposing the SNI, there is still a
risk. For example, some networks may attempt to block applications
that they do not understand, or that they wish users would not use.
In a sense, ALPN filtering could be very similar to the filtering of
specific port numbers exposed in some network. This filtering by
ports has given rise to evasion tactics in which various protocols
are tunneled over HTTP in order to use open ports 80 or 443.
Filtering by ALPN would probably beget the same responses, in which
the applications just move over HTTP, and only the HTTP ALPN values
are used. Applications would not need to do that if the ALPN was
hidden in the same way as the SNI.
It is thus desirable that SNI Encryption mechanisms be also able hide
the ALPN.
2.8.2. Support other transports than HTTP
The TLS handshake is also used over other transports such as UDP with
both DTLS [I-D.ietf-tls-dtls13] and QUIC [I-D.ietf-quic-tls]. The
requirement to encrypt the SNI apply just as well for these
transports as for TLS over TCP.
This points to a requirement for SNI Encryption mechanisms to also be
applicable to non-TCP transports such as DTLS or QUIC.
2.9. Fail to fronting
It is easy to imagine designs in which the client sends some client
hello extension that points to a secret shared by client and hidden
server. If that secret is incorporated into the handshake secret,
the exchange will only succeeds if the connection truly ends at the
hidden server. The exchange will fail if the extension is stripped
by an MITM, and the exchange will also fail if an adversary replays
the extension in a Client Hello.
The problem with that approach is clear. Adversaries that replay the
extension can test whether the client truly wanted to access the
fronting server, or was simply using that fronting server as an
access gateway to something else. The adversaries will not know what
hidden service the client was trying to reach, but they can guess.
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They can also start directly interrogate the user, or other
unpleasant alternatives.
When designing SNI encryption schemes, we have to take into account
attacks that strip parameters from the Client Hello, or replay
attacks. In both cases, the desired behavior is to fall back to a
connection with the fronting server, so there is no visble difference
between a regular connection to that server and an atempt to reach
the hidden server.
In the absence of TLS level SNI encryption, many sites rely on an
"HTTP Co-Tenancy" solution. The TLS connection is established with
the fronting server, and HTTP requests are then sent over that
connection to the hidden service. For example, the TLS SNI could be
set to "fronting.example.com", the fronting server, and HTTP requests
sent over that connection could be directed to "hidden.example.com/
some-content", accessing the hidden service. This solution works
well in practice when the fronting server and the hidden server are
'co-tenant" of the same multiplexed server.
The HTTP fronting solution can be deployed without modification to
the TLS protocol, and does not require using and specific version of
TLS. There are however a few issues regarding discovery, client
implementations, trust, and applicability:
o The client has to discover that the hidden service can be accessed
through the fronting server.
o The client browser's has to be directed to access the hidden
service through the fronting service.
o Since the TLS connection is established with the fronting service,
the client has no proof that the content does in fact come from
the hidden service. The solution does thus not mitigate the
context sharing issues described in Section 2.6.
o Since this is an HTTP level solution, it would not protected non
HTTP protocols such as DNS over TLS [RFC7858] or IMAP over TLS
[RFC2595].
The discovery issue is common to pretty much every SNI encryption
solution, and is also discussed in Section 4.2.3 and Section 5.3.
The browser issue may be solved by developing a browser extension
that support HTTP Fronting, and manages the list of fronting services
associated with the hidden services that the client uses. The multi-
protocol issue can be mitigated by using implementation of other
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applications over HTTP, such as for example DNS over HTTPS
[I-D.hoffman-dns-over-https]. The trust issue, however, requires
specific developments.
3.1. HTTPS Tunnels
The HTTP Fronting solution places a lot of trust in the Fronting
Server. This required trust can be reduced by tunnelling HTTPS in
HTTPS, which effectively treats the Fronting Server as an HTTP Proxy.
In this solution, the client establishes a TLS connection to the
Fronting Server, and then issues an HTTP Connect request to the
Hidden Server. This will establish an end-to-end HTTPS over TLS
connection between the client and the Hidden Server, mitigating the
issues described in Section 2.6.
The HTTPS in HTTPS solution requires double encryption of every
packet. It also requires that the fronting server decrypts and relay
messages to the hidden server. Both of these requirements make the
implementation onerous.
3.2. Delegation Token
Clients would see their privacy compromised if they contacted the
wrong fronting server to access the hidden service, since this wrong
server could disclose their access to adversaries. This can possibly
be mitigated by recording the relation between fronting server and
hidden server in a Delegation Token.
The delegation token would be a form of certificate, signed by the
hidden service. It would have the following components:
o The DNS name of the fronting service
o TTL (i.e. expiration date)
o An indication of the type of access that would be used, such as
direct fronting in which the hidden content is directly served by
the fronting server, or HTTPS in HTTPS, or one of the TLS level
solutions discussed in Section 4 and Section 5
o Triple authentication, to make the barrier to setting up a
honeypot extremely high
1. Cert chain for hidden server certificate (e.g.,
hidden.example.com) up to CA.
2. Certificate transparency proof of the hidden service
certificate (hidden.example.com) from a popular log, with a
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requirement that the browser checks the proof before
connecting.
3. A TLSA record for hidden service domain name
(hidden.example.com), with full DNSSEC chain (also mandatory
to check)
o Possibly, a list of valid addresses of the fronting service.
o Some extension mechanism for other bits
If N multiple domains on a CDN are acceptable fronts, then we may
want some way to indicate this without publishing and maintaining N
separate tokens.
Delegation tokens could be published by the fronting server, in
response for example to a specific query by a client. The client
would then examine whether one of the Delegation Tokens matches the
hidden service that it wants to access.
QUESTION: Do we need a revocation mechanism? What if a fronting
service obtains a delegation token, and then becomes untrustable for
some other reason? Or is it sufficient to just use short TTL?
We propose to provide SNI Privacy by using a form of TLS
encapsulation. The big advantage of this design compared to previous
attempts is that it requires effectively no changes to TLS 1.3. It
only requires a way to signal to the Fronting Server server that the
encrypted application data is actually a ClientHello which is
intended for the hidden service. Once the tunneled session is
established, encrypted packets will be forwarded to the Hidden
Service without requiring encryption or decryption by the Fronting
Service.
4.1. Tunneling TLS in TLS
The proposed design is to encapsulate a second Client Hello in the
early data of a TLS connection to the Fronting Service. To the
outside, it just appears that the client is resuming a session with
the fronting service.
Client Fronting Service Hidden Service
ClientHello
+ early_data
+ key_share*
+ psk_key_exchange_modes
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+ pre_shared_key
+ SNI = fronting
(
//Application data
ClientHello#2
+ KeyShare
+ signature_algorithms*
+ psk_key_exchange_modes*
+ pre_shared_key*
+ SNI = hidden
)
-------->
ClientHello#2
+ KeyShare
+ signature_algorithms*
+ psk_key_exchange_modes*
+ pre_shared_key*
+ SNI = hidden ---->
<Application Data*>
<end_of_early_data> -------------------->
ServerHello
+ pre_shared_key
+ key_share*
{EncryptedExtensions}
{CertificateRequest*}
{Certificate*}
{CertificateVerify*}
{Finished}
<--------------------
{Certificate*}
{CertificateVerify*}
{Finished} ---------------------
[Application Data] <-------------------> [Application Data]
Key to brackets:
* optional messages, not present in all scenarios
() encrypted with Client->Fronting 0-RTT key
<> encrypted with Client->Hidden 0-RTT key
{} encrypted with Client->Hidden 1-RTT handshake
[] encrypted with Client->Hidden 1-RTT key
The way this works is that the Fronting Server decrypts the data in
the client's first flight, which is actually ClientHello#2 from the
client, containing the true SNI and then passes it on to the Hidden
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server. However, the Hidden Server responds with its own ServerHello
which the Fronting Server just passes unchanged, because it's
actually the response to ClientHello#2 rather than to ClientHello#1.
As long as ClientHello#1 and ClientHello#2 are similar (e.g.,
differing only in the client's actual share (though of course it must
be in the same group)), SNI, and maybe EarlyDataIndication), then an
attacker should not be able to distinguish these cases -- although
there may be possible attacks through timing analysis, or by
observing traffic between the Fronting Server and Hidden Server if
they are not colocated.
4.2. Tunneling design issues
The big advantage of this design is that it requires effectively no
changes to TLS. It only requires a way to signal to the Fronting
Server that the encrypted application data is actually a ClientHello
which is intended for the hidden service.
The major disadvantage of this overall design strategy (however it's
signaled) is that it's somewhat harder to implement in the co-
tenanted cases than the simple schemes that carry the "real SNI" in
an encrypted parameter of the Client Hello. That means that it's
somewhat less likely that servers will implement it "by default" and
more likely that they will have to take explicit effort to allow
Encrypted SNI. Conversely, however, these schemes (aside from a
server with a single wildcard or multi-SAN cert) involve more changes
to TLS to deal with issues like "what is the server cert that is
digested into the keys", and that requires more analysis, so there is
an advantage to deferring that. If we have EncryptedExtensions in
the client's first flight it would be possible to define a "Real SNI"
extension later if/when we had clearer analysis for that case.
Notes on several obvious technical issues:
How does the Fronting Server distinguish this case from where the
initial flight is actual application data? See Section 4.2.1 for
some thoughts on this.
Can we make this work with 0-RTT data from the client to the
Hidden server? The answer is probably yes, as discussed in
Section 4.2.2.
What happens if the Fronting Server doesn't gateway, e.g.,
because it has forgotten the ServerConfiguration? In that case,
the client gets a handshake with the Fronting Server, which it
will have to determine via trial decryption. At this point the
Fronting Server supplies a ServerConfiguration and the client can
reconnect as above.
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What happens if the client does 0-RTT inside 0-RTT (as in #2
above) and the Hidden server doesn't recognize the
ServerConfiguration in ClientHello#2? In this case, the client
gets a 0-RTT rejection and it needs to do trial decryption to
know whether the rejection was from the Fronting Server or the
Hidden server.
What happens if the Fronting Server is under a DOS attack, and
chooses to refuse all 0-RTT data?
The client part of that logic, including the handling of question #3
above, is discussed in Section 4.2.3.
4.2.1. Fronting Server logic
The big advantage of this design is that it requires effectively no
changes to TLS. It only requires a way to signal to the Fronting
Server that the encrypted application data is actually a ClientHello
which is intended for the hidden service. The two most obvious
designs are:
o Have an EncryptedExtension which indicates that the inner data is
tunnelled.
o Have a "tunnelled" TLS content type.
EncryptedExtensions would be the most natural, but they were removed
from the ClientHello during the TLS standardization. In Section 4.1
we assume that the second ClientHello is just transmitted as 0-RTT
data, and that the servers use some form of pattern matching to
differentiate between this second ClientHello and other application
messages.
4.2.2. Early data
In the proposed design, the second ClientHello is sent to the
Fronting Server as early data, encrypted with Client->Fronting 0-RTT
key. If the Client follows the second ClientHello with 0-RTT data,
that data could in theory be sent in two ways:
The client could use double encryption. The data is first
encrypted with the Client->Hidden 0-RTT key, then wrapped and
encrypted with the Client->Fronting 0-RTT key. The Fronting
server would decrypt, unwrap and relay.
The client could just encrypt the data with the Client->Hidden
0-RTT key, and ask the server to blindly relay it.
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Each of these ways has its issues. The double encryption scenario
would require two end of early data messages, one double encrypted
and relayed by the Fronting Server to the Hidden Server, and another
sent from Client to Fronting Server, to delimit the end of the double
encrypted stream, and also to ensure that the stream of messages is
not distinguishable from simply sending 0-RTT data to the Fronting
server. The blind relaying is simpler, and is the scenario described
in the diagram of Section 4.1. In that scenario, the Fronting server
switches to relaying mode immediately after unwrapping and forwarding
the second ClientHello. However, the blind relaying requires the
ClientHello to be isolated to a single record.
4.2.3. Client requirements
In order to use the tunneling service, the client needs to identify
the Fronting Service willing to tunnel to the Hidden Service. We can
assume that the client will learn the identity of suitable Fronting
Services from the Hidden Service itself.
In order to tunnel the second ClientHello as 0-RTT data, the client
needs to have a shared secret with the Fronting Service. To avoid
the trap of "well known shared secrets" described in Section 2.2,
this should be a pair wise secret. The most practical solution is to
use a session resumption ticket. This requires that prior to the
tunneling attempt, the client establishes regular connections with
the fronting service and obtains one or several session resumption
tickets.
EDITOR'S NOTE: This section is an alternative design to Section 4.
As the draft progresses, only one of the alternatives will be
selected, and the text corresponding to the other alternative will be
deleted.
We propose to provide SNI Privacy by relying solely on "combined
tickets". The big advantage of this design compared to previous
attempts is that it requires only minimal changes to implementations
of TLS 1.3. These changes are confined to the handling of the
combined ticket by Fronting and Hidden service, and to the signaling
of the Fronting SNI to the client by the Hidden service.
5.1. Session resumption with combined tickets
In this example, the client obtains a combined session resumption
ticket during a previous connection to the hidden service, and has
learned the SNI of the fronting service. The session resumption will
happen as follows:
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Client Fronting Service Hidden Service
ClientHello
+ early_data
+ key_share*
+ psk_key_exchange_modes
+ pre_shared_key
+ SNI = fronting
-------->
// Decode the ticket
// Forwards to hidden
ClientHello ------->
(Application Data*) ---------------------->
ServerHello
+ pre_shared_key
+ key_share*
{EncryptedExtensions}
+ early_data*
{Finished}
<---------------------- [Application Data]
(EndOfEarlyData)
{Finished} ---------------------->
[Application Data] <---------------------> [Application Data]
+ Indicates noteworthy extensions sent in the
previously noted message.
* Indicates optional or situation-dependent
messages/extensions that are not always sent.
() encrypted with Client->Hidden 0-RTT key
{} encrypted with Client->Hidden 1-RTT handshake
[] encrypted with Client->Hidden 1-RTT key
The Fronting server that receives the Client Hello will find the
combined ticket in the pre_shared_key extensions, just as it would in
a regular session resumption attempt. When parsing the ticket, the
Fronting server will discover that the session really is meant to be
resumed with the Hidden server. It will arrange for all the
connection data to be forwarded to the Hidden server, including
forwarding a copy of the initial Client Hello.
The Hidden server will receive the Client Hello. It will obtain the
identity of the Fronting service from the SNI parameter. It will
then parse the session resumption ticket, and proceed with the
resumption of the session.
In this design, the Client Hello message is relayed unchanged from
Fronting server to hidden server. This ensures that code changes are
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confined to the interpretation of the message parameters. The
construction of handshake contexts is left unchanged.
5.2. New Combined Session Ticket
In normal TLS 1.3 operations, the server can send New Session Ticket
messages at any time after the receiving the Client Finished message.
The ticket structure is defined in TLS 1.3 as:
struct {
uint32 ticket_lifetime;
uint32 ticket_age_add;
opaque ticket_nonce<1..255>;
opaque ticket<1..2^16-1>;
Extension extensions<0..2^16-2>;
} NewSessionTicket;
When SNI encryption is enabled, tickets will carry a "Fronting SNI"
extension, and the ticket value itself will be negotiated between
Fronting Service and Hidden Service, as in:
Client Fronting Service Hidden Service
<======= <Ticket Request>
Combined Ticket =======>
[New Session Ticket
<------------------------ + SNI Extension]
<==> sent on connection between Hidden and Fronting service
<> encrypted with Fronting<->Hidden key
[] encrypted with Client->Hidden 1-RTT key
In theory, the actual format of the ticket could be set by mutual
agreement between Fronting Service and Hidden Service. In practice,
it is probably better to provide guidance, as the ticket must meet
three requirements:
o The Fronting Server must understand enough of the combined ticket
to relay the connection towards the Hidden Server;
o The Hidden Server must understand enough of the combined ticket to
resume the session with the client;
o Third parties must not be able to deduce the name of the Hidden
Service from the value of the ticket.
There are three plausible designs, a stateful design, a shared key
design, and a
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In the stateful design, the ticket are just random numbers that the
Fronting server associates with the Hidden server, and the Hidden
server associates with the session context. The shared key design
would work as follow:
o the hidden server and the fronting server share a symmetric key
K_sni.
o the "clear text" ticket includes a nonce, the ordinary ticket used
for session resumption by the hidden service, and the id of the
Hidden service for the Fronting Service.
o the ticket will be encrypted with AEAD, using the nonce as an IV.
o When the client reconnects to the fronting server, it decrypts the
ticket using K_sni and if it succeeds, then it just forwards the
Client Hello to the hidden server indicated in id-hidden-service
(which of course has to know to ignore SNI). Otherwise, it
terminates the connection itself with its own SNI.
The hidden server can just refresh the ticket any time it pleases, as
usual.
This design allows the Hidden Service to hide behind many Fronting
Services, each using a different key. The Client Hello received by
the Hidden Server carries the SNI of the Fronting Service, which the
Hidden Server can use to select the appropriate K_sni.
In the public key design, the Hidden Server encrypts the tickets with
a public key of the Fronting Server. The ticket itself would be
similar to what is used in the shared key design. The compute cost
for a single decryption may be higher, but the Fronting Server would
not need to blindly try multiple decryption keys associated with
multiple Hidden Servers. The Hidden Server would not be able to
decrypt the ession Tickets, which means that it would have to rely on
some kind of stateful storage.
5.3. First session
The previous sections present how sessions can be resumed with the
combined ticket. Clients have that have never contacted the Hidden
Server will need to obtain a first ticket during a first session.
The most plausible option is to have the client directly connect to
the Hidden Service, and then ask for a combined ticket. The obvious
issue is that the SNI will not be encrypted for this first
connection, which exposes the client to surveillance and censorship.
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The client may also learn about the relation between Fronting Service
and Hidden Service through an out of band channel, such as DNS
service, or word of mouth. However, it is difficult to establish a
combined ticket completely out of band, since the ticket must be
associated to two shared secrets, one understood by the Fronting
service and the other shared with the Hidden service to ensure
protection against replay attacks.
An alternative may be to use the TLS-in-TLS service described in
Section 4.1 for the first contact. There will be some overhead due
to tunnelling, but as we discussed in Section 4.2.3 the tunneling
solution allows for safe first contact. Yet another way would be to
use the HTTPS in HTTPS tunneling described in Section 3.1.
The encapsulation protocol proposed in this draft mitigates the known
attacks listed in Section 2. For example, the encapsulation design
uses pairwise security contexts, and is not dependent on the widely
shared secrets described in Section 2.2. The design also does not
rely on additional public key operations by the multiplexed server or
by the fronting server, and thus does not open the attack surface for
denial of service discussed in Section 2.3. The session keys are
negotiated end to end between the client and the protected service,
as required in Section 2.6.
The combined ticket solution also mitigates the known attacks. The
design also uses pairwise security contexts, and is not dependent on
the widely shared secrets described in Section 2.2. The design also
does not rely on additional public key operations by the multiplexed
server or by the fronting server, and thus does not open the attack
surface for denial of service discussed in Section 2.3. The session
keys are negotiated end to end between the client and the protected
service, as required in Section 2.6.
However, in some cases, proper mitigation depends on careful
implementation.
6.1. Replay attacks and side channels
Both solutions mitigate the replay attacks described in Section 2.1
because adversaries cannot decrypt the replies intended for the
client. However, the connection from the fronting service to the
hidden service can be observed through side channels.
To give an obvious example, suppose that the fronting service merely
relays the data by establishing a TCP connection to the hidden
service. An adversary capable of observing all network traffic at
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the fronting server can associate the arrival of an encrypted message
to the fronting service and the TCP handshake between the fronting
server and the hidden service, and deduce which hidden service the
user accessed.
The mitigation of this attack relies on proper implementation of the
fronting service. This may require cooperation from the multiplexed
server.
6.2. Sticking out
The TLS encapsulation protocol mostly fulfills the requirements to
"not stick out" expressed in Section 2.4. The initial messages will
be sent as 0-RTT data, and will be encrypted using the 0-RTT key
negotiated with the fronting service. Adversaries cannot tell
whether the client is using TLS encapsulation or some other 0-RTT
service. However, this is only true if the fronting service
regularly uses 0-RTT data.
The combined token solution almost perfectly fulfills the
requirements to "not stick out" expressed in Section 2.4, as the
observable flow of message is almost exactly the same as a regular
TLS connection. However, adversaries could observe the values of the
PSK Identifier that contains the combined ticket. The proposed
ticket structure is designed to thwart analysis of the ticket, but if
implementations are not careful the size of the combined ticket can
be used as a side channel allowing adversaries to distinguish between
different Hidden Services located behind the same Fronting Service.
6.3. Forward Secrecy
In the TLS encapsulation protocol, the encapsulated Client Hello is
encrypted using the session resumption key. If this key is revealed,
the Client Hello data will also be revealed. The mitigation there is
to not use the same session resumption key multiple time.
The most common implementations of TLS tickets have the server using
Session Ticket Encryption Keys (STEKs) to create an encrypted copy of
the session parameters which is then stored by the client. When the
client resumes, it supplies this encrypted copy, the server decrypts
it, and has the parameters it needs to resume. The server need only
remember the STEK. If a STEK is disclosed to an adversary, then all
of the data encrypted by sessions protected by the STEK may be
decrypted by an adversary.
To mitigate this attack, server implementations of the combined
ticket protocol SHOULD use stateful tickets instead of STEK protected
TLS tickets. If they do rely on STEK protected tickets, they MUST
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ensure that the K_sni keys used to encrypt these tickets are rotated
frequently.
Do we need to register an extension point? Or is it just OK to use
early data?
A large part of this draft originates in discussion of SNI encryption
on the TLS WG mailing list, including comments after the tunneling
approach was first proposed in a message to that list:
<https://mailarchive.ietf.org/arch/msg/tls/
tXvdcqnogZgqmdfCugrV8M90Ftw>.
During the discussion of SNI Encryption in Yokohama, Deb Cooley
argued that rather than messing with TLS to allow SNI encryption, we
should just tunnel TLS in TLS. A number of people objected to this
on the grounds of the performance cost for the Fronting Server
because it has to encrypt and decrypt everything.
After the meeting, Martin Thomson suggested a modification to the
tunnelling proposal that removes this cost. The key observation is
that if we think of the 0-RTT flight as a separate message attached
to the handshake, then we can tunnel a second first flight in it.
The combined ticket approach was first proposed by Cedric Fournet and
Antoine Delignaut-Lavaud.
The delegation token design comes from many people, including Ben
Schwartz, Brian Sniffen and Rich Salz.
Thanks to Daniel Kahn Gillmor for a pretty detailed review of the
initial draft.
9.1. Normative References
[I-D.ietf-quic-tls]
Thomson, M. and S. Turner, "Using Transport Layer Security
(TLS) to Secure QUIC", draft-ietf-quic-tls-09 (work in
progress), January 2018.
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[I-D.ietf-tls-dtls13]
Rescorla, E., Tschofenig, H., and N. Modadugu, "The
Datagram Transport Layer Security (DTLS) Protocol Version
1.3", draft-ietf-tls-dtls13-22 (work in progress),
November 2017.
[I-D.ietf-tls-tls13]
Rescorla, E., "The Transport Layer Security (TLS) Protocol
Version 1.3", draft-ietf-tls-tls13-24 (work in progress),
February 2018.
[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.
9.2. Informative References
[I-D.hoffman-dns-over-https]
Hoffman, P. and P. McManus, "DNS Queries over HTTPS",
draft-hoffman-dns-over-https-01 (work in progress), June
2017.
[RFC2595] Newman, C., "Using TLS with IMAP, POP3 and ACAP",
RFC 2595, DOI 10.17487/RFC2595, June 1999,
https://www.rfc-editor.org/info/rfc2595.
[RFC5246] Dierks, T. and E. Rescorla, "The Transport Layer Security
(TLS) Protocol Version 1.2", RFC 5246,
DOI 10.17487/RFC5246, August 2008,
https://www.rfc-editor.org/info/rfc5246.
[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.
[RFC7590] Saint-Andre, P. and T. Alkemade, "Use of Transport Layer
Security (TLS) in the Extensible Messaging and Presence
Protocol (XMPP)", RFC 7590, DOI 10.17487/RFC7590, June
2015, https://www.rfc-editor.org/info/rfc7590.
[RFC7858] Hu, Z., Zhu, L., Heidemann, J., Mankin, A., Wessels, D.,
and P. Hoffman, "Specification for DNS over Transport
Layer Security (TLS)", RFC 7858, DOI 10.17487/RFC7858, May
2016, https://www.rfc-editor.org/info/rfc7858.