Issues and Requirements
for Server Name Identification (SNI) Encryption in TLS
Private Octopus Inc.
Friday Harbor
WA
98250
United States of America
huitema@huitema.net
Network
This document describes the general problem of encrypting the
Server Name Identification (SNI) TLS parameter. The proposed
solutions hide a hidden service behind a fronting service,
only disclosing the SNI of the fronting service to external
observers. This document lists known attacks against
SNI encryption, discusses the current "HTTP co-tenancy" solution,
and presents requirements for future TLS-layer solutions.
In practice, it may well be that no solution can meet every requirement
and that practical solutions will have to make some compromises.
Introduction
Historically, adversaries have been able to monitor the use of web
services through three primary 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) .
Progressive deployment of solutions like DNS over
TLS and DNS over HTTPS
mitigates the disclosure of DNS information. More and
more services are colocated on multiplexed servers, loosening the
relation between IP address and web service. For example, in virtual hosting
solutions, multiple services can be hosted as co-tenants on the same server,
and the IP address and port do not uniquely identify a service. In cloud or
Content Delivery Network (CDN) solutions, a given platform hosts the services
or servers of a lot of organizations, and looking up what netblock
an IP address belongs to reveals little. However, multiplexed servers
rely on the Server Name Information (SNI) TLS extension to direct connections
to the appropriate service implementation. This protocol element
is transmitted in cleartext. As the other methods of monitoring get
blocked, monitoring focuses on the cleartext SNI. The purpose
of SNI encryption is to prevent that and aid privacy.
Replacing cleartext SNI transmission by an encrypted variant will
improve the privacy and reliability of TLS connections, but the design
of proper SNI encryption solutions is difficult.
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 . In the absence of
a TLS-level solution, the most popular approach to SNI privacy for web
services is HTTP-level fronting, which we discuss in .
This document does not present the design of a solution but
provides guidelines for evaluating proposed solutions. (The review of
HTTP-level solutions in is not
an endorsement of these solutions.)
The need for related work on the encryption of the Application-Layer
Protocol Negotiation (ALPN) parameters of TLS is discussed in
.
History of the TLS SNI Extension
The SNI extension was specified in 2003 in to facilitate management
of "colocation servers", in which multiple services shared the same IP
address. A typical example would be multiple websites served by the
same web server. The SNI extension carries the name of a specific
server, enabling the TLS connection to be established with the desired
server context. The current SNI extension specification can be
found in .
The SNI specification allowed for different types of server names,
though only the "hostname" variant was specified and deployed. In that
variant, the SNI extension carries the domain name of the target
server. The SNI extension is carried in cleartext in the TLS "ClientHello"
message.
Unanticipated Usage of SNI Information
The SNI was defined to facilitate management of servers, but the
developers of middleboxes found out that they could take
advantage of the information. Many examples of such usage are
reviewed in . Other examples came out
during discussions of this document. They include:
- Filtering or censoring specific services for a variety of reasons
- Content filtering by network operators or ISPs blocking specific
websites, for example, to implement parental controls or to prevent access
to phishing or other fraudulent websites
- ISP assigning different QoS profiles to target services
- Firewalls within enterprise networks blocking websites not deemed
appropriate for work
- Firewalls within enterprise networks exempting specific websites from
man-in-the-middle (MITM) inspection, such as healthcare or financial
sites for which inspection would intrude on the privacy of employees
The SNI is probably also included in the general collection of metadata
by pervasive surveillance actors ,
for example, to identify services
used by surveillance targets.
SNI Encryption Timeliness
The cleartext transmission of the SNI was not flagged as a problem
in the Security Considerations sections of , , or
. These specifications did not anticipate the
alternative usage described
in . One reason may be that, when
these RFCs were written, the
SNI information was available through a variety of other means,
such as tracking IP addresses, DNS names, or server certificates.
Many deployments still allocate different IP addresses to different
services, so that different services can be identified by their IP
addresses. However, CDNs commonly
serve a large number of services through a comparatively small
number of addresses.
The SNI carries the domain name of the server, which is also sent as
part of the DNS queries. Most of the SNI usage described in
could also be implemented by monitoring DNS traffic or controlling DNS
usage. But this is changing with the advent of DNS resolvers
providing services like DNS over TLS
or DNS over HTTPS .
The subjectAltName extension of type dNSName of the server certificate
(or in its absence, the common name component) exposes
the same name as the SNI. In TLS versions 1.0 , 1.1 ,
and 1.2 , servers send certificates in cleartext,
ensuring that there would be limited benefits in hiding the SNI. However,
in TLS 1.3 , server certificates are
encrypted in transit.
Note that encryption alone is insufficient to protect server certificates;
see for details.
The decoupling of IP addresses and server names, deployment of DNS
privacy, and protection of server certificate transmissions all
contribute to user privacy in the face of an RFC 7258-style adversary
. Encrypting the SNI
complements this push for privacy and makes it harder to censor or
otherwise provide differential treatment to specific Internet
services.
End-to-End Alternatives
Deploying SNI encryption helps thwart most of the unanticipated SNI usages,
including censorship and pervasive surveillance, but it also
will break or reduce the efficacy of the operational practices and
techniques implemented in middleboxes, as described in . Most of
these functions can, however, be realized by other means. For example, some DNS service
providers offer customers the provision to "opt in" to filtering services
for parental control and phishing protection. Per-stream QoS could be provided by
a combination of packet marking and end-to-end agreements. As
SNI encryption becomes common, we can expect more deployment of such "end-to-end"
solutions.
At the time of this writing, enterprises have the option of installing a
firewall performing SNI filtering to
prevent connections to certain websites. With SNI encryption, this becomes ineffective.
Obviously, managers could block usage of SNI encryption in enterprise computers, but
this wide-scale blocking would diminish the privacy protection of traffic leaving the
enterprise, which may not be desirable.
Enterprise managers could rely instead on filtering software and management
software deployed on the enterprise's computers.
Security and Privacy Requirements for SNI Encryption
Over the past years, there have been multiple proposals to add an SNI encryption
option in TLS. A review of the TLS mailing list archives shows that
many of these proposals appeared promising but were rejected
after security reviews identified plausible attacks. In this section,
we collect a list of these known attacks.
Mitigate Cut-and-Paste Attacks
The simplest SNI encryption designs
replace the cleartext SNI in the initial TLS
exchange 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 cut-and-paste attack, which works
as follows:
- The user starts a TLS connection to the multiplexed server, including an encrypted
SNI value.
- The adversary observes the exchange and copies the encrypted SNI parameter.
- The adversary starts its own connection to the multiplexed server, including in its
connection parameters the encrypted SNI copied from the observed exchange.
- The multiplexed server establishes the connection to the protected service, which sends its certificate, 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 cut-and-paste attacks break that goal by
allowing adversaries to discover that service.
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 user 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.
Prevent SNI-Based Denial-of-Service Attacks
Encrypting the SNI may create extra load for the multiplexed server. Adversaries may mount
denial-of-service (DoS) 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 avenue for DoS attacks,
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.
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 ClientHello packets or specific values of the cleartext 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 were 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.
This requirement to not stick out may be difficult to meet in
practice, as noted in .
Maintain Forward Secrecy
TLS 1.3 is designed to provide forward
secrecy, so that (for example) keys used in past sessions will not be
compromised even if the private key of the server is compromised. The
general concerns about forward secrecy apply to SNI encryption as
well. 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 should be compromised, the adversaries would
be able to process archival records of past connections and retrieve
the protected SNI used in these connections. These designs fail to
maintain forward secrecy of SNI encryption.
Enable Multi-party Security Contexts
We can design solutions in which a fronting
service acts as a relay to reach the protected service. Some of those
solutions involve just one TLS handshake between the client and the fronting service.
The master secret is verified by verifying a certificate provided by
the fronting service but not by the protected service.
These solutions expose the client to a MITM attack by
the fronting service. Even if the client
has some reasonable trust in this service, the possibility of a
MITM attack is troubling.
There are other classes of solutions in which the master secret is verified by
verifying a certificate provided by the protected service. These solutions offer
more protection against a MITM attack by the fronting service.
The
downside is that the client will not verify the identity of the fronting service,
which enables fronting server spoofing attacks, such as the "honeypot" attack
discussed below. Overall, end-to-end TLS to the protected service is preferable,
but it is important to also provide a way to authenticate the fronting service.
The fronting service could be pressured by adversaries.
By design, it could be forced to deny access to
the protected service or to divulge which client accessed it. But
if a MITM attack is possible, the adversaries would also be able to pressure
the fronting service into intercepting or spoofing the communications between
client and protected service.
Adversaries could also 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.
Support Multiple Protocols
The SNI encryption requirement does not stop with HTTP over
TLS.
Multiple other
applications currently use TLS, including, for example, SMTP ,
DNS , IMAP ,
and the Extensible Messaging and Presence Protocol (XMPP) . These applications, too,
will benefit from SNI encryption.
HTTP-only methods, like those described in ,
would not apply there. In fact, even for the HTTPS case, the HTTPS tunneling
service described in is
compatible with HTTP 1.0 and HTTP 1.1
but interacts awkwardly with the multiple streams feature of HTTP/2 .
This points to the need for an application-agnostic solution, which would be
implemented fully in the TLS layer.
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
cleartext 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 networks. 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 were hidden in the same way as the SNI.
In addition to hiding the SNI, it is thus desirable to also hide
the ALPN. Of course, this implies engineering trade-offs. Using the
same technique for hiding the ALPN and encrypting the SNI may result
in excess complexity. It might be preferable to encrypt these
independently.
Supporting Other Transports than TCP
The TLS handshake is also used over other transports, such as UDP
with both DTLS and
QUIC . The requirement to
encrypt the SNI applies 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.
HTTP Co-tenancy Fronting
In the absence of TLS-level SNI encryption, many sites rely on an
"HTTP co-tenancy" solution, often referred to as "domain fronting" . 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"
(accessing the hidden service). This solution works well in
practice when the fronting server and the hidden server
are "co-tenants" of the same multiplexed server.
The HTTP domain fronting solution can be deployed without modification to
the TLS protocol and does not require using any specific version of
TLS. There are, however, a few issues regarding discovery, client
implementations, trust, and applicability:
- The client has to discover that the hidden service can be accessed
through the fronting server.
- The client's browser has to be directed to access the hidden
service through the fronting service.
- Since the TLS connection is established with the fronting service,
the client has no cryptographic proof that the content does, in fact,
come from the hidden service. Thus, the solution does not mitigate the
context sharing issues described in . Note that this is already the case for
co-tenanted sites.
- Since this is an HTTP-level solution, it does not protect non-HTTP
protocols, as discussed in .
The discovery issue is common to most SNI encryption solutions.
The browser issue was solved in by
implementing domain fronting as a pluggable transport for the Tor browser. The
multi-protocol issue can be mitigated by implementing other
applications over HTTP, for example, DNS over HTTPS . The trust issue, however, requires
specific developments.
HTTPS Tunnels
The HTTP domain fronting solution places a lot of trust in the fronting
server. This required trust can be reduced by tunneling 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 .
The HTTPS-in-HTTPS solution requires double encryption of every packet. It
also requires that the fronting server decrypt and relay messages to the
hidden server. Both of these requirements make the implementation onerous.
Delegation Control
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 requires a controlled
way to indicate which fronting server is acceptable by the hidden service.
This problem is similar to the "word of mouth" variant
of the "fronting server
spoofing" attack described in . The spoofing
would be performed by distributing fake advice, such as "to reach
hidden.example.com, use fake.example.com as a fronting
server", when "fake.example.com" is under the control of an
adversary.
In practice, this attack is well mitigated when the hidden service
is accessed through a specialized application. The name of the
fronting server can then be programmed in the code of the
application. But the attack is harder to mitigate when the hidden
service has to be accessed through general-purpose web browsers.
There are several proposed solutions to this problem, such as creating
a special form of certificate to codify the relation between the fronting and
hidden server or obtaining the relation between the hidden and fronting service
through the DNS, possibly using DNSSEC, to avoid spoofing.
The experiment
described in solved the issue by
integrating with the Lantern Internet circumvention tool.
We can observe that CDNs have a similar requirement.
They need to convince the client that "www.example.com" can be accessed
through the seemingly unrelated "cdn-node-xyz.example.net". Most CDNs have
deployed DNS-based solutions to this problem. However, the CDN often
holds the authoritative certificate of the origin. There is, simultaneously,
verification of a relationship between the origin and the CDN (through the
certificate) and a risk that the CDN can spoof the
content from the origin.
Related Work
The ORIGIN frame defined for HTTP/2 can be used to flag content provided by the hidden
server. Secondary certificate authentication can
be used to manage authentication of hidden server content or to
perform client authentication before accessing hidden content.
Security Considerations
This document lists a number of attacks against SNI encryption in Sections
and and presents a list of
requirements to mitigate these attacks. Current HTTP-based solutions
described in only meet some of
these requirements. In practice, it may well be that no solution can meet
every requirement and that practical solutions will have to make some
compromises.
In particular, the requirement to not stick out, presented in
, may have to be lifted,
especially for proposed solutions that could quickly reach large-scale
deployments.
Replacing cleartext SNI transmission by an encrypted variant will
break or reduce the efficacy of the operational practices and techniques
implemented in middleboxes, as described in . As explained in , alternative solutions will have to be developed.
IANA Considerations
This document has no IANA actions.
Informative References
Blocking-resistant communication through domain fronting
Acknowledgements
A large part of this document originated 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:
.
Thanks to for his multiple suggestions, reviews, and edits
to the successive draft versions of this document.
Thanks to for a pretty
detailed review of the initial draft of this document. Thanks to
, ,
, , , , ,
, ,
, , , , and employees of the UK National Cyber
Security Centre for their reviews. Thanks to , , and for helping move this document toward publication.