< draft-ietf-tls-sni-encryption-04.txt   draft-ietf-tls-sni-encryption-05.txt >
Network Working Group C. Huitema Network Working Group C. Huitema
Internet-Draft Private Octopus Inc. Internet-Draft Private Octopus Inc.
Intended status: Informational E. Rescorla Intended status: Informational E. Rescorla
Expires: May 26, 2019 RTFM, Inc. Expires: February 16, 2020 RTFM, Inc.
November 22, 2018 August 15, 2019
Issues and Requirements for SNI Encryption in TLS Issues and Requirements for SNI Encryption in TLS
draft-ietf-tls-sni-encryption-04 draft-ietf-tls-sni-encryption-05
Abstract Abstract
This draft describes the general problem of encryption of the Server This draft describes the general problem of encrypting the Server
Name Identification (SNI) parameter. The proposed solutions hide a Name Identification (SNI) TLS parameter. The proposed solutions hide
Hidden Service behind a Fronting Service, only disclosing the SNI of a Hidden Service behind a fronting service, only disclosing the SNI
the Fronting Service to external observers. The draft lists known of the fronting service to external observers. The draft lists known
attacks against SNI encryption, discusses the current "co-tenancy attacks against SNI encryption, discusses the current "co-tenancy
fronting" solution, and presents requirements for future TLS layer fronting" solution, and presents requirements for future TLS layer
solutions. solutions.
In practice, it may well be that no solution can meet every In practice, it may well be that no solution can meet every
requirement, and that practical solutions will have to make some requirement, and that practical solutions will have to make some
compromises. compromises.
Status of This Memo Status of This Memo
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Internet-Drafts are working documents of the Internet Engineering Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet- working documents as Internet-Drafts. The list of current Internet-
Drafts is at https://datatracker.ietf.org/drafts/current/. Drafts is at https://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress." material or to cite them other than as "work in progress."
This Internet-Draft will expire on May 26, 2019. This Internet-Draft will expire on February 16, 2020.
Copyright Notice Copyright Notice
Copyright (c) 2018 IETF Trust and the persons identified as the Copyright (c) 2019 IETF Trust and the persons identified as the
document authors. All rights reserved. document authors. All rights reserved.
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Table of Contents Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. History of the TLS SNI extension . . . . . . . . . . . . . . 3 2. History of the TLS SNI extension . . . . . . . . . . . . . . 3
2.1. Unanticipated usage of SNI information . . . . . . . . . 3 2.1. Unanticipated usage of SNI information . . . . . . . . . 3
2.2. SNI encryption timeliness . . . . . . . . . . . . . . . . 4 2.2. SNI encryption timeliness . . . . . . . . . . . . . . . . 4
2.3. End-to-end alternatives . . . . . . . . . . . . . . . . . 4 2.3. End-to-end alternatives . . . . . . . . . . . . . . . . . 5
3. Security and Privacy Requirements for SNI Encryption . . . . 5 3. Security and Privacy Requirements for SNI Encryption . . . . 5
3.1. Mitigate Replay Attacks . . . . . . . . . . . . . . . . . 5 3.1. Mitigate Replay Attacks . . . . . . . . . . . . . . . . . 5
3.2. Avoid Widely Shared Secrets . . . . . . . . . . . . . . . 6 3.2. Avoid Widely Shared Secrets . . . . . . . . . . . . . . . 6
3.3. Prevent SNI-based Denial of Service Attacks . . . . . . . 6 3.3. Prevent SNI-based Denial of Service Attacks . . . . . . . 6
3.4. Do not stick out . . . . . . . . . . . . . . . . . . . . 6 3.4. Do not stick out . . . . . . . . . . . . . . . . . . . . 6
3.5. Forward Secrecy . . . . . . . . . . . . . . . . . . . . . 6 3.5. Forward Secrecy . . . . . . . . . . . . . . . . . . . . . 7
3.6. Proper Security Context . . . . . . . . . . . . . . . . . 7 3.6. Multi-Party Security Contexts . . . . . . . . . . . . . . 7
3.7. Fronting Server Spoofing . . . . . . . . . . . . . . . . 7 3.7. Supporting multiple protocols . . . . . . . . . . . . . . 8
3.8. Supporting multiple protocols . . . . . . . . . . . . . . 7 3.7.1. Hiding the Application Layer Protocol Negotiation . . 8
3.8.1. Hiding the Application Layer Protocol Negotiation . . 8 3.7.2. Support other transports than TCP . . . . . . . . . . 8
3.8.2. Support other transports than HTTP . . . . . . . . . 8 4. HTTP Co-Tenancy Fronting . . . . . . . . . . . . . . . . . . 9
4. HTTP Co-Tenancy Fronting . . . . . . . . . . . . . . . . . . 8 4.1. HTTPS Tunnels . . . . . . . . . . . . . . . . . . . . . . 10
4.1. HTTPS Tunnels . . . . . . . . . . . . . . . . . . . . . . 9
4.2. Delegation Control . . . . . . . . . . . . . . . . . . . 10 4.2. Delegation Control . . . . . . . . . . . . . . . . . . . 10
4.3. Related work . . . . . . . . . . . . . . . . . . . . . . 10 4.3. Related work . . . . . . . . . . . . . . . . . . . . . . 11
5. Security Considerations . . . . . . . . . . . . . . . . . . . 10 5. Security Considerations . . . . . . . . . . . . . . . . . . . 11
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 11 6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 11
7. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 11 7. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 11
8. Informative References . . . . . . . . . . . . . . . . . . . 11 8. Informative References . . . . . . . . . . . . . . . . . . . 12
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 13 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 13
1. Introduction 1. Introduction
Historically, adversaries have been able to monitor the use of web Historically, adversaries have been able to monitor the use of web
services through three channels: looking at DNS requests, looking at services through three primary channels: looking at DNS requests,
IP addresses in packet headers, and looking at the data stream looking at IP addresses in packet headers, and looking at the data
between user and services. These channels are getting progressively stream between user and services. These channels are getting
closed. A growing fraction of Internet communication is encrypted, progressively closed. A growing fraction of Internet communication
mostly using Transport Layer Security (TLS) [RFC5246]. Progressive is encrypted, mostly using Transport Layer Security (TLS) [RFC5246].
deployment of solutions like DNS in TLS [RFC7858] mitigates the Progressive deployment of solutions like DNS in TLS [RFC7858] and DNS
disclosure of DNS information. More and more services are colocated over HTTPS [RFC8484] mitigates the disclosure of DNS information.
on multiplexed servers, loosening the relation between IP address and More and more services are colocated on multiplexed servers,
web service. However, multiplexed servers rely on the Service Name loosening the relation between IP address and web service. However,
Information (SNI) to direct TLS connections to the appropriate multiplexed servers rely on the Service Name Information (SNI) TLS
service implementation. This protocol element is transmitted in extension to direct connections to the appropriate service
clear text. As the other methods of monitoring get blocked, implementation. This protocol element is transmitted in clear text.
monitoring focuses on the clear text SNI. The purpose of SNI As the other methods of monitoring get blocked, monitoring focuses on
encryption is to prevent that. the clear text SNI. The purpose of SNI encryption and privacy is to
prevent that.
In the past, there have been multiple attempts at defining SNI In the past, there have been multiple attempts at defining SNI
encryption. These attempts have generally floundered, because the encryption. These attempts have generally floundered, because the
simple designs fail to mitigate several of the attacks listed in simple designs fail to mitigate several of the attacks listed in
Section 3. In the absence of a TLS level solution, the most popular Section 3. In the absence of a TLS-level solution, the most popular
approach to SNI privacy is HTTP level fronting, which we discuss in approach to SNI privacy for web services is HTTP-level fronting,
Section 4. which we discuss in Section 4.
2. History of the TLS SNI extension 2. History of the TLS SNI extension
The SNI extension was standardized in 2003 in [RFC3546] to facilitate The SNI extension was specified in 2003 in [RFC3546] to facilitate
management of "colocation servers", in which a multiple services management of "colocation servers", in which multiple services shared
shared the same IP address. A typical example would be mutiple web the same IP address. A typical example would be mutiple web sites
sites served by the same web server. The SNI extension carries the served by the same web server. The SNI extension carries the name of
name of a specific server, enabling the TLS connection to be a specific server, enabling the TLS connection to be established with
established with the desired server context. The current SNI the desired server context. The current SNI extension specification
extension specification can be found in [RFC6066]. can be found in [RFC6066].
The SNI specification allowed for different types of server names, The SNI specification allowed for different types of server names,
but only the "hostname" variant was standardized and deployed. In though only the "hostname" variant was specified and deployed. In
that variant, the SNI extension carries the domain name of the target that variant, the SNI extension carries the domain name of the target
server. The SNI extension is carried in clear text in the TLS server. The SNI extension is carried in clear text in the TLS
"Client Hello" message. "ClientHello" message.
2.1. Unanticipated usage of SNI information 2.1. Unanticipated usage of SNI information
The SNI was defined to facilitate management of servers, but the The SNI was defined to facilitate management of servers, though the
developer of middleboxes soon found out that they could take developers of middleboxes soon found out that they could take
advantage of the information. Many examples of such usage are advantage of the information. Many examples of such usage are
reviewed in [RFC8404]. They include: reviewed in [RFC8404]. They include:
o Censorship of specific sites by "national firewalls", o Monitoring and identification of specific sites,
o Content filtering by ISP blocking specific web sites in order to o Content filtering by ISP blocking specific web sites in order to
implement "parental controls", or to prevent access to fraudulent implement "parental controls", or to prevent access to phishing or
web sites, such as used for phishing, other fradulent web sites.
o ISP assigning different QOS profiles to target services, o ISP assigning different QoS profiles to target services,
o Enterprise firewalls blocking web sites not deemed appropriate for o Firewalls within enterprise networks blocking web sites not deemed
work, or appropriate for work, or
o Enterprise firewalls exempting specific web sites from MITM o Firewalls within enterprise networks exempting specific web sites
inspection, such as healthcare or financial sites for which from MITM inspection, such as healthcare or financial sites for
inspection would intrude with the privacy of employees. which inspection would intrude with the privacy of employees.
The SNI is probably also included in the general collection of The SNI is probably also included in the general collection of
metadata by pervasive surveillance actors. metadata by pervasive surveillance actors.
2.2. SNI encryption timeliness 2.2. SNI encryption timeliness
The clear-text transmission of the SNI was not flagged as a problem The clear-text transmission of the SNI was not flagged as a problem
in the security consideration sections of [RFC3546], [RFC4366], or in the security consideration sections of [RFC3546], [RFC4366], or
[RFC6066]. These specifications did not anticipate the alternative [RFC6066]. These specifications did not anticipate the alternative
uses and abuses described in Section 2.1. One reason may be that, uses and abuses described in Section 2.1. One reason may be that,
when these RFCs were written, the SNI information was available when these RFCs were written, the SNI information was available
through a variety of other means. through a variety of other means.
Many deployments still allocate different IP addresses to different Many deployments still allocate different IP addresses to different
services, so that different services can be identified by their IP services, so that different services can be identified by their IP
addresses. However, content distribution networks (CDN) commonly addresses. However, content distribution networks (CDN) commonly
serve a large number of services through a small number of addresses. 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 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 part of the DNS queries. Most of the SNI usage described in
Section 2.1 could also be implemented by monitoring DNS traffic or Section 2.1 could also be implemented by monitoring DNS traffic or
controlling DNS usage. But this is changing with the advent of DNS controlling DNS usage. But this is changing with the advent of DNS
resolvers providing services like DNS over TLS [RFC7858] or DNS over resolvers providing services like DNS over TLS [RFC7858] or DNS over
HTTPS [RFC8484]. HTTPS [RFC8484].
The subjectAltName extension of type dNSName of the server The subjectAltName extension of type dNSName of the server
certificate, or in its absence the common name component, expose the certificate, or in its absence the common name component, expose the
same name as the SNI. In TLS versions 1.0 [RFC2246], 1.1 [RFC4346], same name as the SNI. In TLS versions 1.0 [RFC2246], 1.1 [RFC4346],
and 1.2 [RFC5246], the server send their certificate in clear text, and 1.2 [RFC5246], servers send certificates in clear text, ensuring
ensuring that there would be limited benefits in hiding the SNI. But that there would be limited benefits in hiding the SNI. However, in
the transmission of the server certificate is protected in TLS 1.3 TLS 1.3 [RFC8446], server certificates are encrypted in transit.
[RFC8446]. Note that encryption alone is insufficient to protect server
certificates; see Section 3.1 for details.
The decoupling of IP addresses and server names, the deployment of The decoupling of IP addresses and server names, deployment of DNS
DNS privacy, and the protection of server certificates transmissions privacy, and protection of server certificates transmissions all
all contribute to user privacy. Encrypting the SNI now will complete contribute to user privacy in the face of an [RFC3552]-style
this push for privacy and make it harder to censor specific internet adversary. Encrypting the SNI now will complete this push for
services. privacy and make it harder to censor or otherwise provide
differential treatment to specific internet services.
2.3. End-to-end alternatives 2.3. End-to-end alternatives
Deploying SNI encryption will help thwarting most of the Deploying SNI encryption will help thwarting most of the
"unanticipated" SNI usages described in Section 2.1, including "unanticipated" SNI usages described in Section 2.1, including
censorship and pervasive surveillance. It will also thwart functions censorship and pervasive surveillance. It will also thwart functions
that are sometimes described as legitimate. Most of these functions that are sometimes described as legitimate. Most of these functions
can however be realized by other means. For example, some DNS can however be realized by other means. For example, some DNS
service providers offer customers the provision to "opt in" filtering service providers offer customers the provision to "opt in" filtering
services for parental control and phishing protection. Per stream services for parental control and phishing protection. Per-stream
QoS can be provided by a combination of packet marking and end to end QoS can be provided by a combination of packet marking and end-to-end
agreements. As SNI encryption becomes common, we can expect more agreements. As SNI encryption becomes common, we can expect more
deployment of such "end to end" solutions. deployment of such "end-to-end" solutions.
At the moment enterprises have the option of installing a firewall At the moment, enterprises have the option of installing a firewall
performing SNI filtering to prevent connections to certain websites. performing SNI filtering to prevent connections to certain websites.
With SNI encryption this becomes ineffective. Obviously, managers With SNI encryption this becomes ineffective. Obviously, managers
could block usage of SNI encryption in enterprise computers, but this could block usage of SNI encryption in enterprise computers, but this
wide scale blocking would diminish the privacy protection of traffic wide-scale blocking would diminish the privacy protection of traffic
leaving the enterprise, which may not be desirable. Enterprises leaving the enterprise, which may not be desirable. Enterprise
managers could rely instead on filtering software and management managers could rely instead on filtering software and management
software deployed on enterprises computers. software deployed on the enterprise's computers.
3. Security and Privacy Requirements for SNI Encryption 3. Security and Privacy Requirements for SNI Encryption
Over the past years, there have been multiple proposals to add an SNI Over the past years, there have been multiple proposals to add an SNI
encryption option in TLS. Many of these proposals appeared encryption option in TLS. Many of these proposals appeared
promising, but were rejected after security reviews pointed plausible promising, though were rejected after security reviews identified
attacks. In this section, we collect a list of these known attacks. plausible attacks. In this section, we collect a list of these known
attacks.
3.1. Mitigate Replay Attacks 3.1. Mitigate Replay Attacks
The simplest SNI encryption designs replace in the initial TLS The simplest SNI encryption designs replace in the initial TLS
exchange the clear text SNI with an encrypted value, using a key exchange the clear text SNI with an encrypted value, using a key
known to the multiplexed server. Regardless of the encryption used, known to the multiplexed server. Regardless of the encryption used,
these designs can be broken by a simple replay attack, which works as these designs can be broken by a simple replay attack, which works as
follow: follow:
1- The user starts a TLS connection to the multiplexed server, 1- The user starts a TLS connection to the multiplexed server,
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3.3. Prevent SNI-based Denial of Service Attacks 3.3. Prevent SNI-based Denial of Service Attacks
Encrypting the SNI may create extra load for the multiplexed server. Encrypting the SNI may create extra load for the multiplexed server.
Adversaries may mount denial of service attacks by generating random Adversaries may mount denial of service attacks by generating random
encrypted SNI values and forcing the multiplexed server to spend encrypted SNI values and forcing the multiplexed server to spend
resources in useless decryption attempts. resources in useless decryption attempts.
It may be argued that this is not an important DOS avenue, as regular 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 TLS connection attempts also require the server to perform a number
of cryptographic operations. However, in many cases, the SNI of cryptographic operations. However, in many cases, the SNI
decryption will have to be performed by a front end component with decryption will have to be performed by a front-end component with
limited resources, while the TLS operations are performed by the limited resources, while the TLS operations are performed by the
component dedicated to their respective services. SNI based DOS component dedicated to their respective services. SNI-based DOS
attacks could target the front end component. attacks could target the front-end component.
3.4. Do not stick out 3.4. Do not stick out
In some designs, handshakes using SNI encryption can be easily In some designs, handshakes using SNI encryption can be easily
differentiated from "regular" handshakes. For example, some designs differentiated from "regular" handshakes. For example, some designs
require specific extensions in the Client Hello packets, or specific require specific extensions in the Client Hello packets, or specific
values of the clear text SNI parameter. If adversaries can easily values of the clear text SNI parameter. If adversaries can easily
detect the use of SNI encryption, they could block it, or they could detect the use of SNI encryption, they could block it, or they could
flag the users of SNI encryption for special treatment. flag the users of SNI encryption for special treatment.
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The general concerns about forward secrecy apply to SNI encryption The general concerns about forward secrecy apply to SNI encryption
just as well as to regular TLS sessions. For example, some proposed 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 designs rely on a public key of the multiplexed server to define the
SNI encryption key. If the corresponding private key was SNI encryption key. If the corresponding private key was
compromised, the adversaries would be able to process archival compromised, the adversaries would be able to process archival
records of past connections, and retrieve the protected SNI used in records of past connections, and retrieve the protected SNI used in
these connections. These designs failed to maintain forward secrecy these connections. These designs failed to maintain forward secrecy
of SNI encryption. of SNI encryption.
3.6. Proper Security Context 3.6. Multi-Party Security Contexts
We can design solutions in which a fronting service act as a relay to We can design solutions in which a fronting service act as a relay to
reach the protected service. Some of those solutions involve just reach the protected service. Some of those solutions involve just
one TLS handshake between the client and the fronting service. The one TLS handshake between the client and the fronting service. The
master secret is verified by verifying a certificate provided by the master secret is verified by verifying a certificate provided by the
fronting service, but not by the protected service. These solutions fronting service, but not by the protected service. These solutions
expose the client to a Man-In-The-Middle attack by the fronting expose the client to a Man-In-The-Middle attack by the fronting
service. Even if the client has some reasonable trust in this service. Even if the client has some reasonable trust in this
services, the possibility of MITM attack is troubling. service, the possibility of MITM attack is troubling.
There are other classes of solutions in which the master secret is There are other classes of solutions in which the master secret is
verified by verifying a certificate provided by the protected verified by verifying a certificate provided by the protected
service. These solutions offer more protection against a Man-In-The- service. These solutions offer more protection against a Man-In-The-
Middle attack by the fronting service. Middle attack by the fronting service. The downside is the the
client will not verify the identity of the fronting service with
risks discussed in , but solutions will have to mitigate this risks.
Overall, end-to-end TLS to the protected service is preferable.
The fronting service could be pressured by adversaries. By design, The fronting service could be pressured by adversaries. By design,
it could be forced to deny access to the protected service, or to it could be forced to deny access to the protected service, or to
divulge which client accessed it. But if MITM is possible, the divulge which client accessed it. But if MITM is possible, the
adversaries would also be able to pressure the fronting service into adversaries would also be able to pressure the fronting service into
intercepting or spoofing the communications between client and intercepting or spoofing the communications between client and
protected service. protected service.
3.7. Fronting Server Spoofing Adversaries could also mount an attack by spoofing the fronting
service. A spoofed fronting service could act as a "honeypot" for
Adversaries could mount an attack by spoofing the Fronting Service. users of hidden services. At a minimum, the fake server could record
A spoofed Fronting Service could act as a "honeypot" for users of the IP addresses of these users. If the SNI encryption solution
hidden services. At a minimum, the fake server could record the IP places too much trust on the fronting server, the fake server could
addresses of these users. If the SNI encryption solution places too also serve fake content of its own choosing, including various forms
much trust on the fronting server, the fake server could also serve of malware.
fake content of its own choosing, including various forms of malware.
There are two main channels by which adversaries can conduct this There are two main channels by which adversaries can conduct this
attack. Adversaries can simply try to mislead users into believing attack. Adversaries can simply try to mislead users into believing
that the honeypot is a valid Fronting Server, especially if that that the honeypot is a valid fronting server, especially if that
information is carried by word of mouth or in unprotected DNS information is carried by word of mouth or in unprotected DNS
records. Adversaries can also attempt to hijack the traffic to the records. Adversaries can also attempt to hijack the traffic to the
regular Fronting Server, using for example spoofed DNS responses or regular fronting server, using for example spoofed DNS responses or
spoofed IP level routing, combined with a spoofed certificate. spoofed IP level routing, combined with a spoofed certificate.
3.8. Supporting multiple protocols 3.7. Supporting multiple protocols
The SNI encryption requirement does not stop with HTTP over TLS. The SNI encryption requirement does not stop with HTTP over TLS.
Multiple other applications currently use TLS, including for example Multiple other applications currently use TLS, including for example
SMTP [RFC5246], DNS [RFC7858], or XMPP [RFC7590]. These applications SMTP [RFC5246], DNS [RFC7858], or XMPP [RFC7590]. These applications
too will benefit of SNI encryption. HTTP only methods like those too will benefit of SNI encryption. HTTP only methods like those
described in Section 4.1 would not apply there. In fact, even for described in Section 4.1 would not apply there. In fact, even for
the HTTPS case, the HTTPS tunneling service described in Section 4.1 the HTTPS case, the HTTPS tunneling service described in Section 4.1
is compatible with HTTP 1.0 and HTTP 1.1, but interacts awkwardly 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 with the multiple streams feature of HTTP 2.0 [RFC7540]. This points
to the need of an application agnostic solution, that would be to the need of an application-agnostic solution, that would be
implemented fully in the TLS layer. implemented fully in the TLS layer.
3.8.1. Hiding the Application Layer Protocol Negotiation 3.7.1. Hiding the Application Layer Protocol Negotiation
The Application Layer Protocol Negotiation (ALPN) parameters of TLS The Application Layer Protocol Negotiation (ALPN) parameters of TLS
allow implementations to negotiate the application layer protocol allow implementations to negotiate the application layer protocol
used on a given connection. TLS provides the ALPN values in clear used on a given connection. TLS provides the ALPN values in clear
text during the initial handshake. While exposing the ALPN does not text during the initial handshake. While exposing the ALPN does not
create the same privacy issues as exposing the SNI, there is still a create the same privacy issues as exposing the SNI, there is still a
risk. For example, some networks may attempt to block applications risk. For example, some networks may attempt to block applications
that they do not understand, or that they wish users would not use. 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 In a sense, ALPN filtering could be very similar to the filtering of
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the applications just move over HTTP, and only the HTTP ALPN values 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 are used. Applications would not need to do that if the ALPN was
hidden in the same way as the SNI. hidden in the same way as the SNI.
In addition to hiding the SNI, it is thus desirable to also hide the In addition to hiding the SNI, it is thus desirable to also hide the
ALPN. Of course, this implies engineering trade-offs. Using the ALPN. Of course, this implies engineering trade-offs. Using the
same technique for hiding the ALPN and encrypting the SNI may result same technique for hiding the ALPN and encrypting the SNI may result
in excess complexity. It might be preferable to encrypt these in excess complexity. It might be preferable to encrypt these
independently. independently.
3.8.2. Support other transports than HTTP 3.7.2. Support other transports than TCP
The TLS handshake is also used over other transports such as UDP with 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 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 requirement to encrypt the SNI apply just as well for these
transports as for TLS over TCP. transports as for TLS over TCP.
This points to a requirement for SNI Encryption mechanisms to also be This points to a requirement for SNI Encryption mechanisms to also be
applicable to non-TCP transports such as DTLS or QUIC. applicable to non-TCP transports such as DTLS or QUIC.
4. HTTP Co-Tenancy Fronting 4. HTTP Co-Tenancy Fronting
In the absence of TLS level SNI encryption, many sites rely on an In the absence of TLS-level SNI encryption, many sites rely on an
"HTTP Co-Tenancy" solution. The TLS connection is established with "HTTP Co-Tenancy" solution. The TLS connection is established with
the fronting server, and HTTP requests are then sent over that the fronting server, and HTTP requests are then sent over that
connection to the hidden service. For example, the TLS SNI could be connection to the hidden service. For example, the TLS SNI could be
set to "fronting.example.com", the fronting server, and HTTP requests set to "fronting.example.com", the fronting server, and HTTP requests
sent over that connection could be directed to "hidden.example.com", sent over that connection could be directed to "hidden.example.com",
accessing the hidden service. This solution works well in practice accessing the hidden service. This solution works well in practice
when the fronting server and the hidden server are "co-tenant" of the when the fronting server and the hidden server are "co-tenant" of the
same multiplexed server. same multiplexed server.
The HTTP fronting solution can be deployed without modification to The HTTP fronting solution can be deployed without modification to
skipping to change at page 9, line 20 skipping to change at page 9, line 34
TLS. There are however a few issues regarding discovery, client TLS. There are however a few issues regarding discovery, client
implementations, trust, and applicability: implementations, trust, and applicability:
o The client has to discover that the hidden service can be accessed o The client has to discover that the hidden service can be accessed
through the fronting server. through the fronting server.
o The client browser's has to be directed to access the hidden o The client browser's has to be directed to access the hidden
service through the fronting service. service through the fronting service.
o Since the TLS connection is established with 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 client has no cryptographic proof that the content does in
the hidden service. The solution does thus not mitigate the fact come from the hidden service. The solution does thus not
context sharing issues described in Section 3.6. mitigate the context sharing issues described in Section 3.6.
o Since this is an HTTP level solution, it would not protect non o Since this is an HTTP-level solution, it would not protect non-
HTTP protocols such as DNS over TLS [RFC7858] or IMAP over TLS HTTP protocols such as DNS over TLS [RFC7858] or IMAP over TLS
[RFC2595]. [RFC2595].
The discovery issue is common to pretty much every SNI encryption The discovery issue is common to most SNI encryption solutions. The
solution. The browser issue may be solved by developing a browser browser issue may be solved by developing a browser extension that
extension that support HTTP Fronting, and manages the list of support HTTP Fronting, and manages the list of fronting services
fronting services associated with the hidden services that the client associated with the hidden services that the client uses. The multi-
uses. The multi-protocol issue can be mitigated by using protocol issue can be mitigated by using implementation of other
implementation of other applications over HTTP, such as for example applications over HTTP, such as for example DNS over HTTPS [RFC8484].
DNS over HTTPS [RFC8484]. The trust issue, however, requires The trust issue, however, requires specific developments.
specific developments.
4.1. HTTPS Tunnels 4.1. HTTPS Tunnels
The HTTP Fronting solution places a lot of trust in the Fronting The HTTP Fronting solution places a lot of trust in the Fronting
Server. This required trust can be reduced by tunnelling HTTPS in Server. This required trust can be reduced by tunnelling HTTPS in
HTTPS, which effectively treats the Fronting Server as an HTTP Proxy. HTTPS, which effectively treats the Fronting Server as an HTTP Proxy.
In this solution, the client establishes a TLS connection to the In this solution, the client establishes a TLS connection to the
Fronting Server, and then issues an HTTP Connect request 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 Hidden Server. This will establish an end-to-end HTTPS over TLS
connection between the client and the Hidden Server, mitigating the connection between the client and the Hidden Server, mitigating the
skipping to change at page 10, line 14 skipping to change at page 10, line 30
4.2. Delegation Control 4.2. Delegation Control
Clients would see their privacy compromised if they contacted the Clients would see their privacy compromised if they contacted the
wrong fronting server to access the hidden service, since this wrong wrong fronting server to access the hidden service, since this wrong
server could disclose their access to adversaries. This requires a server could disclose their access to adversaries. This requires a
controlled way to indicate which fronting ferver is acceptable by the controlled way to indicate which fronting ferver is acceptable by the
hidden service. hidden service.
This problem is both similar and different from the "fronting server This problem is both similar and different from the "fronting server
spoofing" attack described in Section 3.7. Here, the spoofing would spoofing" attack described in Section 3.6. Here, the spoofing would
be performed by distributing fake advice, such as "to reach example be performed by distributing fake advice, such as "to reach example
hidden.example.com, use fake.example.com as a fronting server", when hidden.example.com, use fake.example.com as a fronting server", when
"fake.example.com" is under the control of an adversary. "fake.example.com" is under the control of an adversary.
In practice, this attack is well mitigated when the hidden service is In practice, this attack is well mitigated when the hidden service is
accessed through a specialized application. The name of the fronting accessed through a specialized application. The name of the fronting
server can then be programmed in the code of the application. But server can then be programmed in the code of the application. But
the attack is much harder to mitigate when the hidden service has to the attack is much harder to mitigate when the hidden service has to
be accessed through general purpose web browsers. The browsers will be accessed through general purpose web browsers. The browsers will
need a mechanism to obtain the fronting server indication in a secure need a mechanism to obtain the fronting server indication in a secure
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There are several proposed solutions to this problem, such as There are several proposed solutions to this problem, such as
creating a special form of certificate to codify the relation between creating a special form of certificate to codify the relation between
fronting and hidden server, or obtaining the relation between hidden fronting and hidden server, or obtaining the relation between hidden
and fronting service through the DNS, possibly using DNSSEC to avoid and fronting service through the DNS, possibly using DNSSEC to avoid
spoofing. spoofing.
We can observe that content distribution network have a similar We can observe that content distribution network have a similar
requirement. They need to convince the client that "www.example.com" requirement. They need to convince the client that "www.example.com"
can be accessed through the seemingly unrelated "cdn-node- can be accessed through the seemingly unrelated "cdn-node-
xyz.example.net". Most CDN have deployed DNS-based solutions to this xyz.example.net". Most CDNs have deployed DNS-based solutions to
problem. this problem.
4.3. Related work 4.3. Related work
The ORIGIN frame defined for HTTP/2 [RFC8336] can be used to flag The ORIGIN frame defined for HTTP/2 [RFC8336] can be used to flag
content provided by the hidden server. Secondary certificate content provided by the hidden server. Secondary certificate
authentication [I-D.ietf-httpbis-http2-secondary-certs] can be used authentication [I-D.ietf-httpbis-http2-secondary-certs] can be used
to manage authentication of hidden server content, or to perform to manage authentication of hidden server content, or to perform
client authentication before accessing hidden content. client authentication before accessing hidden content.
5. Security Considerations 5. Security Considerations
Replacing clear text SNI transmission by an encrypted variant will Replacing clear text SNI transmission by an encrypted variant will
improve the privacy and reliability of TLS connections, but the improve the privacy and reliability of TLS connections, but the
design of proper SNI encryption solutions is difficult. This design of proper SNI encryption solutions is difficult. This
document does not present the design of a solution, but provide document does not present the design of a solution, but provides
guidelines for evaluating proposed solutions. guidelines for evaluating proposed solutions.
This document lists a number of attacks against SNI encryption in This document lists a number of attacks against SNI encryption in
Section 3, and also in Section 4.2, and presents a list of Section 3, and also in Section 4.2, and presents a list of
requirements to mitigate these attacks. The current HTTP based requirements to mitigate these attacks. The current HTTP based
solutions described in Section 4 only meet some of these solutions described in Section 4 only meet some of these
requirements. In practice, it may well be that no solution can meet requirements. In practice, it may well be that no solution can meet
every requirement, and that practical solutions will have to make every requirement, and that practical solutions will have to make
some compromises. some compromises.
skipping to change at page 11, line 32 skipping to change at page 11, line 48
7. Acknowledgements 7. Acknowledgements
A large part of this draft originates in discussion of SNI encryption A large part of this draft originates in discussion of SNI encryption
on the TLS WG mailing list, including comments after the tunneling on the TLS WG mailing list, including comments after the tunneling
approach was first proposed in a message to that list: approach was first proposed in a message to that list:
<https://mailarchive.ietf.org/arch/msg/tls/ <https://mailarchive.ietf.org/arch/msg/tls/
tXvdcqnogZgqmdfCugrV8M90Ftw>. tXvdcqnogZgqmdfCugrV8M90Ftw>.
Thanks to Daniel Kahn Gillmor for a pretty detailed review of the Thanks to Daniel Kahn Gillmor for a pretty detailed review of the
initial draft. Thanks to Stephen Farrell, Mark Orchezowski, Martin initial draft. Thanks to Stephen Farrell, Martin Rex Martin Thomson
Rex and Martin Thomson for their reviews. and employees of the UK National Cyber Security Centre for their
reviews.
8. Informative References 8. Informative References
[I-D.ietf-httpbis-http2-secondary-certs] [I-D.ietf-httpbis-http2-secondary-certs]
Bishop, M., Sullivan, N., and M. Thomson, "Secondary Bishop, M., Sullivan, N., and M. Thomson, "Secondary
Certificate Authentication in HTTP/2", draft-ietf-httpbis- Certificate Authentication in HTTP/2", draft-ietf-httpbis-
http2-secondary-certs-03 (work in progress), October 2018. http2-secondary-certs-04 (work in progress), April 2019.
[I-D.ietf-quic-tls] [I-D.ietf-quic-tls]
Thomson, M. and S. Turner, "Using Transport Layer Security Thomson, M. and S. Turner, "Using TLS to Secure QUIC",
(TLS) to Secure QUIC", draft-ietf-quic-tls-16 (work in draft-ietf-quic-tls-22 (work in progress), July 2019.
progress), October 2018.
[I-D.ietf-tls-dtls13] [I-D.ietf-tls-dtls13]
Rescorla, E., Tschofenig, H., and N. Modadugu, "The Rescorla, E., Tschofenig, H., and N. Modadugu, "The
Datagram Transport Layer Security (DTLS) Protocol Version Datagram Transport Layer Security (DTLS) Protocol Version
1.3", draft-ietf-tls-dtls13-30 (work in progress), 1.3", draft-ietf-tls-dtls13-32 (work in progress), July
November 2018. 2019.
[RFC2246] Dierks, T. and C. Allen, "The TLS Protocol Version 1.0", [RFC2246] Dierks, T. and C. Allen, "The TLS Protocol Version 1.0",
RFC 2246, DOI 10.17487/RFC2246, January 1999, RFC 2246, DOI 10.17487/RFC2246, January 1999,
<https://www.rfc-editor.org/info/rfc2246>. <https://www.rfc-editor.org/info/rfc2246>.
[RFC2595] Newman, C., "Using TLS with IMAP, POP3 and ACAP", [RFC2595] Newman, C., "Using TLS with IMAP, POP3 and ACAP",
RFC 2595, DOI 10.17487/RFC2595, June 1999, RFC 2595, DOI 10.17487/RFC2595, June 1999,
<https://www.rfc-editor.org/info/rfc2595>. <https://www.rfc-editor.org/info/rfc2595>.
[RFC3546] Blake-Wilson, S., Nystrom, M., Hopwood, D., Mikkelsen, J., [RFC3546] Blake-Wilson, S., Nystrom, M., Hopwood, D., Mikkelsen, J.,
and T. Wright, "Transport Layer Security (TLS) and T. Wright, "Transport Layer Security (TLS)
Extensions", RFC 3546, DOI 10.17487/RFC3546, June 2003, Extensions", RFC 3546, DOI 10.17487/RFC3546, June 2003,
<https://www.rfc-editor.org/info/rfc3546>. <https://www.rfc-editor.org/info/rfc3546>.
[RFC3552] Rescorla, E. and B. Korver, "Guidelines for Writing RFC
Text on Security Considerations", BCP 72, RFC 3552,
DOI 10.17487/RFC3552, July 2003,
<https://www.rfc-editor.org/info/rfc3552>.
[RFC4346] Dierks, T. and E. Rescorla, "The Transport Layer Security [RFC4346] Dierks, T. and E. Rescorla, "The Transport Layer Security
(TLS) Protocol Version 1.1", RFC 4346, (TLS) Protocol Version 1.1", RFC 4346,
DOI 10.17487/RFC4346, April 2006, DOI 10.17487/RFC4346, April 2006,
<https://www.rfc-editor.org/info/rfc4346>. <https://www.rfc-editor.org/info/rfc4346>.
[RFC4366] Blake-Wilson, S., Nystrom, M., Hopwood, D., Mikkelsen, J., [RFC4366] Blake-Wilson, S., Nystrom, M., Hopwood, D., Mikkelsen, J.,
and T. Wright, "Transport Layer Security (TLS) and T. Wright, "Transport Layer Security (TLS)
Extensions", RFC 4366, DOI 10.17487/RFC4366, April 2006, Extensions", RFC 4366, DOI 10.17487/RFC4366, April 2006,
<https://www.rfc-editor.org/info/rfc4366>. <https://www.rfc-editor.org/info/rfc4366>.
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