| Network Working Group | C. Huitema |
| Internet-Draft | Private Octopus Inc. |
| Intended status: Informational | E. Rescorla |
| Expires: May 26, 2019 | RTFM, Inc. |
| November 22, 2018 |
Issues and Requirements for SNI Encryption in TLS
draft-ietf-tls-sni-encryption-04
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 lists known attacks against SNI encryption, discusses the current "co-tenancy fronting" 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.
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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 3. In the absence of a TLS level solution, the most popular approach to SNI privacy is HTTP level fronting, which we discuss in Section 4.
The SNI extension was standardized in 2003 in [RFC3546] to facilitate management of "colocation servers", in which a multiple services shared the same IP address. A typical example would be mutiple web sites 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 [RFC6066].
The SNI specification allowed for different types of server names, but only the "hostname" variant was standardized and deployed. In that variant, the SNI extension carries the domain name of the target server. The SNI extension is carried in clear text in the TLS "Client Hello" message.
The SNI was defined to facilitate management of servers, but the developer of middleboxes soon found out that they could take advantage of the information. Many examples of such usage are reviewed in [RFC8404]. They include:
The SNI is probably also included in the general collection of metadata by pervasive surveillance actors.
The clear-text transmission of the SNI was not flagged as a problem in the security consideration sections of [RFC3546], [RFC4366], or [RFC6066]. These specifications did not anticipate the alternative uses and abuses described in Section 2.1. One reason may be that, when these RFCs were written, the SNI information was available through a variety of other means.
Many deployments still allocate different IP addresses to different services, so that different services can be identified by their IP addresses. However, content distribution networks (CDN) commonly serve a large number of services through a 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 Section 2.1 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 [RFC7858] or DNS over HTTPS [RFC8484].
The subjectAltName extension of type dNSName of the server 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], and 1.2 [RFC5246], the server send their certificate in clear text, ensuring that there would be limited benefits in hiding the SNI. But the transmission of the server certificate is protected in TLS 1.3 [RFC8446].
The decoupling of IP addresses and server names, the deployment of DNS privacy, and the protection of server certificates transmissions all contribute to user privacy. Encrypting the SNI now will complete this push for privacy and make it harder to censor specific internet services.
Deploying SNI encryption will help thwarting most of the "unanticipated" SNI usages described in Section 2.1, including censorship and pervasive surveillance. It will also thwart functions that are sometimes described as legitimate. Most of these functions can however be realized by other means. For example, some DNS service providers offer customers the provision to "opt in" filtering services for parental control and phishing protection. Per stream QoS can 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 moment 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. Enterprises managers could rely instead on filtering software and management software deployed on enterprises computers.
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.
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.
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.
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.
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.
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.
We can design solutions in which 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 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 Man-In-The-Middle attack by the fronting service. Even if the client has some reasonable trust in this services, the possibility of 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 Man-In-The-Middle attack by 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 MITM 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 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.
The SNI encryption requirement does 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 4.1 would not apply there. In fact, even for 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 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.
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.
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.
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.
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", 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 any specific version of TLS. There are however a few issues regarding discovery, client implementations, trust, and applicability:
The discovery issue is common to pretty much every SNI encryption solution. 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 applications over HTTP, such as for example DNS over HTTPS [RFC8484]. The trust issue, however, requires specific developments.
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 3.6.
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.
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 ferver is acceptable by the hidden service.
This problem is both similar and different from the "fronting server spoofing" attack described in Section 3.7. Here, the spoofing would be performed by distributing fake advice, such as "to reach example 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 much harder to mitigate when the hidden service has to be accessed through general purpose web browsers. The browsers will need a mechanism to obtain the fronting server indication in a secure way.
There are several proposed solutions to this problem, such as creating a special form of certificate to codify the relation between fronting and hidden server, or obtaining the relation between hidden and fronting service through the DNS, possibly using DNSSEC to avoid spoofing.
We can observe that content distribution network 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 CDN have deployed DNS-based solutions to this problem.
The ORIGIN frame defined for HTTP/2 [RFC8336] can be used to flag content provided by the hidden server. Secondary certificate authentication [I-D.ietf-httpbis-http2-secondary-certs] can be used to manage authentication of hidden server content, or to perform client authentication before accessing hidden content.
Replacing clear text 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. This document does not present the design of a solution, but provide guidelines for evaluating proposed solutions.
This document lists a number of attacks against SNI encryption in Section 3, and also in Section 4.2, and presents a list of requirements to mitigate these attacks. The current HTTP based solutions described in Section 4 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 Section 3.4 may have to be lifted, especially for proposed solutions that could quickly reach large scale deployments.
This draft does not require any IANA action.
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>.
Thanks to Daniel Kahn Gillmor for a pretty detailed review of the initial draft. Thanks to Stephen Farrell, Mark Orchezowski, Martin Rex and Martin Thomson for their reviews.