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Encrypt it or lose it: how encrypted SNI works

by Alessandro Ghedini.

Today we announced support for encrypted SNI, an extension to the TLS 1.3 protocol that improves privacy of Internet users by preventing on-path observers, including ISPs, coffee shop owners and firewalls, from intercepting the TLS Server Name Indication (SNI) extension and using it to determine which websites users are visiting.

Encrypted SNI, together with other Internet security features already offered by Cloudflare for free, will make it harder to censor content and track users on the Internet. Read on to learn how it works.

SNWhy?

The TLS Server Name Indication (SNI) extension, originally standardized back in 2003, lets servers host multiple TLS-enabled websites on the same set of IP addresses, by requiring clients to specify which site they want to connect to during the initial TLS handshake. Without SNI the server wouldn’t know, for example, which certificate to serve to the client, or which configuration to apply to the connection.

The client adds the SNI extension containing the hostname of the site it’s connecting to to the ClientHello message. It sends the ClientHello to the server during the TLS handshake. Unfortunately the ClientHello message is sent unencrypted, due to the fact that client and server don’t share an encryption key at that point.

TLS 1.3 with Unencrypted SNI

This means that an on-path observer (say, an ISP, coffee shop owner, or a firewall) can intercept the plaintext ClientHello message, and determine which website the client is trying to connect to. That allows the observer to track which sites a user is visiting.

But with SNI encryption the client encrypts the SNI even though the rest of the ClientHello is sent in plaintext.

TLS 1.3 with Encrypted SNI

So how come the original SNI couldn’t be encrypted before, but now it can? Where does the encryption key come from if client and server haven’t negotiated one yet?

If the chicken must come before the egg, where do you put the chicken?

As with many other Internet features the answer is simply “DNS”.

The server publishes a public key on a well-known DNS record, which can be fetched by the client before connecting (as it already does for A, AAAA and other records). The client then replaces the SNI extension in the ClientHello with an “encrypted SNI” extension, which is none other than the original SNI extension, but encrypted using a symmetric encryption key derived using the server’s public key, as described below. The server, which owns the private key and can derive the symmetric encryption key as well, can then decrypt the extension and therefore terminate the connection (or forward it to a backend server). Since only the client, and the server it’s connecting to, can derive the encryption key, the encrypted SNI cannot be decrypted and accessed by third parties.

It’s important to note that this is an extension to TLS version 1.3 and above, and doesn’t work with previous versions of the protocol. The reason is very simple: one of the changes introduced by TLS 1.3 (not without problems) meant moving the Certificate message sent by the server to the encrypted portion of the TLS handshake (before 1.3, it was sent in plaintext). Without this fundamental change to the protocol, an attacker would still be able to determine the identity of the server by simply observing the plaintext certificate sent on the wire.

The underlying cryptographic machinery involves using the Diffie-Hellman key exchange algorithm which allows client and server to generate a shared encryption key over an untrusted channel. The encrypted SNI encryption key is thus calculated on the client-side by using the server’s public key (which is actually the public portion of a Diffie-Hellman semi-static key share) and the private portion of an ephemeral Diffie-Hellman share generated by the client itself on the fly and discarded immediately after the ClientHello is sent to the server. Additional data (such as some of the cryptographic parameters sent by the client as part of its ClientHello message) is also mixed into the cryptographic process for good measure.

The client’s ESNI extension will then include, not only the actual encrypted SNI bits, but also the client’s public key share, the cipher suite it used for encryption and the digest of the server’s ESNI DNS record. On the other side, the server uses its own private key share, and the public portion of the client’s share to generate the encryption key and decrypt the extension.

While this may seem overly complicated, this ensures that the encryption key is cryptographically tied to the specific TLS session it was generated for, and cannot be reused across multiple connections. This prevents an attacker able to observe the encrypted extension sent by the client from simply capturing it and replaying it to the server in a separate session to unmask the identity of the website the user was trying to connect to (this is known as “cut-and-paste” attack).

However a compromise of the server’s private key would put all ESNI symmetric keys generated from it in jeopardy (which would allow observers to decrypt previously collected encrypted data), which is why Cloudflare’s own SNI encryption implementation rotates the server’s keys every hour to improve forward secrecy, but keeps track of the keys for the previous few hours to allow for DNS caching and replication delays, so that clients with slightly outdated keys can still use ESNI without problems (but eventually all keys are discarded and forgotten).

But wait, DNS? For real?

The observant reader might have realized that simply using DNS (which is, by default, unencrypted) would make the whole encrypted SNI idea completely pointless: an on-path observer would be able to determine which website the client is connecting to by simply observing the plaintext DNS queries sent by the client itself, whether encrypted SNI was used or not.

But with the introduction of DNS features such as DNS over TLS (DoT) and DNS over HTTPS (DoH), and of public DNS resolvers that provide those features to their users (such as Cloudflare’s own 1.1.1.1), DNS queries can now be encrypted and protected by the prying eyes of censors and trackers alike.

However, while responses from DoT/DoH DNS resolvers can be trusted, to a certain extent (evil resolvers notwithstanding), it might still be possible for a determined attacker to poison the resolver’s cache by intercepting its communication with the authoritative DNS server and injecting malicious data. That is, unless both the authoritative server and the resolver support DNSSEC[1]. Incidentally, Cloudflare’s authoritative DNS servers can sign responses returned to resolvers, and the 1.1.1.1 resolver can verify them.

What about the IP address?

While both DNS queries and the TLS SNI extensions can now be protected by on-path attackers, it might still be possible to determine which websites users are visiting by simply looking at the destination IP addresses on the traffic originating from users’ devices. Some of our customers are protected by this to a certain degree thanks to the fact that many Cloudflare domains share the same sets of addresses, but this is not enough and more work is required to protect end users to a larger degree. Stay tuned for more updates from Cloudflare on the subject in the future.

Where do I sign up?

Encrypted SNI is now enabled for free on all Cloudflare zones using our name servers, so you don’t need to do anything to enable it on your Cloudflare website. On the browser side, our friends at Firefox tell us that they expect to add encrypted SNI support this week to Firefox Nightly (keep in mind that the encrypted SNI spec is still under development, so it’s not stable just yet).

By visiting encryptedsni.com you can check how secure your browsing experience is. Are you using secure DNS? Is your resolver validating DNSSEC signatures? Does your browser support TLS 1.3? Did your browser encrypt the SNI? If the answer to all those questions is “yes” then you can sleep peacefully knowing that your browsing is protected from prying eyes.

Conclusion

Encrypted SNI, along with TLS 1.3, DNSSEC and DoT/DoH, plugs one of the few remaining holes that enable surveillance and censorship on the Internet. More work is still required to get to a surveillance-free Internet, but we are (slowly) getting there.

[1]: It's important to mention that DNSSEC could be disabled by BGP route hijacking between a DNS resolver and the TLD server. Last week we announced our commitment to RPKI and if DNS resolvers and TLDs also implement RPKI, this type of hijacking will be much more difficult.

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