How to configure HTTPS securely?

Wait, the *Secure* Socket Layer in HTTPS can be insecure? Yep, in the age of total surveillance, it can.

Good news: To the best of our knowledge, there is also secure SSL still. (But don't trust me on these instructions with your life or the life of your website users – you have to become your own expert!). The considerations below take into account both secrecy and server performance. The tips are in decreasing order of importance.

(1) Your users need an uncompromized computer!

Because if we'd start with compromized hardware, all is lost anyway. The malware can simply grab your communications from the browser screen and send it to the surveillance body. No need to break SSL, then. But of course, that would also be simple: that malware would also have an easy job to hide man-in-the-middle attacks by preventing the tools mentioned below from detecting SSL certificate changes.

The best first tip for having a non-compromized computer is having two: one for daily work, one for only the high-value communications. And you would not go near any threat on the Internet with the second one.

(2) Enforce HTTPS for all connections

For performance reasons, you might think about using SSL connections only for login (password transmission), or at least only while users are logged in (also protecting the content they post, and the session cookie which else can be used for session hijacking). However, surveillance can derive lots of metadata, behavioral data etc. also from looking at what people read while not being logged in. With proper SSL speed optimization, the server load of enforcing SSL everywhere should be manageable.

(3) Throw out insecure SSL cipher suites

Configure your webserver to not use:

  • the old utmost crap "export" cipher suites
  • plain DES (triple DES is ok though)
  • RC2
  • RC4 (which is kind of broken)

See the source for these recommendations. Note: The SSL Labs SSL Test is a nice site to check if your configuration works as intended.

From the remaining ones, all ciper suites that use at least 128 bit keys for symmetrical keys are ok. This is roughly equivalent to the security of 3072 bits RSA keys [source, p. 64], which are a sufficient protection against brute force attacks even beyond 2030 (as we will see below). Which means, encrypted data recorded now could only be broken some time after 2030. This is further protected by the fact that the symmetrical keys are only used for one SSL session, and using brute force attacks to decrypt one such small session from 20+ years ago is almost certainly not worth the effort in 2040 or so.

In a few years, when all browsers will support higher grade AES cipher suites and so on, you would of course switch to only allow at least 192 or 256 bits of security. Which is equivalent to 7680 resp. 15360 bit RSA keys [source, p. 64] and comes at relatively neglible performance costs of needing about 30% more CPU time for the same data throughput [source].

(4) Use only DHE Perfect Forward Secrecy key cipher suites

We want to use perfect forward secrecy (PFS) cipher suites. PFS means: when the private key of the server is leaked at some time, recorded communication of the past still cannot be decrypted. It only allows the attacker to impersonate the server for negotiating keys for new sessions, until the SSL certificate expires (which is hopefully soon).

Here is how to configure your webserver for using PFS cipher suites.

However, not all PFS ciphers are the same. As Bruce Schneier writes: "Prefer conventional discrete-log-based systems over elliptic-curve systems; the latter have constants that the NSA influences when they can." [source] This means, use Diffie-Hellman key exchange (DHE) cipher suites, not Elliptic-curve Diffie-Hellman key exchange (ECDHE) [source]. The danger of weak ECDHE ciphers is that recorded, encrypted communication could be broken later with limited effort attacks (albeit only session by session). The downside of DHE, on the other side, is "only" that it is slower.

(If you really want to use ECDHE for performance reasons, offer it only with elliptic curves that are safe. I am not sure if browsers support any safe curves [see] or how to restrict what curves your OpenSSL installation will offer to the client [see]. Tell us in the comments when you find it out.)

(5) Always store your private key in encrypted form

Which should mean, store it in encrypted form. This will require you to enter a passphrase when restarting your webserver process. But assuming that your webserver is stable, this would only be done when you are at the server anyway. It is quite common practice to store the private key in plain text so that it is only readable by root, but that is a severe vulnerability that would allow a remote attacker or somebody with physical access to the server to do a man-in-the-middle attack that does not need a certificate change.

(6) Use a 2048 bit private key

Because 1024 bit RSA private keys are considered too weak already.

A 2048 bit RSA private key for your sever is however enough. You want to avoid a needlessly longer key because SSL handshake performance exponentially degrades with increasing key length [source; test results].

2048 bits are enough because: With only forward secrecy cipher suits available, a broken or compromized private key of the server only means that an attacker can impersonate the server from that moment on (that is, can do a man-in-the-middle attack without causing a SSL certificate change). But he can not decipher past recorded communications. So the key strength has only to be enough to prevent brute force attacks during the certificate's validity. Which means 2048 bits until 2030, and 3072 bits afterwards [source]. But keep the validity period short of course (say, a year).

(7) Let your users monitor for SSL certificate changes

The problem with powerful surveillance bodies is that they are powerful: it is credibly alleged that three letter agencies can deploy man-in-the-middle attacks effortlessly on large scales by having backdoors in consumer DSL routers [source]. This also allows to compromise SSL connections, as follows. The router usually acts as a DNS server, and forwards requests to DNS servers of the telecommunications provider. When enabled by the backdoor, this allows DNS requests to be deflected to another DNS server, resulting (for example) in a site with a fraudulent SSL certificate being served to you (that will forward to the real site, but monitor your communications) [source, p. 23].

Such an attack might use a different SSL certificate signed by the same Certification Authority (CA) – or a different CA, it does not really matter since browsers by default do not notify users when a site's CA changed. But after all, the certificaate is different, and that is how it can be detected. Because: they simply cannot use these man-in-the-middle attacks on SSL for anything beyond targeted operations [source]. When used permanently, the attacks would be detected for example by site owners who notice the difference between wha certificate their site should serve to the world and what they see when visiting it in their browser.

In any case, it means that we should assume the CA-based certification mechanism to be completely broken, and should not rely on it. Until it is going to be replaced with a distributed, reliable mechanism (maybe a PGP style trust network? or registering the public key / domain name pairs on the Namecoin blockchain?), we have to make do with verifying the SSL certificate yourself (see below), or where this is too much effort, by tracking certificate changes.

Tracking certificate changes provides a decent protection against man-in-the-middle attacks, since these require exchanging the certificate, as shown above. You should monitor both the differences from what certificate everone else sees (using the Perspectives Firefox plugin) and from what certificate you saw in the past (using the Certificate Patrol Firefox plugin). From practical experience, Certificate Patrol is however not useful on large websites (Google, Facebook Twitter etc.) since these tend to use multiple certificates, from multiple CAs, and exchange them frequently. This makes using Certificate Patrol annoying; it would be better to have an option in it that lets you switch it on only for sites that you "do not want to be surveilled on".

(8) Let your users verify the SSL public key themselves

It is alleged that three-letter agencies collude with Certification Authorities (CAs) to get a second, different certificate signed by them for every new one they sign [source, p. 23]. Which will allow the man-in-the-middle attacks on SSL explained above. It is not known if this extends to any CA outside the US (where they can be subpoenaed …). But for practical purposes, let's just assume that all CAs are compromised.

A HTTPS certificate merely says that a website's traffic goes to whoever controls the domain plus whoever controls the CA. Even worse: since browsers by default do not notify a user when the HTTPS certificate changed and the new comes from a different CA, without manual checks at each page load a user can never be sure if the traffic does not perhaps go to certain three-letter agencies, as it can be safely assumed that at least some CAs are controlled by three-letter agencies. So, while HTTPS certificates are still good enough to exclude ordinary criminals, they are not up against massive surveillance.

(Also of course, you cannot trust the CAs yourself: never let them generate the private key for you, or upload your private key to them. With that precaution, even using a CA that is controlled by a three-letter agency is not a problem. They do not get you server's private key, they just sign your server's public key to attest it belongs to your server. Which is a correct statement, and not affected by being compromised. The compromised CA could create a duplicate certificate for a spy agency's own private key, but whether they use that or one from a different CA is just a matter of taste since a browser does not warn users against CA changes.)

In practical terms: You may proceed using a certfificate from a compromised CA for your "normal" website visitors, while at the same time also warning everyone that the HTTPS CA scheme is broken and users should not rely on it, instead should verify the SSL public key themselves. Tell your users to not simply trust the CA certificate represented by a happy Firefox displaying a lock to them. Tell them to verify by themselves, at the first time using the site, that the certificate is the one issued by the site operators instead of possibly a man-in-the-middle. Together with being alerted about certificate changes (see above), this provides a very decent protection against man-in-the-middle attacks.

Verifying can be difficult, depending on the size of your user community, but as tracking SSL certificate changes is usually enough by itself it does not have to be a very thorough vaidation. So you have different options. Here are some proposals, by increasing security:

  • Publish the fingerprint of the correct SSL certificate on the same website.
  • In the signup message, include the SSL certificate's fingerprint. This protects against later, dynamic modification of your website content by attackers.
  • As above, but also sign the signup message with the organization's GPG key, and publish the GPG public key on keyservers.
  • As above, but let users manually verify your GPG public key fingerprint (and also the current SSL certificate fingerprint while you're at it). This can be done without personal meeting of business card handovers etc., simply by creating a video live link, making sure it's not a pre-recorded video (some talking …) and letting the site representative both speak and sign the finger print characters at the same time. This of course mandates that the site representative is well-known, ideally by having met in person before.

(9) Use a self-signed certificate

With the above technique of still using CA-certified public keys but warning users against trusting them, most users would not see or simpply not follow the warning. They can be forced to deal with untrustable certificates though if you simply use a self-signed certificate. It will cause the browser to promt users to add a security exception, so they will have to verify the certificate before doing so.

(10) Let your users monitor IP address changes

If an attacker can get hold of the server's original SSL private key, he can impersonate the server without the tools above detecting an SSL certificate change. However, a similar tool would detect an IP address change. And you would announce the IP address of your server, and changes to it, for users to verify them, in analogy to how you want them to verify the SSL certificate fingerprint above. (And when you're at it, you might even want to switch to a self-signed certificate that would then also include the IP address, for free. At least on a second, synonymous domain or subdomain, for the users who know what they are doing.)

(11) Exchange your SSL private key and certificate frequently

Not sure about this one. It seems better to invalidate a SSL private key after a month than a year, which will prohibit an attacker from going on for long with man-in-the-middle attacks using your original private key that are undetectable by the above SSL certificate change monitoring. However, it introduces a task for users to manually verify the fingerprint of a new SSL certificate every month or so. Which is unrealistic for nearly all public-facing websites. Also it might not be needed because a man-in-the-middle attack could still be detected by IP address change monitoring proposed above. (However maybe even IP addresses can be bent by the secret services? I just don't know.)

(12) Put your server at a secure location

Even if you have encrypted your private key as stored on the server, there is a chance that it might be read from a memory dump. Which can be obtained when having physical access to your machine, or remote access to the virtualization system if you are on a VPS host (virtual private host). So at least do not rent a server in a country where three-letter agencies have easy access to company secrets, and also don't host at large hosting companies. Ideally of course, place the server physically at your home. And there into an intrusion-protected room. With lots of concrete around. In your basement. But ahh well … sorry, now I became paranoid about it all 😛







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