Hanno's blog

I recently switched my personal web page and my blog to deliver content exclusively encrypted via HTTPS. I want to take this opportunity to give some facts about enabling TLS encryption by default and problems you may face.

First of all the non-problems: Enabling HTTPS by default is almost never a significant performance problem. If people tell me that they can not possibly enable HTTPS due to performance reasons the first thing I ask is if they believe this or if they have real benchmark data showing this. If you don't believe me on that, I can quote Adam Langley from Google here: "In January this year (2010), Gmail switched to using HTTPS for everything by default. Previously it had been introduced as an option, but now all of our users use HTTPS to secure their email between their browsers and Google, all the time. In order to do this we had to deploy no additional machines and no special hardware. On our production frontend machines, SSL/TLS accounts for less than 1% of the CPU load, less than 10KB of memory per connection and less than 2% of network overhead."

Enabling HTTPS may cause a number of compatibility issues you may not instantly think about. First of all, we know that IPs in the IPv4 space are limited and expensive these days, so many people probably can't afford having a distinct IP for their web page. The solution to that is a TLS extension called SNI (Server Name Indication) which allows to have different certificates for different domain names on the same IP. It works in all major browsers and has been working for quite some time. The only major browser you'll face these days that doesn't support SNI is the Android 2.x browser.

There are some subtle issues with SNI. One is that browsers have fallback modes if they cannot connect via TLS and that may lead to a connection downgrade to SSLv3. And that ancient protocol doesn't support extensions and thus no SNI. So you may have irregular certificate errors if you are on a bad connection. A solution to that on the server side is to just disable SSLv3. It will make SNI much more reliable.

I don't really have a clear picture how many browsers will fail with SNI. There are probably a number of embedded devices out there like smart TVs with browsers or things alike that have problems. If you have any experiences feel free to post them in the comments.

The first issue I only noticed after I switched to HTTPS: I had an application called RSS Graffiti set up to automatically post all articles I write to a facebook fan page. After changing to HTTPS only it silently stopped working. Re-adding my feed didn't work. I now found a similar service called dlvr.it that I now use to post my RSS feed to facebook. I can only assume that this is a glimpse of a much bigger problem: There are probably tons of applications and online services out there not prepared for an encrypted Internet. If we want more people to deploy encryption by default we need to find these issues, document them and hopefully put enough pressure on their developers to fix them.

Another yet unfixed issue is the Yandex Bot. Yandex is a search engine and although you may never have heard of it it's probably one of the few companies in this area that can claim to be a serious competitor to Google. The reason you may not know it is that it's mostly operating in Russian language. Depending on who your page visitors are this may matter more or less.

The Yandex Bot speaks SSL but according to the Qualys SSL test it only supports the ancient SSLv3. So you have a choice between three possibilities: Don't enable HTTPS by default, enable HTTPS with a shitty configuration supporting ancient technology that will cause trouble for SNI or enable HTTPS with a sane configuration and get no traffic from the leading Russian search engine. None of them sounds very good to me.

Another issue is third party content. For security reasons today's browsers block all active HTTP content (CSS, JavaScript etc.) on HTTPS webpages. This isn't much of a problem for me, but it's a problem for webpages that rely on advertising because from what I hear most advertisement providers don't support HTTPS yet (Google being a laudable exception here). This is the main reason you won't see many news webpages enforcing HTTPS. However, I still have passive third party HTTP content on my blog. That's why you'll probably see a yellow warning sign in front of the URL in some browsers.

A number of people seem to be confused how to correctly install certificate chains for TLS servers. This happens quite often on HTTPS sites and to avoid having to explain things again and again I thought I'd write up something so I can refer to it. A few days ago flattr.com had a missing certificate chain (fixed now after I reported it) and various pages from the Chaos Computer Club have no certificate chain (not the main page, but several subdomains like events.ccc.de and frab.cccv.de). I've tried countless times to tell someone, but the problem persists. Maybe someone in charge will read this and fix it.

Web browsers ship a list of certificate authorities (CAs) that are allowed to issue certificates for HTTPS websites. The whole system is inherently problematic, but right now that's not the point I want to talk about. Most of the time, people don't get their certificate from one of the root CAs but instead from a subordinate CA. Every CA is allowed to have unlimited numbers of sub CAs.

The correct way of delivering a certificate issued by a sub CA is to deliver both the host certificate and the certificate of the sub CA. This is neccesarry so the browser can check the complete chain from the root to the host. For example if you buy your certificate from RapidSSL then the RapidSSL cert is not in the browser. However, the RapidSSL certificate is signed by GeoTrust and that is in your browser. So if your HTTPS website delivers both its own certificate by RapidSSL and the RapidSSL certificate, the browser can validate the whole chain.

However, and here comes the tricky part: If you forget to deliver the chain certificate you often won't notice. The reason is that browsers cache chain certificates. In our example above if a user first visits a website with a certificate from RapidSSL and the correct chain the browser will already know the RapidSSL certificate. If the user then surfs to a page where the chain is missing the browser will still consider the certificate as valid. Such certificates with missing chain have been called transvalid, I think the term was first used by the EFF for their SSL Observatory.

Now the CCC uses certificates from CAcert.org. Two more issues pop up here that make things even more complicated. First of all, the root certificate of CAcert is not in browsers, users have to manually import it. But CAcert offers both their root (Class 1) and sub (Class 3) certificate on the same webpage and doesn't really tell users that they usually only have to import the root. So everyone who imports both certificates will see transvalid CAcert certificates as valid. The second issue that pops up is that browsers sometimes do weird things when it comes to certificate error messages. I have no idea why exactly this is happening, but if you have the CAcert root installed and use Chromium to surf to a page with a transvalid CAcert certificate, it'll warn you about a weak signature algorithm. This doesn't make any sense, I can only assume that it has something to do with the fact that the CAcert root is self-signed with MD5 (which isn't a security issue, because self-signatures don't really have any meaning, they're just there because X.509 doesn't allow certificates without a signature).

So how can you check if you have a transvalid certificate? One way is to use a fresh browser installation without anything cached. If you then surf to a page with a transvalid certificate, you'll get an error message (however, as we've just seen, not neccessarily a meaningful one). An easier way is to use the SSL Test from Qualys. It has a line "Chain issues" and if it shows "None" you're fine. If it shows "Incomplete" then your certificate is most likely transvalid. If it shows anything else you have other things to look after (a common issues is that people unneccesarily send the root certificate, which doesn't cause issues but may make things slower). The Qualys test test will tell you all kinds of other things about your TLS configuration. If it tells you something is insecure you should probably look after that, too.

Last weekend I was at the Easterhegg in Stuttgart, an event organized by the Chaos Computer Club. I had a talk with the title "How broken is TLS?"

This was quite a lucky topic. I submitted the talk back in January, so I had no idea that the Heartbleed bug would turn up and raise the interest that much. However, it also made me rework large parts of the talk, because after Heartbleed I though I had to get a much broader view on the issues. The slides are here as PDF, here as LaTeX and here on Slideshare.

There's also a video recording here (media.ccc.de) and also on Youtube.

I also had a short lightning talk with some thoughs on paperless life, however it's only in German. Slides are here (PDF), here (LaTeX) and here (Slideshare). (It seems there is no video recording, if it appears later I'll add the link.)

tl;dr A very short key exchange crashes Chromium/Chrome. Other browsers accept parameters for a Diffie Hellman key exchange that are completely nonsense. In combination with recently found TLS problems this could be a security risk.

People who tried to access the webpage https://demo.cmrg.net/ recently with a current version of the Chrome browser or its free pendant Chromium have experienced that it causes a crash in the Browser. On Tuesday this was noted on the oss-security mailing list. The news spread quickly and gave this test page some attention. But the page was originally not set up to crash browsers. According to a thread on LWN.net it was set up in November 2013 to test extremely short parameters for a key exchange with Diffie Hellman. Diffie Hellman can be used in the TLS protocol to establish a connection with perfect forward secrecy.

For a key exchange with Diffie Hellman a server needs two parameters, those are transmitted to the client on a connection attempt: A large prime and a so-called generator. The size of the prime defines the security of the algorithm. Usually, primes with 1024 bit are used today, although this is not very secure. Mostly the Apache web server is responsible for this, because before the very latest version 2.4.7 it was not able to use longer primes for key exchanges.

The test page mentioned above tries a connection with 16 bit - extremely short - and it seems it has caught a serious bug in chromium. We had a look how other browsers handle short or nonsense key exchange parameters.

Mozilla Firefox rejects connections with very short primes like 256 bit or shorter, but connections with 512 and 768 bit were possible. This is completely insecure today. When the Chromium crash is prevented with a patch that is available it has the same behavior. Both browsers use the NSS library that blocks connections with very short primes.

The test with the Internet Explorer was a bit difficult because usually the Microsoft browser doesn't support Diffie Hellman key exchanges. It is only possible if the server certificate uses a DSA key with a length of 1024 bit. DSA keys for TLS connections are extremely rare, most certificate authorities only support RSA keys and certificates with 1024 bit usually aren't issued at all today. But we found that CAcert, a free certificate authority that is not included in mainstream browsers, still allows DSA certificates with 1024 bit. The Internet Explorer allowed only connections with primes of 512 bit or larger. Interestingly, Microsoft's browser also rejects connections with 2048 and 4096 bit. So it seems Microsoft doesn't accept too much security. But in practice this is mostly irrelevant, with common RSA certificates the Internet Explorer only allows key exchange with elliptic curves.

Opera is stricter than other browsers with short primes. Connections below 1024 bit produce a warning and the user is asked if he really wants to connect. Other browsers should probably also reject such short primes. There are no legitimate reasons for a key exchange with less than 1024 bit.

The behavior of Safari on MacOS and Konqueror on Linux was interesting. Both browsers accepted almost any kind of nonsense parameters. Very short primes like 17 were accepted. Even with 15 as a "prime" a connection was possible.

No browser checks if the transmitted prime is really a prime. A test connection with 1024 bit which used a prime parameter that was non-prime was possible with all browsers. The reason is probably that testing a prime is not trivial. To test large primes the Miller-Rabin test is used. It doesn't provide a strict mathematical proof for primality, only a very high probability, but in practice this is good enough. A Miller-Rabin test with 1024 bit is very fast, but with 4096 bit it can take seconds on slow CPUs. For a HTTPS connection an often unacceptable delay.

At first it seems that it is irrelevant if browsers accept insecure parameters for a key exchange. Usually this does not happen. The only way this could happen is a malicious server, but that would mean that the server itself is not trustworthy. The transmitted data is not secure anyway in this case because the server could send it to third parties completely unencrypted.

But in connection with client certificates insecure parameters can be a problem. Some days ago a research team found some possibilities for attacks against the TLS protocol. In these attacks a malicious server could pretend to another server that it has the certificate of a user connecting to the malicious server. The authors of this so-called Triple Handshake attack mention one variant that uses insecure Diffie Hellman parameters. Client certificates are rarely used, so in most scenarios this does not matter. The authors suggest that TLS could use standardized parameters for a Diffie Hellman key exchange. Then a server could check quickly if the parameters are known - and would be sure that they are real primes. Future research may show if insecure parameters matter in other scenarios.

The crash problems in Chromium show that in the past software wasn't very well tested with nonsense parameters in cryptographic protocols. Similar tests for other protocols could reveal further problems.

The mentioned tests for browsers are available at the URL https://dh.tlsfun.de/.

This text is mostly a translation of a German article I wrote for the online magazine Golem.de.

Yesterday, we had a meeting at CAcert Berlin where I had a little talk about how to almost-perfectly configure your HTTPS server. Motivation for that was the very nice Qualys SSL Server test, which can remote-check your SSL configuration and tell you a bunch of things about it.

While playing with that, I created a test setup which passes with 100 points in the Qualys test. However, you will hardly be able to access that page, which is mainly due to it's exclusive support for TLS 1.2. All major browsers fail. Someone from the audience told me that the iPhone browser was successfully able to access the page. To safe the reputation of free software, someone else found out that the Midori browser is also capable of accessing it. I've described what I did there on the page itself and you may also read it here via http.

Here are my slides "SSL, X.509, HTTPS - How to configure your HTTPS server" as ODP, as PDF and on Slideshare.

And some links mentioned in the slides:
Check SSL and SSH weak keys due to broken random numbers
EFF SSL Observatory
Sovereign Keys proect

Some great talks on the mentioned topics by others:
Facthacks Talk 29c3
MD5 considered harmful today - Creating a rogue CA Certificate
Is the SSLiverse a safe place?

Update: As people seem to find these browser issue interesting: It's been pointed out that the iPad Browser also works. Opera with TLS 1.2 enabled seems to work for some people, but not for me (maybe Windows-only). luakit and epiphany also work, but they don't check certificates at all, so that kind of doesn't count.

https is likely the most widely used cryptographic protocol. It's based on X.509 certificates. There's a living debate how useful this concept is at all, mainly through the interesting findings of the EFF SSL Observatory. But that won't be my point today.

Pretty much all webpage certificates use RSA and sadly, the vast majority still use insecure hash algorithms. But it is rarely known that the X.509 standards support a whole bunch of other public key algorithms.

I've set up a page with a couple of test-cases for less-often used algorithm combinations. At the moment, it's mainly focused on RSASSA-PSS, but I plan to add elliptic curve algorithms soon. As I won't get any certificate authority to sign me certificates with anything else than classic RSA, I created my own testing root CA.

I'd be very interested to get some feedback. If you happen to have some interesting OS/Browser combination, please import the root certificate and send me a screenshot where I can see how many green ticks there are (post a link to the screenshot in the commends or send it via email).

At the moment, I'm especially looking for people to test:

• Internet Explorer 9 on Windows 7
• Safari on latest MacOS X
• Internal browser on iPhone (I don't know if it's possible to install a new certificate authority there)

I got selected for this years Google Summer of Code with a project for the implementation of RSA-PSS in the nss library. RSA-PSS will also be the topic of my diploma thesis, so I thought I'd write some lines about it.

RSA is, as you may probably know, the most widely used public key cryptography algorithm. It can be used for signing and encryption, RSA-PSS is about signing (something similar, RSA-OAEP, exists for encryption, but that's not my main topic).

The formula for the RSA-algorithm is S = M^k mod N (S is the signature, M the input, k the private key and N the product of two big prime numbers). One important thing is that M is not the Message itself, but some encoding of the message. A simple way of doing this encoding is using a hash-function, for example SHA256. This is basically how old standards (like PKCS #1 1.5) worked. While no attacks exist against this scheme, it's believed that this can be improved. One reason is that while the RSA-function accepts an input of size N (which is the same length as the keysize, for example 2048/4096 bit), hash-functions usually produce much smaller inputs (something like 160/256 bit).

An improved scheme for that is the Probabilistic Signature Scheme (PSS), (Bellare/Rogaway 1996/1998). PSS is "provable secure". It does not mean that the outcoming algorithm is "provable secure" (that's impossible with today's math), but that the outcome is as secure as the input algorithm RSA and the used hash function (so-called "random oracle model"). A standard for PSS-encryption is PKCS #1 2.1 (republished as RFC 3447) So PSS in general is a good idea as a security measure, but as there is no real pressure to implement it, it's still not used very much. Just an example, the new DNSSEC ressource records just published last year still use the old PKCS #1 1.5 standard.

For SSL/TLS, standards to use PSS exist (RFC 4055, RFC 5756), but implementation is widely lacking. Just recently, openssl got support for PSS verification. The only implementation of signature creation I'm aware of is the java-library bouncycastle (yes, this forced me to write some lines of java code).

The nss library is used by the Mozilla products (Firefox, Thunderbird), so an implementation there is crucial for a more widespread use of PSS.

A big problem with web security in the past was that it was impossible to have https-hosts with more than one certificate per IP. This is due to the protocol design of https, which needs to establish an ssl-connection with the certificate before the hostname is transferred.

There is a solution though, called Server Name Indication (SNI) and part of TLS. Strange enough, client compatibility isn't that much of a problem. Firefox, Opera and IE already support it in their current versions, konqueror will with kde4, I've no information when it'll hit safari. Oh, and I haven't testet w3m, lynx, links and wget yet, but if you want, feel free to add your experiences to the comments :-)

The problem was that until some weeks ago, openssl didn't support SNI, apachen mod_ssl didn't, lighttpd didn't. Only GnuTLS, but mod_gnutls is considered unstable by it's authors. With OpenSSL 0.9.8f, TLS Extensions and with them SNI landet in openssl, apache still needs patches.

We've now implemented SNI on schokokeks.org, which you can test:
https://www.schokokeks.org/
https://www.hboeck.de/
https://www.fabian-fingerle.de/

If your browser supports SNI, you should see different certificates, all on the same IP. All certs are cacert-signed, they also have a Wiki page from the VhostTaskForce for SNI and alternative solutions.