Andrew Ayer

A surprisingly hard, and widely misunderstood, problem with SSL certificates is figuring out what organization (called a certificate authority, or CA) issued a certificate. This information is useful for several reasons:

• You've discovered an unauthorized certificate for your domain via Certificate Transparency logs and need to contact the certificate authority to get the certificate revoked.
• You've discovered a certificate via Certificate Transparency and want to know if it was issued by one of your authorized certificate providers.
• You're a researcher studying the certificate ecosystem and want to count how many certificates each certificate authority has issued.

On the surface, this looks easy: every certificate contains an issuer field with human-readable attributes, including an organization name. Problem solved, right?

Not so fast: a certificate's issuer field is frequently a lie that tells you nothing about the organization that really issued the certificate. Just look at the certificate chain currently served by ...

My preferred method for deploying webapps is to have the webapp listen directly on port 443, without any sort of standalone web server or HTTP reverse proxy in front. I have had it with standalone web servers: they're all over-complicated and I always end up with an awkward bifurcation of logic between my app's code and the web server's config. Meanwhile, my preferred language, Go, has a high-quality, memory-safe HTTPS server in the standard library that is well suited for direct exposure on the Internet.

However, only one process at a time can listen on a given IP address and port number. In a world of ubiquitous IPv6, this wouldn't be a problem - each of my servers has literally trillions of IPv6 addresses, so I could easily dedicate one IPv6 address per webapp. Unfortunately, IPv6 is not ubiquitous, and due to the shortage of IPv4 addresses, it would ...

Did you know that you can use the ssh-keygen command to sign and verify signatures on arbitrary data, like files and software releases? Although this feature isn't super new - it was added in 2019 with OpenSSH 8.0 - it seems to be little-known. That's a shame because it's super useful and the most viable alternative to PGP for signing data. If you're currently using PGP to sign data, you should consider switching to SSH signatures.

Here's why I like SSH signatures:

• It's not PGP. For years, security professionals have been sounding the alarm on PGP, including its most popular implementation, GnuPG/GPG. PGP is absurdly complex, has an awful user experience, and is full of crufty old cryptography which shouldn't be touched with a ten foot pole.

• SSH is everywhere, and people already have SSH keys. If you use Debian Bullseye or Ubuntu 20.04 or newer, you already have ...

Before adding a dependency to one of my software projects, I do some basic vetting of the dependency. Among the things I check are:

• How is the code licensed?
• Who are the authors?
• Are there any serious unresolved issues in the issue tracker?
• Is there a history of serious bugs in the issue tracker?
• What kind of code review process is used for pull requests?

Finally, I do a cursory review of the code. I look for anything blatantly insecure or malicious, and try to get a feel for the quality of the code base. I look for "Brown M&Ms" - minor inattention to detail that might indicate a larger problem.

I repeat the above recursively on transitive dependencies as many times as necessary. I also repeat the cursory code review any time I upgrade a dependency.

This is quite a bit of work, but is necessary to ...

I'm not actually sure when SSLMate was born. I got the idea, registered the domain name, and wrote the first lines of code in August 2013, but I put it on the backburner until March 2014. I think I "launched" in early April, but since I thought of SSLMate as a side project mainly for my own use, I didn't do anything special.

I do know that I sold my first certificate on April 13, 2014, four years ago to this day. I sold it to a friend who needed to replace his certificates after Heartbleed and was fed up with how hard his certificate authority was making it. It turns out a lot of people were generally fed up with how hard certificate authorities made things, and in the last four years, SSLMate has exceeded my wildest expectations and become my full-time job.

Sadly, certificate resellers have a deservedly ...

Certificate Transparency, the effort to detect misissued SSL certificates by publishing all certificates in public logs, only works if TLS clients reject certificates that are not logged. Otherwise, certificate authorities could just not log the certificates that they misissue. TLS clients accomplish this by requiring that a certificate be accompanied by a "signed certificate timestamp" (SCT), which is a promise by a log to include the certificate within 24 hours of the SCT's issuance timestamp. (This period is called the Maximum Merge Delay. It doesn't have to be 24 hours, but it is for all logs currently trusted by Chrome.) The SCT can be embedded in the certificate, the OCSP response (which the TLS server must staple), or the TLS handshake.

But an SCT is only a promise. What if the log breaks its promise and never includes the certificate? After all, certificate authorities promise not to issue bad ...

Last week, Nick Sullivan launched mitm.watch, a website that purports to tell you whether or not your HTTPS connection is being intercepted by a man-in-the-middle (MitM). mitm.watch uses Caddy's HTTPS MitM Detection Feature, which implements the techniques described in this paper. Basically, Caddy compares the browser name and version number advertised by the User-Agent header to the properties of the TLS handshake initiated by the client (e.g. ciphersuites). If the TLS handshake doesn't match the known properties of the purported browser, then the TLS handshake was probably not initiated by the browser, but by a man-in-the-middle. Caddy's documentation suggests that you could display an error message if a MitM is detected. mitm.watch displays either a green "No MITM!" page, or a red "Likely MITM!" page.

Unfortunately, there is a significant and intractable shortcoming to MitM detection: a MitM can defeat the detection by making its TLS implementation ...

The following command, when run as any user, will crash systemd:

NOTIFY_SOCKET=/run/systemd/notify systemd-notify ""

After running this command, PID 1 is hung in the pause system call. You can no longer start and stop daemons. inetd-style services no longer accept connections. You cannot cleanly reboot the system. The system feels generally unstable (e.g. ssh and su hang for 30 seconds since systemd is now integrated with the login system). All of this can be caused by a command that's short enough to fit in a Tweet.

Edit (2016-09-28 21:34): Some people can only reproduce if they wrap the command in a while true loop. Yay non-determinism!

The bug is remarkably banal. The above systemd-notify command sends a zero-length message to the world-accessible UNIX domain socket located at /run/systemd/notify. PID 1 receives the message and fails an assertion that the message length is greater than zero. Despite ...

Cryptography is notorious for its sharp edges. It's easy to make a minor mistake that totally dooms your security. The situation is improving thanks to the development of easier-to-use libraries like libsodium which provide a high-level interface instead of forcing the user to combine basic building blocks. However, you still need to know exactly what security guarantees your cryptographic primitives provide and be sure not to go beyond their guarantees.

As an example of what can go wrong when you assume too much from a primitive, consider the duplicate signature key selection attack which I discovered in ACME, the protocol used by Let's Encrypt. The vulnerability was severe and would have allowed attackers to obtain SSL certificates for domains they didn't control. Fortunately, it was mitigated before Let's Encrypt was publicly trusted, and was definitively fixed a couple weeks ago.

The vulnerability was caused by a misuse of ...

The first version of LibreSSL portable, 2.0.0, was released a few days ago (followed soon after by 2.0.1). Despite the 2.0.x version numbers, these are only preview releases and shouldn't be used in production yet, but have been released to solicit testing and feedback. After testing and examining the codebase, my feedback is that the LibreSSL PRNG is not robust on Linux and is less safe than the OpenSSL PRNG that it replaced.

Consider a test program, fork_rand. When linked with OpenSSL, two different calls to RAND_bytes return different data, as expected:

$ cc -o fork_rand fork_rand.c -lcrypto $ ./fork_rand Grandparent (PID = 2735) random bytes = f05a5e107f5ec880adaeead26cfff164e778bab8e5a44bdf521e1445a5758595 Grandchild (PID = 2735) random bytes = 03688e9834f1c020765c8c5ed2e7a50cdd324648ca36652523d1d71ec06199de

When the same program is linked with LibreSSL, two different calls to RAND_bytes return the same data, which is a catastrophic failure of the PRNG:

$ cc -o fork_rand fork_rand.c libressl-2.0.1/crypto/.libs/libcrypto.a -lrt ...

Ever since Heartbleed, I've been thinking of ways to better isolate OpenSSL so that a vulnerability in OpenSSL won't result in the compromise of sensitive information. This blog post will describe how you can protect the private key by isolating OpenSSL private key operations in a dedicated process, a technique I'm using in titus, my open source high-isolation TLS proxy server.

If you're worried about OpenSSL vulnerabilities, then simply terminating TLS in a dedicated process, such as stunnel, is a start, since it isolates sensitive web server memory from OpenSSL, but there's still the tricky issue of your private key. OpenSSL needs access to the private key to perform decryption and signing operations. And it's not sufficient to isolate just the key: you must also isolate all intermediate calculations, as Akamai learned when their patch to store the private key on a "secure heap" was ripped to shreds ...