FreeBSD, NGINX, SSL and the ChaCha20 cipher suites

In this post, I’ll be describing the journey of enabling the stronger ChaCha20 cipher suites on my FreeBSD NGINX reverse proxy. I’m using SSL Labs SSLTest to get the details on what is being offered, and for sanity-testing the SSL configuration in general. I’m using FreeBSD 10.2.

In nginx.conf:

http {
  (...)
  ssl_protocols       TLSv1 TLSv1.1 TLSv1.2;
  ssl_ciphers         EECDH+CHACHA20:EECDH+AES128:RSA+AES128:EECDH+AES256:RSA+AES256:!MD5;
  ssl_prefer_server_ciphers on;
  (...)
}

This clearly shows the server prefers the EECDH+CHACHA20 cipher suite. But according to the SSL test, it’s not being offered. Why could this be? I’m using whatever OpenSSL version is in the FreeBSD 10.2 base system, so I go poke it to find which ciphers it supports:

# /usr/bin/openssl version
OpenSSL 1.0.1p-freebsd 9 Jul 2015
# /usr/bin/openssl ciphers | grep -i chacha
#

Apparently, it doesn’t support the ChaCha cipher suites. Ok, then it’s obvious why NGINX isn’t offering those cipher suites. So I go over to my package build server and change its configuration around a little. I add security/libressl to the build list, and I instruct it to build the reverse proxy’s packages with the following make.conf options:

# Build ports against security/libressl
WITH_OPENSSL_PORT=      yes
OPENSSL_PORT=           security/libressl

I instruct the package builder to run a build for the reverse proxy, and it seems to be doing what I wanted. All ports linking to OpenSSL (including NGINX) are being rebuilt, and LibreSSL is built too.

Once the build server is done, I tell the reverse proxy to force an update and upgrade. (pkg update -f and pkg upgrade -f), restart NGINX, and test again.

It’s still not serving CHACHA20. WHAT!? I check if NGINX actually linked against LibreSSL:

# /usr/local/bin/openssl version
LibreSSL 2.2.4
# ldd /usr/local/bin/openssl
/usr/local/bin/openssl:
        libthr.so.3 => /lib/libthr.so.3 (0x800888000)
        libssl.so.35 => /usr/local/lib/libssl.so.35 (0x800aac000)
        libcrypto.so.35 => /usr/local/lib/libcrypto.so.35 (0x800d12000)
        libc.so.7 => /lib/libc.so.7 (0x801110000)

# ldd /usr/local/sbin/nginx
/usr/local/sbin/nginx:
        libthr.so.3 => /lib/libthr.so.3 (0x8008db000)
        libcrypt.so.5 => /lib/libcrypt.so.5 (0x800aff000)
        libpcre.so.1 => /usr/local/lib/libpcre.so.1 (0x800d1f000)
        libssl.so.35 => /usr/local/lib/libssl.so.35 (0x800f96000)
        libcrypto.so.35 => /usr/local/lib/libcrypto.so.35 (0x8011fc000)
        libz.so.6 => /lib/libz.so.6 (0x8015fa000)
        libc.so.7 => /lib/libc.so.7 (0x801810000)

Yes, it is linked against LibreSSL. And /usr/local/bin/openssl ciphers does list CHACHA20. This is very strange. While trying to figure out what’s going on here, I strike up a conversation with Allan Jude on IRC and casually mention my troubles. He mentions something about ChaCha20 being slower (not a bad thing), citing https://wiki.freebsd.org/SSHPerf, and mention some black magic that is using the OpenSSL client to check the SSL session handshake. So I do that.

# /usr/local/bin/openssl s_client -host HOSTNAME -port 443
(...)
SSL-Session:
    Protocol  : TLSv1.2
    Cipher    : ECDHE-RSA-CHACHA20-POLY1305
(...)

What? This is saying it’s using CHACHA20! Why isn’t the SSL test saying so? And then Allan casually says the SSL report does state the preferred cipher is CHACHA20. I go refresh the page, and indeed it does.

So to make a long story short, I had to link NGINX with LibreSSL to get CHACHA20 support, once that was done, I checked the now stale report to see CHACHA20 wasn’t supported, even though it actually was.

Lesson of the story: Be thorough. Very thorough. Messing up a simple step can make you stare at the screen wondering if you’ve lost your sanity. Also, sharing the trouble can be entertaining for those you share it with, and save you time. Win-win. :)

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