SSL operation failed with code 1: dh key too small

... error:14082174:SSL routines:SSL3_CHECK_CERT_AND_ALGORITHM:dh key too small

The error number you are interested in is the OpenSSL error 0x14082174.

The SSL3_CHECK_CERT_AND_ALGORITHM is usually seen when enabling export grade ciphers. It may be showing up again in non-export grade negotiations due to Logjam (see below).

I'm assuming DH Key is too small is the main problem, but I have no idea what that means. I've googled Diffie–Hellman key exchange, along with the message "key too small" but I haven't had much luck.

That's due to the recent Logjam attack from the paper Imperfect Forward Secrecy: How Diffie-Hellman Fails in Practice.

You should use 2048-bit Diffie-Hellman groups or larger. You should not be using 512-bit or 1024-bit Diffie-Hellman groups.

The first thing to check for is your cipher list string. It should be similar to:

"HIGH:!aNULL:!MD5:!RC4"

It will avoid the export grade ciphers, and use modern ciphers. But you will also need to ensure your DH callback is not using a weak/small field size. For that, you need to check the server configuration.

Some folks are "solving" the issue with kRSA. kRSA is a key transport scheme, not a key agreement scheme. The RSA key transport scheme does not provide forward secrecy, and its use is usually discouraged. In fact, its going to be removed from TLS 1.3.

I can only say "usually discouraged" because it depends on the data being protected. If you have SSL/TLS to guard downloads of a publicly available file, then its probably OK to use. If your website has a login, then its probably a little risky to use it because the password is secret data (unlike the publicly downloadable file).

To avoid key transport and pass those Qualsys SSL Labs tests for web server configurations and forward secrecy, use:

"HIGH:!aNULL:!kRSA:!MD5:!RC4"

In your Apache configuration file, it would look like so:

# cat /etc/httpd/conf.d/ssl.conf | grep SSLCipherSuite# SSLCipherSuite HIGH:MEDIUM:!aNULL:!MD5SSLCipherSuite HIGH:!aNULL:!kRSA:!MD5:!RC4

I seem to recall wget rejected small groups quite some time before the paper was released. It might make a good test case for your site.

There's also an improved sslscan, which tests for lots of things. That might make a good QA tool, too.