Refuse weak DH groups

[KellerFuchs]

There are at least two kinds of weak DH groups that lua-http silently accepts:

• [ ] groups with small subgroups;
• [ ] composite groups.

I'm not sure whether OpenSSL can check for those at all.

[daurnimator]

I'm not sure how to check these... https://github.com/chromium/badssl.com/issues/40 indicates that I should check the DH parameters.

I can't see in the OpenSSL API how to get the DH params (I'd preferably want them as an EVP_PKEY*) out of an SSL*; anyone able to jump in here?

[daurnimator]

I can't see in the OpenSSL API how to get the DH params (I'd preferably want them as an EVP_PKEY*) out of an SSL*; anyone able to jump in here?

SSL_get_server_tmp_key is the function I was looking for. Introduced in 1.0.2.

Thanks to http://openssl.6102.n7.nabble.com/How-to-enforce-DH-field-size-in-the-client-td60442.html

I'll note that openssl seems to have some built in checks for DH keys available. grep for SSL_R_DH_KEY_TOO_SMALL.

[daurnimator]

luaossl now has ssl:getServerTemporaryKey()

[daurnimator]

I think this also needs https://github.com/wahern/luaossl/issues/135