Adding this as a monkey patch to my app did work to let me use google oauth without the insecure context workarounds:

abstract class OpenSSL::SSL::Context
  class Client < Context
    def initialize(method : LibSSL::SSLMethod = Context.default_method)
      super(method)

      self.verify_mode = OpenSSL::SSL::VerifyMode::PEER
      {% if LibSSL::OPENSSL_102 %}
        self.default_verify_param = "ssl_server"
        {% end %}
    end
  end

  class Server < Context
    def initialize(method : LibSSL::SSLMethod = Context.default_method)
      super(method)

      add_options(OpenSSL::SSL::Options::CIPHER_SERVER_PREFERENCE)
      {% if LibSSL::OPENSSL_102 %}
        self.default_verify_param = "ssl_client"
        {% end %}

      set_tmp_ecdh_key(curve: LibCrypto::NID_X9_62_prime256v1)
    end
  end
end

I suppose it's fine. Any reference anywhere about that?

@ysbaddaden No, I couldn't find a good reference about this. I didn't test the server part and I wonder what it's actually verifying there? I guess is the client certificate in case there is one. Can client requests still be made without a client certificate?

Documentation is available in the x509 man page under CERTIFICATE EXTENSIONS:
https://www.openssl.org/docs/manmaster/man1/x509.html

SSL certificates may have a "purpose" to add another level of trust and verification to the certificate. For example a certificate may be trusted as a SSL Client (or SSL Server) and should be refused when it acts for another purposes.

I assume the X509 context verification is meant to verify the received certificate, which means a server will verify a client certificate, and a client will verify a server certificate. I assume we got the purposes wrong? The OpenSSL API makes it confusing, since we set the purpose on the SSL context (client/server) but must set an opposite value (server/client)...

Maybe we should document this, and properly explain in the commit message (since the value is weird, a git blame will help).

I agree that a bit of docs should be added though.

@waj @RX14 @ysbaddaden @jhass I've updated the commit's message to reflect why are we changing this, with the relevant OpenSSL/x509 RFC docs linked.

I've also rebased this to be included in a future 0.24.2 release.

Do you think this is OK? Any ideas on how to clarify further?

Ensure this is cherry-picked into master please.

This is still not cherry-picked into master. This is why I dislike all these weird release branches.