Using OpenSSL in Delphi webservice - available protocols and ciphers

Question

I'm a bit confused about using OpenSSL in my Delphi webservice in relation to the available ciphers for a HTTPS connection.

Setup:

• My webservice runs on a client's server. OpenSSL is installed there. The webservice uses Indy (a TIdHTTPWebBrokerBridge) and the OpenSSL DLLs (with TIdServerIOHandlerSSLOpenSSL) to load the client's certificate
• Our Android/iOS apps connect to this webservice over HTTPS
• The client has configured a domain and IP that the app users can connect to and reach my webservice. If we test that domain using e.g. the SSLLabs server test we get an overview of the supported ciphers and protocols (SSLLabs even mimics handshakes from devices and browsers and shows what ciphers were negotiated).

Question: Is there anything my webservice (in combination with OpenSSL) has to do/can do to influence the available ciphers for the TLS handshake between app and webservice? Is there anything additional that needs to be setup with OpenSSL?

I thought the answer is 'no', i.e. that it is just the server setup that (in the handshake with the app through Android/iOS) determines which cipher to use from the available server ones. Is this a correct assumption? Or do I miss something?
(As a matter of fact, I am not actually interested in limiting or expanding the available ciphers, but the client insists that something "should be done" in/with the webservice/OpenSSL to have it communicate "safely" with the apps. The SSLLabs test shows that their domain only supports TLS 1.0 and ciphers with the RSA key exchange mechanism, so e.g. no Perfect Forward Secrecy. To me, that looks like something that needs to be fixed anyway).

Notes:

• This SO question suggests I may have to do something, but it has no answers.

• I posted an earlier somewhat related question, but that has no answers.

• This SO post states OpenSSL honors the client's cipher preference, not the server's, during the SSL handshake, which again suggest there are things I can do?

• I had some doubts whether this question is in the proper place here (also because Why we are not customer support), but since this may be relevant to more programmers I decided to put it on SO.

Answer 1

You can specify available ciphers via TIdServerIOHandlerSSLOpenSSL.SSLOptions.CipherList (as well as SSL/TLS versions via TIdServerIOHandlerSSLOpenSSL.SSLOptions.SSLVersions).

If you want Perfect Forward Secrecy, you has to create DHParam keys using openssl.exe (fill TIdServerIOHandlerSSLOpenSSL.SSLOptions.DHParamsFile by result file name). If you want not only DHE, but ECDHE ciphers you need to call some additional openssl api, see a Support for Perfect Forward Secrecy in SSL with indy 10 for example.