How to reject weak DH parameters in an OpenSSL cli

问题:

Currently OpenSSL in client mode stops handshake only if the keylength of server selected DH parameters is less than 768 bit (hardcoded in source).

In my client I want to stop handshake if the keylength of Server selected DH parameters is less than 2048-bit. The preferred way would be to set via API, e.g. option setting exposed by OpenSSL.

Is there any way to set the minimum key length using public APIs?

回答1:

Is there any way to set the minimum key length using public APIs?

Yes (or maybe I should say, "I believe so"). Use your Diffie-Hellman callback. The callback is set with SSL_CTX_set_tmp_dh_callback and SSL_set_tmp_dh_callback.

Usually the Diffie-Hellman callback is used on the server to generate its keys. But according to OpenSSL's SSL_CTX_set_tmp_dh_callback(3) man page, its "... to be used when a DH parameters are required for tmp_dh_callback...".

For an example of using the callback in the context of a server (which should be similar to using it in a client), see 'No Shared Cipher' Error with EDH-RSA-DES-CBC3-SHA. It performs key length checks.