Set a strong group size for Diffie-Hellman key exchange
If your service uses Diffie-Hellman key exchange, set the group size to a minimum of 1024 bits. A group size of 2048 bits or greater is recommended.
If your app or service doesn't meet this requirement, devices that use iOS 9 or OS X El Capitan might not be able to connect to it. For example, Safari might say that it can't establish a secure connection.
Mac computers using OS X El Capitan display the message
SSLHandshake failed (-9850) in the Console app when trying to connect to a service that doesn't meet these requirements.