techblog 

OpenSSL provides a set of engine functions to allow you to access cryptographic modules. I have used these before to access a Sun SCA6000 via pkcs11. In those examples I always assumed the necessary configuration settings were in the openssl config file. However some settings would be better set on the fly. For example, you wouldn’t want the password required to access the keystore to be kept on disk in the config file.

To pass the password (pin) to an engine you can do something like this

 /* Send PIN to engine */
    if(!ENGINE_ctrl_cmd_string(e, "PIN", "nominet1:abc123", 0)){
        printf("Error sending PIN to engine");
        ENGINE_free(e);
        return;
    }

Thanks to Stephen Henson for pointing me in the correct direction.