SSL Server setup DH/ECDH

[Matt Caswell]

On 06/08/2019 09:42, Chitrang Srivastava wrote:
> Hi,
> 
> I am implementing HTTPs server using openssl 1.1.1b.
> Is it mandatory to setup these API's while creating ssl context ?
> 
> SSL_CTX_set_tmp_ecdh
> 
> SSL_CTX_set_tmp_dh

By default OpenSSL will automatically use ECDH if appropriate and choose a
suitable group so there is no need to call SSL_CTX_set_tmp_ecdh() unless you
want more control over which specific group is used.

OpenSSL will not use DH unless you specifically configure it. If you want to
make use of DH based ciphersuites then you must either call SSL_CTX_set_tmp_dh()
or SSL_CTX_set_dh_auto() (or the SSL_* equivalents). Calling the former enables
you to configure any arbitrary DH group that you choose. Calling the latter will
enable the built-in DH groups.

It is not mandatory to call any of the above.

> 
> Also any suggestion what all options one should set while setting up server like
> SSL_CTX_set_options like SSL_OP_NO_SSLv2 |SSL_OP_NO_SSLv3

Don't use the protocol version specific options at all. Use
SSL_CTX_set_min_proto_version() if you want to specify a minimum protocol
version. SSLv2 is no longer supported at all. SSLv3 is compiled out by default.

Other options that are worth considering are SSL_OP_NO_RENEGOTIATION and
(possibly) SSL_OP_CIPHER_SERVER_PREFERENCE. Generally you don't need the others
unless there is a specific problem you are trying to solve.

Matt