How to use Kex diffie-hellman-group-exchange-sha256 ?

[Antenore Gatta <antenore@xxxxxxxxxxxx>]

Hi all,

I'm trying to connect to a host using the Kex
diffie-hellman-group-exchange-sha256 , but even if I force the Kex, I
got this trace:

> [SSH] ssh_kex_select_methods: kex error : no match for method kex
> algos: server [diffie-hellman-group-exchange-sha256], client
> [curve25519-sha256@xxxxxxxxxx,ecdh-sha2-nistp256,diffie-hellman-group14-sha1,diffie-hellman-group1-sha1]
> [SSH] ssh_connect: current state : 9

In fact the diffie-hellman-group-exchange-sha256 kex is nopt
recognized:

> [SSH] SSH_OPTIONS_KEY_EXCHANGE does not have a valid value:
> diffie-hellman-group-exchange-sha256 

Is there any special flag to enable
diffie-hellman-group-exchange-sha256 at compile time? 

At the moment libssh, in our flatpak, is compiled with:

> -DCMAKE_BUILD_TYPE:STRING=Release
> -DWITH_NACL:BOOL=ON
> -DNACL_INCLUDE_DIR:PATH=/app/include/
> -DNACL_LIBRARY:PATH=/app/lib/nacl/libnacl.a
> -DWITH_EXAMPLES:BOOL=OFF


Thanks in advance!!!

Best regards

Antenore