Michael Albinus <michael.albinus@gmx.de> writes:

Hi everybody,

>> ssh is able to identify the host using host keys:
>>
>> /etc/ssh/ssh_host_ed25519_key.pub
>> /etc/ssh/ssh_host_rsa_key.pub
>
> Thanks, these can be accessed via ssh-keyscan. Will use it in Tramp.

I've implemented a proof-of-concept, see appended patch to
tramp-sh.el. It is towards Emacs 29.0.50, but might also apply for Emacs
28.0.90 (for testing). After loading Tramp, there is a new user option
tramp-use-scp-direct-remote-copying which must be set to non-nil.

Direct scp copying between two remote servers shall happen then. Could
people test it? Setting tramp-verbose to 6 shall show the scp command in
the debug buffer, like in my case

--8<---------------cut here---------------start------------->8---
16:41:38.755570 tramp-do-copy-or-rename-file-out-of-band (6) # scp -p -T -R -q -r gandalf:/home/albinus/Downloads/CentOS-8.4.2105-x86_64-dvd1.iso detlef:/tmp/CentOS-8.4.2105-x86_64-dvd1.iso
--8<---------------cut here---------------end--------------->8---

Best regards, Michael.