I did some update on an old debian system and then SSH server refused to let me in.
I got the message:
sshd: fatal: no matching cipher found:
Just have a look in your /var/log/auth file and determine what cipher the client and server are using. This error message means there is no common cipher between the client and the server.
I added in /etc/ssh/sshd_config file the lines:
Then I got a new error message:
sshd: fatal: Unable to negotiate a key exchange method [preauth]
Just precise key exchange method by adding the following line in your /etc/ssh/sshd_config file: