Fix the SSH post-quantum key exchange warning
Your ssh client warns that the connection is not using a post-quantum key exchange. Find which algorithm you negotiate, and fix the right side.
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The history of SSH, from telnet to OpenSSH
A 1995 password sniffing attack in Helsinki produced SSH. Here is the verified timeline from telnet and rlogin to OpenSSH and post-quantum defaults.
Fix SSH Too Many Authentication Failures
Too many authentication failures means your SSH agent offered every key it holds and the server cut you off. Diagnose it with ssh -v and fix it for good.
Post-Quantum SSH: What Changed on Ubuntu
OpenSSH already negotiates a post-quantum key exchange by default. Check what your Ubuntu box really uses, and learn why host keys are still classical.
Fix SSH Permission denied (publickey)
Permission denied (publickey) is five different faults. Read the ssh -v output, find which one you have, and fix it without locking yourself out.