Introduction
Imagine you're working on your GitHub project, and suddenly your git pull command throws a "Permission denied (publickey)" error. You've changed nothing on your side, your key is still registered on GitHub, and everything works on another device with a different key. What happened?
The Context
This scenario has been experienced by many developers recently. The SSH key, though properly registered and functional, gets rejected by GitHub. The issue seems to lie in the absence of a .pub file, which, when generated, immediately resolves the problem.
Why a .pub File?
The presence of a .pub file alters the authentication flow used by OpenSSH. With a .pub file, the client first probes the public key and waits for the server's OK before signing the authentication. Without a .pub, OpenSSH skips this step and sends a fully signed authentication request directly. According to RFC 4252, both methods are valid, but GitHub appears to have recently changed its server policy to only accept the first method.
A GitHub Update?
Developers have noticed a change in the SSH server identification from GitHub. The new server banner indicates a software change, which could explain the modification in the accepted authentication flow. This change might be why a working setup in the morning stops working in the afternoon without any user-side modifications.
How to Fix the Problem
The solution is simple: ensure your private SSH key has a corresponding .pub file. Just generate the .pub file with the following command:
``bash ssh-keygen -y -f ~/.ssh/your_key_name > ~/.ssh/your_key_name.pub ``
It may seem trivial, but this small adjustment has resolved the issue for many users.
Conclusion
This case highlights the importance of staying updated with policy changes on platforms we use daily. Managing SSH keys is crucial for security and repository access, so understanding how these elements work together is essential.
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