Key exists, but does not match

If your screen looks like this 

identity_sign: private key /home/cgaucho/.ssh/id_ed25519 contents do not match public
git@github.com: Permission denied (publickey).
fatal: Could not read from remote repository.

You might also see this after trying git clone, git pull, or git push.

What this usually means

Your private key and public key are not a matching pair anymore.

So GitHub rejects the login, even though a key file exists.

How to fix it

  1. Recreate your SSH keys using the lecture notebook steps (lec02, or lec01 if needed). Look for instructions with ssh-keygen.
  2. Set global Git config again (the commands that set your user.email and user.name).
  3. Copy the correct new public key from ~/.ssh/id_ed25519.pub.
  4. In GitHub, delete the old SSH key and add the new one you just copied.

Try again

Rerun your previous Git command (for example, git clone).

Reassurance note

This is a common setup issue. You do not need to debug every line. Recreating the key pair and replacing the old GitHub key usually fixes it.