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In many environments, a stubborn SSH key rejection despite correct file permissions points to a mismatch between what the server expects and what the client offers. Start by confirming the server’s authentication method setup, verifying that it accepts public key authentication, and that the authorized_keys file and ~/.ssh directory on the server have the correct ownership and modes. On the client side, ensure you are using the appropriate private key with the right passphrase, and that no extra spaces, line breaks, or corrupted characters exist in your public key entry. A small syntax error can cause the server to dismiss a perfectly valid key, so carefully review the key format and paths involved in the SSH command.

After you verify the basics, inspect the SSH server logs for concrete error messages that indicate why a key is rejected. On Unix-like systems, this typically involves examining /var/log/auth.log, /var/log/secure, or journal entries via journalctl, focusing on lines tied to the SSH daemon during the attempted connection. Look for messages that mention “pubkey,” “authentication failure,” or “Permission denied (publickey).” These messages can reveal whether the server rejected the key due to permissions, an unsupported key type, or a policy restriction. If logs point to an unsupported key type, consider regenerating a compatible key with the right algorithm (for example, RSA, ECDSA, or Ed25519) that the server accepts.

One frequent cause is mismatched owners or overly permissive file permissions on the server’s home directory and the .ssh directory or files. The standard guideline expects the user's home directory to be owned by the user with restrictive permissions, and the .ssh directory to be readable, writable, and executable only by the user. The authorized_keys file must be readable by the user but not writable by others. If you see a permission mismatch, correct it with careful chown and chmod adjustments, ensuring that SSH can traverse the path to the key without tripping over security safeguards. After changing permissions, reload or restart the SSH service to apply the new policy and test again with verbose output to confirm the outcome.

Client-side permission issues can equally block authentication. Ensure your private key file on the client is readable only by you, not by others, and that it has restrictive permissions such as 600. If you are using an SSH agent, verify that the correct key is loaded; conveyance mistakes or multiple keys may cause the server to receive an unexpected key. Running ssh-add -l helps confirm which identities the agent is offering. If the agent presents the wrong key, remove that identity and re-add the intended one. Also confirm that the key pair was generated with a compatible algorithm and that the public key was correctly appended to the server’s authorized_keys file.

A subtle but real obstacle is agent forwarding and per-session restrictions. If your SSH client relies on an agent, ensure agent forwarding is either enabled if required or disabled to reduce exposure if it isn’t. Misconfigured forwarding can cause authentication to fail because the server cannot access the forwarded key during the session initiation. Review your SSH config files (for example, ~/.ssh/config and /etc/ssh/ssh_config) for mismatches in IdentityFile directives, which key files are tried first, and any patterns that could skip the intended key. If multiple identities exist, specify the full path to the correct private key in the SSH command to avoid the agent’s automatic suggestions.

Network-level constraints can masquerade as authentication failures. Firewalls, network ACLs, or VPN policies might transform or drop the authentication handshake, especially if the server expects a particular source address or if intermediate proxies terminate TLS or modify SSH handshakes. Confirm there is no IP-based restriction blocking your client, and examine SSH server configuration for Match blocks that restrict which users or addresses may authenticate. If you operate behind NAT or use a corporate proxy, test from a simple, direct network segment to isolate whether the issue is network-related. Document any recent changes to the network environment that could affect the SSH handshake.

Sometimes a server-side change, such as a recent update or a policy tweak, can invalidate working keys. Review server audits or package management logs to see if OpenSSH or related libraries were updated. A newer SSH daemon may enforce stricter defaults or deprecate older key types, causing a previously valid key to fail. If you suspect this, temporarily adjust the sshd_config to a more permissive setting, test, and then plan a proper upgrade path. Remember to revert to secure defaults after verification. Keep a change log to track what was modified and why, which helps prevent regressions in the future.

Another frequent culprit is a mismatch between the key type and what the server supports. Some servers are configured to accept only Ed25519 or RSA keys, while your key might be of a different type. Recreate your key using a supported algorithm with a sufficiently long and safe key length, then re-add the public key to the server. After generating the new key, ensure the client references the new private key in your SSH command or config file. Don’t forget to remove older keys from the agent and the server to avoid accidental conflicts during authentication attempts, and verify permissions throughout the process.

If you still face issues, enable verbose debugging on the client to gather deeper insights. Run ssh -vvvvv user@host to obtain a detailed log of the authentication sequence, including which keys are offered and why the server rejects them. Carefully parse the output for lines indicating which part of the handshake fails, such as “Offering public key: …” or “Auth via public key failed.” The diagnostic trace helps you pinpoint whether the problem lies with key matching, agent issues, or server-side policy. Use these findings to adjust client parameters, reload daemons, or update user permissions accordingly, ensuring that every adjustment moves you closer to a stable, authorized connection.

In parallel, verify the server’s authorized_keys contents directly. Confirm that the public key on the server is exactly the counterpart to your local private key, with no stray characters or line breaks that could corrupt the key. If you previously edited the authorized_keys file, revert any brittle edits and reappend the clean key using a reliable method, such as ssh-copy-id or a secure file transfer, rather than manual paste. After updating the server’s authorized_keys, test again with a fresh session to determine whether the issue has shifted from a local problem to a server-side one, or vice versa.

If permissions, key types, and network policies all appear correct, consider a holistic, repeatable workflow to avoid recurring failures. Create a checklist that begins with verifying the key pair and its presence in the client agent, then moves through server permissions and authorized_keys integrity, followed by SSH configuration alignment on both ends. Add a step for temporary verbose logging to capture future incidents. Maintain versioned backups of SSH configuration files, and document any policy changes in a central knowledge base. A structured approach minimizes guesswork, reduces downtime, and makes it easier to identify the root cause rapidly should the situation reoccur.

Finally, when troubleshooting in production, adopt a minimal-risk stance. Apply fixes in small, reversible increments, test after each change, and monitor authentication behavior closely. If a change improves the situation, validate across multiple users or hosts to ensure consistency. When in doubt, isolate the variables by testing from a clean client environment or a different server instance, which helps separate client misconfigurations from server-side constraints. By embracing a methodical, incremental approach, you empower secure access with confidence and resilience, even amid complex authentication ecosystems.