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How SSH public-key login is supposed to work
The client uses the private key to prove it can access it; the server checks whether the corresponding public key is authorized for the account. Possessing a keypair is therefore only one part of login. The destination host, remote username, client identity selection, authorized-key lookup, and server policy must also line up. OpenBSD’s ssh(1) manual describes this distinction.
Diagnose in sequence. If SSH cannot reach the intended host, changing a user key will not fix that earlier failure. If it connects but rejects public-key authentication, move on to identity and server checks.
1. Confirm the destination, port, and account
Check the exact command and any SSH alias before inspecting keys. A valid key can be rejected if it is being offered to the wrong machine or account.
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- Verify the hostname or IP address and the remote username in the command, such as
ssh -v user@host. - If you use a host alias, inspect the matching client configuration, including the resolved hostname, user, port, and identity settings. OpenSSH client options are documented in ssh_config(5); vendor builds and other SSH clients may differ.
- Separate connection errors from authentication errors. A failure before a session reaches authentication points to an earlier connection or target issue, not proof that the key is bad.
2. Read the client’s verbose output
Run a diagnostic attempt with increased verbosity:
ssh -v user@host
OpenSSH documents -v as a way to increase client output. If needed, increase verbosity further using the levels supported by your installed client; consult its local manual because options can vary. Look for whether public-key authentication is attempted, which identity files are considered, and whether an identity is offered or rejected. The output helps locate the stage, but a client message alone may not explain a server-side policy decision.
Client-side output and server logs answer different questions. The client can show what it tried; a server administrator may be able to see why the server rejected an offered key. OpenSSH’s server manual documents debug logging at DEBUG level or higher, but the user may not have access to those logs. Do not post unredacted hostnames, usernames, or verbose logs publicly, and never share a private key, passphrase, or agent socket.
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3. Check which identity the client can use
Verbose output may show that the intended identity was never offered. Check the configured identity path and whether the client is selecting another key. OpenSSH client configuration can affect identity selection and agent use; inspect the relevant host block and your installed client’s effective configuration rather than assuming a default.
For a file-backed private key
- Confirm that the private key file exists at the path the client is using and that your account can read it.
- OpenSSH ignores private-key files accessible by others. Its ssh(1) manual describes private-key files and their corresponding public-key files, typically named with a
.pubsuffix. File names and paths are not proof that the correct identity is being used. - Do not loosen permissions broadly or delete existing keys as a diagnostic shortcut. Check the specific file and follow the guidance for your operating system and SSH implementation.
For an agent-held identity
An SSH agent is an identity source, not a key generator. OpenSSH says the agent initially has no private keys; identities can be added with ssh-add, or made available by the client when configured with AddKeysToAgent. See ssh-agent(1) and ssh_config(5).
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- Check that the environment running SSH can reach the expected agent and that the intended identity is loaded.
- If you use a terminal, remote session, or other environment that differs from your usual desktop session, verify that it sees the same agent and identity.
- If the log shows no suitable identity offered, investigate the local identity path or agent state before asking the server administrator to change policy.
4. Verify the remote account and authorized-key source
Once the client offers the intended key, the server must find the corresponding public key in an authorized-key source for the correct account. Confirm the account name first, then ask an administrator to check the source the server actually uses—not just the default file you expect.
OpenSSH’s sshd_config(5) manual documents AuthorizedKeysFile: it can name one or more files, use paths relative to the user’s home directory, or be set to none. A key absent from the active source will not be authorized even if a similarly named file exists elsewhere.
5. Check server permissions and authentication policy
If the public key is present but rejected, the server’s account and authentication rules may explain why. Ask a server administrator to inspect the effective settings for the account and connection, including applicable global directives and Match blocks. OpenSSH configuration behavior can vary by release and local configuration, so the file on disk alone may not reveal which rule applies.
- Path ownership and permissions: Check the user’s home path and authorized-key files against the server’s requirements. Do not “fix” a rejection by making directories or files writable to everyone.
- Public-key authentication: Verify that the active server configuration permits it.
- Account access: Check allowed or denied users and groups, and whether account restrictions prevent login.
- Required authentication methods: A server may require more than a public key, so a valid key alone may not complete authentication.
- Revocation: Check whether the key is covered by the server’s revoked-key configuration.
These controls are documented in OpenBSD’s sshd_config(5). The exact directives available and their behavior depend on the installed OpenSSH version and any vendor-specific service or appliance configuration.
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6. Investigate algorithms or FIDO requirements only when indicated
Consider algorithm compatibility when the client or server output points to a key-type or negotiation problem. Do not treat it as the default explanation for every rejection. Likewise, FIDO-specific requirements matter only if the identity is an authenticator-hosted key and the configuration or logs point to them.
OpenSSH documents authenticator-hosted ECDSA and Ed25519 key types. Its server configuration manual also describes FIDO-related touch-required and verify-required controls, which can require physical presence or user verification such as a PIN. These controls do not apply to ordinary non-FIDO key types. Check that the client, authenticator interface, and server support the selected method before treating a missing touch or verification step as the cause. See ssh(1) and sshd_config(5).
What to share with a server administrator
If the client output shows that it offered the expected identity but authentication still fails, the next useful evidence may be server-side. OpenSSH notes that the server may inform the client of errors that prevented public-key authentication after authentication completes using a different method. That information is not necessarily visible during a failed attempt, so a server administrator may need to consult permitted logs.
Quick Recap
- The approximate time of the attempt and the intended host and account, shared privately with the administrator.
- A redacted excerpt showing the connection and authentication stage, with public IPs, usernames, and host details removed if you are posting outside a trusted support channel.
- The key’s public-key fingerprint or public key only if the administrator requests it through an appropriate channel; never send the private key or passphrase.
- The client and server OpenSSH versions if available, since manuals and settings may differ across releases and vendor builds.
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