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SSH key authentication proves that the client can create a valid signature with the private key corresponding to a public key the server accepts for the account. The private key is not sent to the server. Password authentication works differently: it sends the password value inside an SSH authentication packet, but the SSH transport encrypts that packet when confidentiality is active. Keys change the authentication proof and the credential-management risks; they do not make a poorly secured SSH account automatically safe.
What an SSH key proves
RFC 4252 describes public-key authentication as proof of possession: “With this method, the possession of a private key serves as authentication.” The server must also check that the corresponding public key is valid for the user and that the signature is valid. RFC 4252
- The server identifies acceptable keys. In a common OpenSSH setup, it checks configured authorized-keys files. The documented default filenames are
.ssh/authorized_keysand.ssh/authorized_keys2. A key’s presence in an authorized-keys source grants authentication rights subject to its options and server policy. OpenSSH sshd(8) - The client accesses the matching private key. It may be stored locally, accessed through an agent, or held by a supported hardware authenticator.
- The client signs the authentication data. The signature includes the SSH session identifier and authentication request fields, binding it to that session and request rather than presenting a reusable password string. RFC 4252
- The server verifies both authorization and proof. The associated public key must be accepted for that user, and the signature must verify. Server policy may require another authentication method as well.
This establishes that the client can use an authorized private-key credential for the requested authentication. It does not establish that a particular person is at the keyboard, that the client device is uncompromised, or that nobody has copied the private key.
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How password authentication differs
With SSH password authentication, the client sends the password value in the authentication packet. That does not mean the password is ordinarily exposed in cleartext to a passive network observer: RFC 4252 specifies that the packet is encrypted by the SSH transport, and says both endpoints should check that the transport provides confidentiality. RFC 4252
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| Question | Public-key authentication | Password authentication |
|---|---|---|
| What the client presents | A public-key identity and a signature made with the corresponding private key. | The password value inside an SSH authentication packet. |
| What the server checks | That the key is authorized for the account and the signature is valid. | That the supplied password passes the server’s configured password validation. |
| Effect of SSH transport encryption | Authentication takes place within the SSH connection. | The authentication packet is encrypted when the transport provides confidentiality. |
The difference is not that one method uses encryption and the other does not. SSH protects the connection with its transport; the methods differ in what the client uses as its authentication proof.
Why keys can be preferable—and where the risk moves
A key-based login avoids presenting the password itself as the authentication proof. Instead, the client demonstrates use of a private credential by signing data bound to the SSH session. This changes the risks an attacker must address, but it is not a universal guarantee that keys are safer under every configuration. The outcome depends on the account, server policy, client device, key storage, and recovery practices.
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- Private-key exposure: Someone who obtains an usable private key may gain access until the key is removed or revoked. Limit access to the key file or agent; OpenSSH documents that a private-key file should not be readable by others. OpenSSH ssh-keygen(1)
- Passphrase trade-off: A passphrase encrypts the private portion of a local key. If forgotten, it cannot be recovered; the user must create a replacement key and install its public key again. OpenSSH ssh-keygen(1)
- Client compromise: Malware or an attacker controlling the client may be able to use credentials available to that device, even if the key file itself is protected.
- Overbroad authorization: An authorized key grants access under the configured key options and server policy. OpenSSH supports per-key restrictions such as forced commands and source-address constraints. OpenSSH sshd(8)
- Weak recovery or revocation: OpenSSH supports revoked-key files and key revocation lists, but administrators must maintain the revocation mechanism and remove or reject exposed credentials. OpenSSH sshd(8)
- Password-related risks remain distinct: Password reuse, guessing, exposure at endpoints, and account password policy still matter when password authentication is enabled. SSH transport encryption does not address those endpoint or account-management risks.
Using a key with another authentication method
Key authentication does not have to be the only step. OpenSSH’s AuthenticationMethods setting can require sequences such as publickey,password or publickey,keyboard-interactive. The server’s configured policy determines whether an additional method is required. OpenSSH sshd_config(5)
Optional: hardware-backed FIDO2 SSH keys
OpenSSH supports FIDO authenticator-backed key types including ecdsa-sk and ed25519-sk. The authenticator must be present to sign; it can keep a device-specific private key non-exportable. User-presence or user-verification requirements are configurable for supported FIDO keys. This is an optional way to strengthen key custody, not a prerequisite for ordinary SSH key authentication. OpenSSH ssh-keygen(1)
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Compatibility depends on the client, operating system, firmware, and key model. Yubico’s documentation says FIDO support generally requires OpenSSH 8.2 or later; its verify-required PIN-per-use option requires OpenSSH 8.4 or later. The bundled macOS OpenSSH may lack FIDO support, Windows has a separate version requirement, and some key types depend on device firmware. Check the specific setup before choosing a hardware authenticator. Yubico: Securing SSH with the YubiKey
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