An SSH key proves that the client can make a valid, session-bound signature with a private key whose matching public key the server authorizes for the account. The private key is not sent to the server. That is a different authentication proof from a password, but it does not make every key-based setup safer by default: private-key custody, client security, server policy, and recovery still matter.
What an SSH key proves
Public-key authentication involves both a credential and a server-side authorization decision. The client must be able to use a private key, and the server must accept the corresponding public key for the requested account. A valid signature alone is not enough if the key is not authorized; an authorized public key is not enough if the client cannot produce a valid signature.
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- The server identifies allowed keys. In a common OpenSSH setup, it checks configured authorized-keys files, whose documented default names include
.ssh/authorized_keysand.ssh/authorized_keys2. OpenSSH’ssshd(8)manual describes the server-side handling. - The client accesses the matching private key. It may be stored locally, provided by an agent, or held by a supported hardware authenticator.
- The client signs authentication data. The signature includes the SSH session identifier and authentication request fields, tying the proof to that session and request rather than presenting a reusable password string. RFC 4252 specifies the public-key method.
- The server checks authorization and the signature. Both checks must succeed for public-key authentication to succeed; server policy may also require another authentication method.
In RFC 4252’s words, “With this method, the possession of a private key serves as authentication.” The server still has to check that the key is valid for the user and that the signature is valid.
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How password authentication differs
With password authentication, the client supplies the password value in an SSH authentication packet. That does not mean SSH ordinarily sends the password in cleartext over the network: the packet is carried within the SSH transport, which encrypts it when confidentiality is provided. RFC 4252 says both endpoints should check that the transport offers confidentiality before using password authentication. Read the RFC’s password-authentication requirements.
| Question | Public-key authentication | Password authentication |
|---|---|---|
| What the client presents | A public-key identity and a signature made using the corresponding private key. | The password value in the SSH authentication packet. |
| What the server checks | That the key is authorized for the account and the signature is valid. | The supplied password against the server’s configured password-validation mechanism. |
| What transport encryption does | The authentication occurs within the SSH connection. | The password packet is encrypted by the SSH transport when confidentiality is active; it is not ordinarily visible as plaintext to a passive network observer. |
| Primary credential-management concern | Protecting the private key, controlling which public keys are authorized, and removing or revoking compromised keys. | Preventing guessing, reuse, and exposure at endpoints, and maintaining sound password policy. |
The distinction is not “encrypted versus unencrypted.” It is what the authentication method asks the client to prove. A key-based login uses a signature created by a client-held credential; password authentication supplies the password value inside the protected SSH connection.
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- POWERFUL SECURITY KEY: The YubiKey 5C NFC is the most versatile physical passkey, protecting your digital life from phishing attacks. It ensures only you can access your accounts
- WORKS WITH 1000+ ACCOUNTS: Compatible with popular accounts like Google, Microsoft, and Apple. A single YubiKey 5C NFC secures 100+ of your favorite accounts, including email, password managers, and more
- FAST & CONVENIENT LOGIN: Plug in your YubiKey 5C NFC via USB and tap it, or tap it against your phone (NFC), to authenticate. No batteries, no internet connection, and no extra fees required
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- PRIMARY & SPARE KEYS: Just like having a spare house key, we recommend buying two YubiKeys - one for daily use and one as a spare. That way you’ll never get locked out of your accounts
Where the security benefit comes from—and where it stops
Key authentication changes the credential an attacker would need to obtain or misuse: a password is not the proof presented, while the private key must remain under the client’s control. The practical advantage depends on how that key and the server are managed. There is no established universal outcome or quantified advantage for every SSH deployment; the protocol and implementation documentation explain the mechanics, not an empirical head-to-head security rate.
- Private-key exposure can grant access. Limit who and what can read a private-key file or use its agent. OpenSSH documents that private-key files should not be readable by others. OpenSSH’s
ssh-keygen(1)manual covers key handling. - A passphrase protects a local key file, not a compromised device. OpenSSH says a passphrase encrypts the private portion of the key. If it is forgotten, it cannot be recovered; generate a replacement and install its public key again.
- Authorization is a server-side control. A public key is not secret, but adding it to an authorized-keys source grants authentication rights subject to its options and server policy. OpenSSH supports per-key restrictions such as forced commands and source-address constraints.
- Revocation needs operational upkeep. OpenSSH can reject revoked keys through a revoked-key file or key revocation list, but administrators must maintain that mechanism and remove compromised or unwanted authorization.
- A compromised client remains a risk. Malware or an attacker with access to a logged-in device, key agent, or hardware authenticator may be able to misuse authentication capabilities even if the private key is not exportable.
These controls are documented in the sshd(8) and ssh-keygen(1) manuals.
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- WORKS WITH 1000+ ACCOUNTS: Compatible with popular accounts like Google, Microsoft, and Apple. A single YubiKey 5 NFC secures 100+ of your favorite accounts, including email, password managers, and more
- FAST & CONVENIENT LOGIN: Plug in your YubiKey 5 NFC via USB and tap it, or tap it against your phone (NFC), to authenticate. No batteries, no internet connection, and no extra fees required
- MOST SECURE PASSKEY: Supports FIDO2/WebAuthn, FIDO U2F, Yubico OTP, OATH-TOTP/HOTP, Smart card (PIV), and OpenPGP. That means it’s versatile, working almost anywhere you need it
- PRIMARY & SPARE KEYS: Just like having a spare house key, we recommend buying two YubiKeys - one for daily use and one as a spare. That way you’ll never get locked out of your accounts
Can SSH require both a key and a password?
Yes. OpenSSH’s AuthenticationMethods setting can require sequential methods, including publickey,password or publickey,keyboard-interactive. The server must be configured to require the sequence; merely having both methods enabled does not mean both are required. See OpenSSH’s sshd_config(5) documentation.
Do you need a hardware security key?
No. Hardware-backed FIDO2 SSH keys are an optional way to keep a device-specific private key non-exportable. OpenSSH supports authenticator-backed key types ecdsa-sk and ed25519-sk; the authenticator must be present to sign, and supported keys can be configured for user-presence or user-verification requirements. OpenSSH documents these key types and options.
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- WORKS WITH 1000+ ACCOUNTS: Compatible with Google, Microsoft, and Apple. A single Security Key NFC secures 100 of your favorite accounts, including email, password managers, and more.
- FAST & CONVENIENT LOGIN: Plug in your Security Key NFC via USB-A and tap it, or tap it against your phone (NFC) to authenticate. No batteries, no internet connection, and no extra fees required.
- TRUSTED PASSKEY TECHNOLOGY: Uses the latest passkey standards (FIDO2/WebAuthn & FIDO U2F) but does not support One-Time Passwords. For complex needs, check out the YubiKey 5 Series.
- BUILT TO LAST: Made from tough, waterproof, and crush-resistant materials. Manufactured in Sweden and programmed in the USA with the highest security standards.
Compatibility depends on the client software, operating system, device model, and firmware. 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 its own version requirement, and some key types depend on device firmware. Check the Yubico SSH compatibility guidance for the exact setup rather than assuming every client or device supports the same features.
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