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Public/private key login is a way to prove your identity without sending your private key to the server. In SSH, your client uses a private key to sign an authentication request; the server checks that signature with the matching public key and confirms that the key is accepted for your account.
What does public/private key login mean?
It is an authentication method based on a mathematically related pair of keys. In SSH public-key user authentication, the private key is the credential the client uses to create a signature. The public key is the shareable counterpart the server can use to verify that signature. The SSH standard describes the method this way: “With this method, the possession of a private key serves as authentication.” (RFC 4252, section 7)
This explanation uses SSH as its concrete example. Other systems may use public/private keys differently, so the details should not be assumed to apply to every login protocol.
Which key goes on the server, and which key stays private?
- Public key: Add or register this key with the server or service for the relevant account. It can be shared without exposing the private key.
- Private key: Keep this on the client and protect it. The client uses it to sign authentication data; it does not send the private key itself.
Microsoft’s OpenSSH for Windows key-management guidance says a stolen private key can enable sign-in as its owner to SSH servers that accept it, and advises that private-key files remain protected.
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How does SSH public-key login work?
- The client requests authentication as a named user and identifies a public key.
- If the server recognizes that key as an acceptable authenticator for the user, the client signs data for the authentication request with the matching private key. The signature is tied to the SSH session and request, rather than being a reusable password.
- The server checks that the public key is acceptable for the account and verifies the signature. If both checks succeed, public-key authentication succeeds; server policy may still require another authentication method.
A copied public key alone does not demonstrate that someone has the corresponding private key. The server needs both an acceptable account-to-key association and a valid signature.
Does key login still use a password or passphrase?
Not necessarily an SSH account password. A passphrase can encrypt a private-key file and must be supplied locally to unlock the key before the client can sign. That passphrase protects the key at rest; it is distinct from the server account password.
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SSH also defines password authentication as a separate method. In that method, the password is sent within the protected SSH transport; with public-key authentication, the client uses a private-key signature that the server verifies. Neither method is automatically safer in every setup: key protection, configuration, server policy, and implementation all matter. (RFC 4252; RFC 4251)
What does the key prove—and what does it not prove?
A valid signature shows that the client can use the private key matching the offered public key. The server’s account and key checks determine whether that credential is acceptable for the requested user. Authentication does not by itself grant unrestricted access: what the user can do after login depends on server and service authorization policy.
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SSH also separates user authentication from server identity. The SSH transport protocol handles server authentication, confidentiality, and integrity; the user-authentication protocol authenticates the client to the server. Successfully logging in with a user key is therefore not, by itself, the same as verifying that you connected to the intended server. (RFC 4251)
A passphrase-protected key adds a local unlock condition, but that alone does not establish that a deployment meets its multifactor-authentication requirements. An SSH server can require an additional authentication method.
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Do you need a hardware key for SSH public-key login?
No. Ordinary SSH public-key login can use key files; the protocol does not require a smartcard or hardware key. Hardware-backed credentials can be an option where an organization needs stronger control over key use. RFC 4251 notes that a private-key passphrase is not enforceable as policy and suggests smartcards or similar technology where enforceable passphrase protection is needed. Compatibility depends on the particular SSH client, server, and credential, so a hardware device should not be assumed to work everywhere. (RFC 4251)
Windows-specific support caveat
Microsoft’s OpenSSH for Windows documentation states that its documented key-based authentication supports local Windows and Active Directory accounts, but not Microsoft Entra ID accounts. This is a limitation of that documented Windows implementation, not a general limitation of SSH. Check the current platform guidance for account and configuration support before setting up access. (Microsoft Learn)
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