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SSH (Secure Shell) is a protocol for secure remote login and other network services over an untrusted network. It protects traffic between a client and server, verifies the server’s identity, and then authenticates the user account. Those last two checks are separate: the server’s host key identifies the server to you; a user key or another accepted method identifies you to the server.
What SSH does
The IETF describes SSH as “a protocol for secure remote login and other secure network services over an insecure network” in the abstract of RFC 4252, published in January 2006. SSH is a protocol, not a particular paid product. It can carry remote login and other network services through protected connections.
SSH is organized into three layers. The transport layer negotiates algorithms, authenticates the server, and provides confidentiality and integrity. The user-authentication layer checks the client’s account credentials. The connection layer carries one or more logical channels, allowing the established connection to support multiple streams or services. These roles are specified in RFC 4251 and RFC 4253.
How an SSH connection authenticates both sides
- The client connects and negotiates transport settings. The client and server agree on algorithms for the connection.
- The client checks the server’s identity. During transport setup, the server proves possession of its host key. The client uses that key, together with any trusted prior knowledge of it, to identify the server.
- The client requests a user-authentication method. The request includes a username and an authentication method. The server decides which methods it will accept for that account and may reject a request while indicating which methods can be tried next.
- The server verifies the user’s credentials. For a public key, it checks authorization and a signature; for a password, it validates the password according to its configuration and policy.
- The server reports success when authentication is complete. A server may require more than one method, so a successful individual method does not always finish the exchange.
The transport stage is about whether you are talking to the intended server. User authentication is about whether the server should grant access to a particular account. An unknown or changed host-key warning concerns the first question, not whether your login key is valid.
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Host keys and user keys identify different parties
| Credential | Whose identity it helps verify | Who checks it | Where it is used |
|---|---|---|---|
| Server host key | The SSH server | The client | Transport setup |
| User public key and corresponding private key | The client’s user account | The server | User authentication |
On a first connection, a client may not yet know the server’s host key. If it warns that the key is unknown or has changed, verify the server’s fingerprint through a trusted channel, such as an administrator or an independently obtained record. Do not accept a changed value blindly: doing so can undermine your ability to distinguish the intended server from an impostor. The role of prior host-key knowledge is described in RFC 4251.
How public-key authentication works
In public-key login, the client has a public key and a matching private key. The public key can be authorized for an account on the server; the private key should remain under the client’s control. The client proves it has the private key by producing a digital signature—it does not send the private key as the proof.
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- The client identifies the account and offers a public key.
- The server checks whether that key is authorized for the account. Depending on the exchange, it may first indicate whether the key is acceptable.
- The client signs authentication data using the associated private key. The signature covers the SSH session identifier and request fields, tying the proof to that session and request.
- The server verifies the signature with the offered public key. If the key is authorized and the signature is valid, the method succeeds, subject to any additional authentication the server requires.
This is a signing operation, not encryption of the login proof with the user’s SSH key. In particular, Ed25519 is a signing algorithm, not an encryption algorithm. RFC 8709 defines the SSH algorithm names ssh-ed25519 and ssh-ed448 for signing rather than encryption and recommends their implementation in standard SSH implementations.
Public-key and password authentication compared
| Question | Public-key authentication | Password authentication |
|---|---|---|
| What the client supplies as proof | A signature proving possession of the private key; the private key itself is not sent as proof. | The password is carried in the authentication request inside the protected SSH transport. |
| What the server verifies | Whether the public key is authorized for the account and whether the signature is valid. | The password against the server’s password database and policy. |
| Important security assumption | The endpoints holding the private key must not be compromised. A passphrase protecting a stored key can reduce risk. | RFC 4251 warns that a compromised server can expose a valid username/password combination. |
| What determines availability | The server must enable and accept the method, and the client must be able to use the key. | The server must enable and accept the method; suitability depends on deployment and policy. |
Neither method is universally available or appropriate. RFC 4252 requires implementations to support public-key authentication, while password and host-based methods are optional. A particular server’s configuration and operational policy determine what it actually permits.
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Passphrases, agents, forwarding, and hardware authenticators
Protecting a private-key file
A passphrase can encrypt a private key stored on disk, so someone who obtains the file may still need the passphrase to use it. It is a safeguard, not a guarantee: it does not protect a key while it is in use on a compromised device. RFC 4251 notes that passphrases mitigate risk but do not by themselves enforce a policy; where enforceable protection is needed, it points to smartcards or similar technology.
Using an SSH agent
An SSH agent can hold identities or perform key operations on the client’s behalf, reducing repeated prompts to unlock a key. OpenBSD’s ssh_config(5) manual documents identity files and agent identities. Configuration details and defaults can vary by client release, so consult the manual for the version installed on your system.
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Forwarding an agent to a remote host
Agent forwarding lets a remote system request key operations through an SSH connection without directly receiving the private-key material. However, while forwarding is available, the remote host can ask the agent to perform operations. Forward only when you trust the remote environment and need the feature. The IETF’s RFC 9987 describes SSH agent operations and forwarding.
Authenticator-hosted keys
Some SSH key types use a hardware authenticator. The OpenBSD ssh-keygen(1) manual lists ecdsa-sk and ed25519-sk types and documents USB HID support for FIDO authenticator-hosted keys. A physical security key is optional, not a general SSH requirement. Confirm operating-system, client, server, and device compatibility before choosing this route; available types and defaults can change between releases.
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What to verify when SSH login does not work
- Host-key warning: Check the server fingerprint through a trusted channel. This is a server-identity issue, separate from your user credential.
- Public key rejected: Confirm that the intended public key is authorized for the right account and that the client is offering the matching private key.
- Method unavailable: The server may not enable the method you are trying. Its configuration controls accepted methods; a client cannot make the server accept one by itself.
- Additional prompt after a successful key check: The server may require another authentication method before declaring the exchange complete.
- Key type or authenticator incompatibility: Check the manuals and supported algorithms for the installed client and server rather than assuming every release supports the same options.
- Unexpected agent behavior: Check which identities the client can access and whether agent forwarding is enabled; avoid forwarding through hosts you do not trust.
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