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What SSH is—and what it does
SSH, or Secure Shell, is a protocol suite for securely connecting to another computer over a network. A shell is one thing it can carry, but SSH can also run a remote command, forward network connections, or provide access to subsystems. Its architecture separates those functions into a transport layer, a user-authentication layer, and a connection layer. See the IETF’s RFC 4251.
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This separation matters because encryption, server identity, and account login are not the same operation. SSH protects traffic using negotiated transport keys; it does not use a user’s public key as the traffic cipher.
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How an SSH connection works, step by step
- The client connects and negotiates algorithms. The client and server exchange protocol identification and agree on compatible algorithms for key exchange, server host-key authentication, encryption, integrity, and hashing. SSH is extensible, so the available choices depend on both implementations and their policies. The transport process is specified in RFC 4253.
- They establish session keys and verify the server. Key exchange derives session keys for protecting the connection. During this process, the server proves its identity with a host key. The client must have a trusted basis for associating that server name with that host key—for example, a key it previously recorded or a host certificate signed by a trusted authority. RFC 4251 states: “The server host key is used during key exchange to verify that the client is really talking to the correct server.”
- SSH protects the transport. Once key exchange is complete, negotiated symmetric encryption and integrity protection secure data in transit. This transport protection is established before the user’s account authentication.
- The user authenticates. The client requests SSH’s user-authentication service. The server checks whether the offered method is permitted by policy and whether the account is authorized. Methods specified by SSH include public-key, password, and host-based authentication; a server may also require additional authentication. The protocol is described in RFC 4252.
- SSH opens channels for work. Once authentication succeeds, the connection protocol can carry a shell, a remote command, TCP/IP forwarding, X11 forwarding, or a subsystem as logical channels over the protected transport. These channels are defined in RFC 4254.
How SSH public-key authentication works
With public-key authentication, the client proves possession of a private key without sending that private key to the server. It signs authentication data bound to the SSH session. The server checks that the corresponding public key is authorized for the requested account and verifies the signature. A valid signature alone is not enough if the key is not authorized for that user.
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The key used to authenticate a user is distinct from the server’s host key. The host key helps the client decide which server it reached during transport setup; a user key can help the server decide whether that user may log in after the transport is established. SSH’s architecture and authentication roles are described in RFC 4251 and RFC 4252.
Host-key trust: what the warning means
Encryption protects traffic from passive observers, but it does not by itself prove that the encrypted connection is to the intended server. If a client accepts an attacker’s host key without checking it, an active intermediary could impersonate the server. The client therefore needs to verify the server’s host identity, commonly by comparing it with a locally remembered key or validating a host certificate against a trusted authority. RFC 4251 describes these trust models and does not recommend omitting host-key verification.
- On first connection: When possible, compare the presented host key or fingerprint with information obtained through a trusted, independent channel before accepting it.
- On an unexpected key change: Stop and determine whether the server was rebuilt or rekeyed, or whether interception is possible. Do not dismiss the warning automatically.
SSH algorithms are negotiated, not universal
There is no single cipher or key type that every SSH connection uses. Client and server negotiate from the algorithms they support and allow. For examples of the standards’ extensibility, RFC 8709 specifies Ed25519 and Ed448 public-key algorithms for SSH and notes that OpenSSH 6.5 introduced Ed25519 for server and user authentication. RFC 8731 specifies Curve25519 and Curve448 for SSH key exchange. These standards do not establish which algorithms are enabled by default in every current client or server; defaults are implementation- and version-specific.
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What SSH encryption does not protect you from
- An unverified server identity: An encrypted session can still be established with the wrong server if the client does not properly verify the host key.
- A compromised endpoint: SSH authentication and transport protection cannot make a compromised client or server trustworthy. An endpoint may expose credentials, session data, or services available through the connection.
- An exposed private key: Someone who obtains a private key may impersonate its holder wherever that key remains authorized. A passphrase can protect a private-key file, and RFC 4251 notes that smartcards or similar technology may make passphrase use enforceable. This is not a guarantee of compatibility with every SSH setup.
- Unsafe forwarding: Forwarded connections can make additional services reachable. Operators should limit permitted channels and destinations according to local security policy.
- Every possible algorithm configuration: Security properties depend on the negotiated method and implementation. Do not assume that every SSH configuration has the same properties, including perfect forward secrecy.
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