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1Clear out junk files and repair common Windows errors2Fix the driver behind crashes, sound loss and screen glitches3Repair Windows errors before they cause bigger problemsSSH (Secure Shell) protects a remote session in stages. First, the client and server negotiate algorithms and perform a host-authenticated key exchange. They then derive fresh, directional session keys and turn on encryption and integrity checks. Only after that protected transport exists does the client send a password or prove possession of a user private key. Finally, SSH multiplexes shells, commands, forwarding, and subsystems as channels inside the same encrypted connection.
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What SSH is securing
SSH is a protocol suite for remote login and other network services across an untrusted network. It is not just an encryption wrapper around a terminal. Its transport layer authenticates the server, negotiates cryptographic algorithms, establishes keys, and protects packets. Its user-authentication layer verifies the person or account requesting access. Its connection layer carries one or more logical channels for interactive shells, one-off commands, port forwarding, X11 forwarding, and subsystems such as file-transfer services.
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The architecture is defined for SSH version 2. RFC 4251 describes these as three major components, while RFCs 4252, 4253, and 4254 specify user authentication, transport and key exchange, and channels. Implementations can support different algorithms, so the exact cipher and key-exchange method depend on what both endpoints allow and what local policy selects.
The three SSH layers at a glance
| Layer | What it does | When it matters |
|---|---|---|
| Transport | Negotiates algorithms, authenticates the server with its host key, performs key exchange, and supplies confidentiality and integrity. | Before user credentials are accepted and throughout the connection. |
| User authentication | Proves that the requested account may log in, using public-key, password, or another method permitted by the server. | After the encrypted transport is active. |
| Connection | Multiplexes shells, commands, forwarding requests, X11, and subsystems into logical channels. | After authentication, and repeatedly while the session runs. |
What happens when you connect with SSH
The following sequence describes a normal SSHv2 connection. Message names such as SSH_MSG_KEXINIT and SSH_MSG_NEWKEYS are protocol messages, not commands you type.
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1. TCP connection and identification strings
The client first opens a reliable TCP stream to the server’s SSH port, commonly port 22 but not necessarily. Each side sends an identification string that declares SSH protocol support and an implementation version. This exchange is still unencrypted. It tells the peers that they are speaking SSH and establishes where the binary transport protocol begins; it does not authenticate either endpoint.
2. Algorithm negotiation with SSH_MSG_KEXINIT
Each endpoint sends an SSH_MSG_KEXINIT packet containing ordered lists of supported choices. The lists cover key-exchange methods, server host-key algorithms, encryption algorithms for each direction, integrity or MAC algorithms, and compression methods. The peers select the first mutually acceptable option according to the protocol rules and their local preferences or security policy.
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This is why two SSH connections can use different cryptography without changing the command you run. A server may disable older algorithms, while a client may have its own minimums and preference order. If the lists have no acceptable overlap, negotiation stops before login and the client reports that no matching algorithm, host-key type, or cipher is available.
3. Host-authenticated key exchange creates K and H
Next, the selected key-exchange method establishes a shared secret commonly denoted K and computes an exchange hash H. In a typical Diffie–Hellman-style exchange, the client and server contribute ephemeral values. Neither side sends the final shared secret over the network; each derives it independently from the exchanged values.
The server signs the exchange transcript, including H, with its SSH host private key. The client verifies that signature using the server’s host public key. This step binds the newly negotiated secret to a server identity and prevents an attacker from silently substituting a different endpoint, provided the client has a trustworthy host-key record or verifies the fingerprint through another channel.
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Host authentication is separate from user authentication. The host key answers, “Am I talking to the intended server?” It does not identify the human or account that will log in.
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4. Deriving separate session keys
SSH derives encryption and integrity material from K, H, and the session identifier. The first exchange hash becomes that stable session identifier. Derivation produces distinct keys for client-to-server and server-to-client traffic, rather than reusing one key in both directions. Implementations also derive separate values for encryption and integrity when the negotiated mode requires them.
Using derived session keys gives the data stream fresh, connection-specific cryptographic material. A user’s long-term private key is not used as the bulk-encryption key, and the server host private key is not copied to the client. Those long-term keys authenticate identities; ephemeral exchange values and derived keys protect the actual stream.
5. SSH_MSG_NEWKEYS switches on protection
After both sides have the required key material, each sends an SSH_MSG_NEWKEYS message. Packets sent after the switch use the negotiated encryption and integrity algorithms. Depending on the selected mode, integrity may be provided by a separate MAC or by an authenticated-encryption mode. The receiver checks the integrity result and rejects a packet that was altered, truncated, or otherwise fails verification.
The identification strings and early negotiation messages are therefore not confidential. The application data and credentials are protected only after the new keys take effect. An observer can usually see that an SSH connection is being negotiated, but cannot read the subsequent password, commands, or output when the cryptography and endpoint verification are sound.
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6. Requesting user authentication
With the transport protected, the client requests the ssh-userauth service and offers methods allowed by the server. Public-key authentication is common: the client demonstrates possession of a private key corresponding to a public key that the account is authorized to use. The private key itself is not sent to the server. The proof is tied to the session so it cannot simply be copied into an unrelated connection.
Password authentication is also sent through the already encrypted transport. SSH does not need to expose the password during the network exchange, but the password still has to be protected at the endpoints and the server must store or process credentials safely. Servers can require multiple methods, such as a public key followed by a one-time code, and can reject methods disabled by policy.
If authentication fails, the transport can remain encrypted while the client tries another permitted method, subject to server attempt limits. A successful authentication does not create a second tunnel; it authorizes the account to use the transport that is already in place.
7. Starting the connection service and opening channels
After authentication, the client requests the ssh-connection service and opens one or more channels. A channel is a logical, flow-controlled stream inside the encrypted transport. An interactive terminal uses a shell channel; a single command uses an execution channel; port forwarding, X11 forwarding, and subsystems use their own channel types.
Multiple channels can share one TCP connection. For example, a client can keep a shell open while forwarding a local port and running a separate command. Channel identifiers, window sizes, and open or close messages keep these streams separate even though they are carried in the same encrypted packet stream. RFC 4254 summarizes this design as channels multiplexed into a single encrypted tunnel.
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What “SSH encryption” actually means
Asymmetric cryptography for identity and agreement
Public-key cryptography appears during host authentication and, when selected, user authentication. The server signs the key-exchange exchange hash with its host key. A user key can sign an authentication proof. These operations are valuable for proving possession and identity, but they are comparatively expensive for continuous traffic.
Symmetric encryption for the data stream
Once the handshake derives session keys, SSH normally uses symmetric encryption for packets because it is efficient for sustained traffic. The protocol negotiates the algorithm rather than mandating one cipher for every installation. Modern implementations may offer authenticated-encryption modes such as AES-GCM, counter-mode ciphers paired with an integrity algorithm, elliptic-curve key exchange, and SHA-2-based integrity options. Which option is actually used is determined by the overlap between client and server configuration.
Integrity and ordering
Confidentiality hides packet contents; integrity protection detects modification. SSH packet processing includes sequence tracking and integrity verification so an attacker cannot alter a command or response unnoticed. Integrity does not make a compromised endpoint trustworthy: malware on the client or server can read data before it is encrypted or after it is decrypted.
Is SSH encrypted before you log in?
Not during the entire connection setup. The initial identification exchange and algorithm negotiation occur before the session keys are activated. The host-authenticated key exchange then creates and verifies the shared secret, and SSH_MSG_NEWKEYS marks the point after which negotiated encryption and integrity apply. The user-authentication exchange, including a password method, is sent after that point and is protected by the transport.
This ordering matters when interpreting packet captures or verbose logs: seeing plaintext-looking negotiation metadata before a login prompt is expected. Seeing credentials or command data after new keys have been installed would indicate a serious implementation or configuration failure.
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Host keys, user keys, and the first-connection warning
Server host keys
A server has one or more host key pairs. The private portion stays on the server; the public portion is presented during key exchange. SSH clients commonly store an accepted host key in a local known_hosts database. On a first connection, the client displays the server name, key type, and fingerprint and asks whether to trust it.
Accepting without checking the fingerprint leaves room for a man-in-the-middle attack at the moment trust is established. A safer process is to obtain the expected fingerprint from an administrator, console access, a separately secured inventory system, or another out-of-band channel, then compare it before accepting. If a known host key later changes, stop and investigate rather than deleting the old entry automatically; legitimate causes include a rebuilt server, a deliberate key rotation, or a host-name reuse, but interception is also possible.
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A user’s private key is held by the client, often protected with a passphrase. The corresponding public key is placed in the account’s authorized-key configuration on the server. During login, the client signs session-bound data and the server verifies it with the stored public key. Losing control of the private key is therefore different from a host-key change: it can allow unauthorized user authentication and should trigger key revocation or removal from authorized lists.
How to observe the process safely
OpenSSH’s verbose mode shows the stages without revealing private key material. Run a normal connection with one, two, or three v characters, such as ssh -vvv user@server, and read the output for identification, KEXINIT algorithm selection, host-key verification, NEWKEYS, authentication methods, and channel creation. The exact wording varies by client version.
Use verbose output to diagnose, not to bypass verification. A message saying that the host key is unknown calls for fingerprint checking. A message saying that the host key changed calls for an investigation of the endpoint and your stored record. Do not “fix” either condition by blindly accepting a new key.
Common failures and what they mean
| Symptom | Likely stage | Useful response |
|---|---|---|
| Connection refused or timed out | TCP reachability, listener, firewall, or routing | Confirm the address and port, verify that the SSH service is listening, and check network policy before examining keys. |
| No matching key-exchange, host-key, or cipher algorithm | Algorithm negotiation | Compare client and server policy and enable a mutually supported, currently approved option rather than re-enabling obsolete algorithms indiscriminately. |
| Host key verification failed | Server identity check | Compare the presented fingerprint with a trusted record and investigate unexpected changes. |
| Permission denied (publickey) | User authentication | Check the account name, authorized public key, private-key access and passphrase, and server authentication policy. |
| Password rejected after a successful handshake | User authentication policy or credentials | Confirm the password, account status, allowed methods, and any multi-factor requirement; the encrypted transport itself is not the problem. |
| Shell works but forwarding is refused | Channel or forwarding policy | Check server-side forwarding settings, requested channel type, and account restrictions. |
Fresh keys and long-lived sessions
The initial exchange creates session-specific key material. SSH can perform another key exchange during a long-lived connection, replacing traffic keys while preserving the authenticated session. Rekeying limits how much data one key protects and refreshes the cryptographic context without forcing the user to log in again. The host identity and session identifier continue to bind the connection to its original authenticated exchange.
Security decisions are configuration decisions
The SSH protocol defines negotiation and authentication frameworks, not one universally correct configuration. Administrators should set an explicit policy for acceptable key-exchange methods, host-key algorithms, ciphers, integrity algorithms, compression, forwarding, and authentication methods. Clients should keep their software current, protect private keys with appropriate passphrases or hardware-backed controls, restrict who can read key files, and remove obsolete authorized keys.
Encryption cannot compensate for a weak trust process. Verify host fingerprints, protect endpoint accounts, limit forwarding and channel capabilities to what users need, and monitor failed authentication attempts. Treat a private-key compromise, unexpected host-key change, or unexplained algorithm downgrade as a security event rather than a routine warning.
Quick Recap
A compact mental model
- Connect: TCP and identification establish an SSH conversation, but not secrecy.
- Agree: KEXINIT lists produce a mutually permitted cryptographic suite.
- Prove the server: the host key signs the exchange hash while both sides derive the shared secret.
- Protect: derived directional keys become active at NEWKEYS, adding encryption and integrity.
- Authenticate the user: a public-key proof, password, or other allowed method is sent through that protected transport.
- Open channels: shells, commands, forwarding, and subsystems share one encrypted connection.
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