SSHhandshake

The SSH handshake is the initial communication process between a client and a server during an SSH (Secure Shell) session establishment. It is a critical component of the SSH protocol, ensuring secure authentication and encrypted communication before any data transfer occurs. The handshake follows a standardized sequence outlined in the SSH protocol specifications, primarily defined in RFC 4250, RFC 4251, RFC 4252, and RFC 4253.

The process begins with the client sending a client version string, identifying the SSH protocol version it supports, typically "SSH-2.0-..." followed by the software name and version. The server responds with its own version string, confirming compatibility. If the versions are incompatible, the connection is terminated. Once compatibility is established, the client and server exchange key exchange algorithms, encryption methods, and integrity algorithms to agree on a secure configuration for the session.

Next, the client generates a random number and sends it to the server, which also generates its own random number. These numbers, along with the agreed-upon algorithms, form the basis for deriving shared secrets using key exchange algorithms like Diffie-Hellman (DH) or elliptic curve Diffie-Hellman (ECDH). The shared secret enables the client and server to establish symmetric encryption keys for secure communication.

Following key exchange, the server authenticates itself to the client using a host key, typically a cryptographic public key (e.g., RSA or ECC). The client verifies the server’s host key against a trusted list of known keys to prevent man-in-the-middle attacks. If verification succeeds, the client may proceed with user authentication, which can involve password-based or public-key-based methods.

Once authentication is complete, the handshake concludes, and encrypted data transfer begins. The entire process ensures that the connection is secure, confidential, and authenticated before any sensitive data is exchanged. The SSH handshake is designed to be resistant to common attacks, such as eavesdropping, tampering, and impersonation, by leveraging cryptographic principles.