The SSH handshake process is a series of steps that occur when a client initiates a connection to an SSH server. This process establishes a secure communication channel between the client and server, ensuring confidentiality and integrity. Here's an overview of the SSH handshake process:
1. Connection Initialization:
The SSH handshake begins when a client initiates a connection to an SSH server. The client sends a connection request to the server, indicating its intention to establish an SSH connection.
2. Server Greeting (Banner):
Upon receiving the connection request, the server responds with a greeting message known as the "banner." This message typically includes information about the server and may be customizable by system administrators. The banner is displayed to the user, providing some information about the server.
3. Key Exchange Initiation:
The key exchange initiation phase is a crucial step in the SSH handshake. The client and server negotiate cryptographic parameters and exchange information necessary to establish secure communication. This includes selecting algorithms for key exchange, encryption, and integrity checks.
4. Key Exchange:
During the key exchange phase, the client and server perform a key exchange algorithm to agree on a shared secret. This shared secret will be used to derive session keys for encrypting the data exchanged during the SSH session.
5. Public Key Authentication (Optional):
If public key authentication is configured on the server, the client may provide its public key to the server. The server checks whether the client's public key is listed in the authorized keys file. If the key is found, the client is authenticated.
6. User Authentication:
The user authentication phase involves the client proving its identity to the server. This can be done using various methods, including password authentication, public key authentication, or more advanced methods like multi-factor authentication.
7. Establishment of Secure Channel:
Once the key exchange and authentication phases are successfully completed, a secure channel is established between the client and server. This channel is encrypted using the derived session keys, ensuring that data transmitted between the client and server is secure and cannot be easily intercepted.
8. Session Creation:
With the secure channel in place, the client and server can create an SSH session. This session allows the client to execute commands on the server, transfer files securely, or perform other authorized activities.
9. Encrypted Data Exchange:
From this point forward, all data exchanged between the client and server is encrypted using the established session keys. This ensures the confidentiality and integrity of the communication.
10. Connection Termination:
When the SSH session is complete, the connection can be terminated gracefully. The client and server exchange termination messages, and the secure channel is closed.
The SSH handshake process is a critical aspect of SSH security, as it establishes a secure and encrypted communication channel while ensuring the authentication and integrity of the involved parties. The use of cryptographic keys, strong algorithms, and secure protocols makes SSH a widely trusted and secure method for remote access and file transfer.