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Transport

The SSH transport layer handles algorithm negotiation between the server and client over TCP/IP. Negotiation begins when the SSH client and server send each other textual information that identifies their SSH version. If they both agree that the versions are compatible, the client and server exchange lists that specify the algorithms that they support for key exchange, encryption, data integrity through a message authentication code (MAC), and compression. Each party sends two lists. One list has the algorithms supported for transmission; the other has the algorithms supported for receipt. The algorithms are specified in order of preference in each list. The client and server use the algorithm for each process that matches the client’s highest preference and is supported by the server. If no intersection is found, the negotiation attempt fails and the connection is terminated.

If algorithm negotiation is successful, the server sends its public host key to the client for authentication so the client can be certain that it is connected to the intended host rather than to an imposter. The client compares the key to its host key database. The client authenticates the server if the key is found in the database. If the key is not present, then the client can accept or reject this new, unknown key depending on how you have configured the client. For more information, see Host Key Management.

When the client authenticates the server’s host key, it begins the transport key exchange process by sending the key data required by the negotiated set of algorithms. The server responds by sending its own key data set. If both sides agree that the keys are consistent and authentic, the keys are applied so that all subsequent messages between client and server are encrypted, authenticated, and compressed according to the negotiated algorithms.

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