Secure Shell or SSH

Secure Shell or SSH is a point-to-point tunneling protocol that is designed to pass encrypted traffic between two specific hosts. Along with tunneling traffic at the presentation layer, SSH also can provide application layer functionality, including a login shell similar to telnet (over the encrypted tunnel), and FTP-type services. The sftp system is an implementation of an FTP-like protocol over SSH. Used primarily on Linux and Unix based systems to access shell accounts, SSH was designed as a replacement for TELNET and other insecure remote shells, which send information, notably passwords, in plaintext, leaving them open for interception.
The SSH protocol addresses each of the basic security concepts.
SSH uses public-key cryptography to authenticate the remote computer and allow the remote computer to authenticate the user, if necessary. Note that SSH is a protocol that can be used for many applications. Some of the applications may require features that are only available or compatible with specific SSH clients or servers. For example, using the SSH protocol to implement a VPN is possible, but presently only with the OpenSSH server and client implementation.
Confidentiality: Each SSH connection is encrypted, preventing an eavesdropper from viewing information. For added security, SSH periodically reexchanges keys to ensure that a compromise of one set of keys does not compromise the entire session. In contrast, CTCP, IPsec, and IPv6 only exchange keys at the beginning of the connection.
Authentication: Before establishing a connection, the client must authenticate the server and the server must authenticate the client. Client authentication can be any combination of certificates (keys), passwords, or digital tokens. Although SSH is usually used with one-part authentication (a password or a key), it can support two- and three-part authentication systems. The server only uses a certificate to authenticate with the client.
Authorization: SSH limits the traffic that can enter the tunnel and can restrict how data exits the tunnel. For remote login access, SSH restricts authorization to the user’s login privileges.
Integrity: SSH uses encryption and cryptographic checksums to validate each packet. Any packet that fails an integrity check is viewed as an attack, which terminates the connection.
Nonrepudiation: Each packet is cryptographically signed using an HMAC, ensuring that the data actually came from the sender. SSH has three primary uses: establish a secure network tunnel, provide a VPN with port-forwarding characteristics, and supply application-layer login functionality.

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