To help validate identities, SSH has a key management capacity and related agents. When configured with public key authentication, your key proves your identity to remote SSH hosts. An SSH-based identity consists of two parts: a public key and a private key. The private SSH key is the user's identity for outbound SSH connections and should be kept confidential. When a user initiates an SSH or SCP session to a remote host or server, he or she is said to be the SSH client. Through a mathematical algorithm, a private key is like your electronic identification card; the public key is like the lock or gate mechanism that you present your ID card to. Your private key says, "This really is Fred Smythe"; the public key says, "Yes, you are indeed the real Fred Smythe; you are now authenticated: Please enter."
Your public key represents who you will allow inbound access to through your gate or lock. Public keys need not be kept secret; they cannot be used to compromise a system or for unwarranted access into a system. On a Linux or UNIX system, these private and public key pairs are stored in ASCII text files; on Windows systems, some programs store the key pairs as text files, some in the Windows registry.
Multiple identifications using multiple private keys can be created with an SSH Protocol 2 configuration. Let's look at how to generate, set up, and configure an SSH private and public key pair on typical Linux hosts (see Figure 5).
Figure 5. Diagram of the SSH private-public key pair transactions, as defined within the SSH defined architecture model
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