Lock or unlock the entire agent with a passphrase.Sign a message with a key stored in the agent.Add a key (regular or constrained) from a smart card (public key only).Add a constrained key pair (public and decrypted private keys).Add a regular key pair (public and decrypted private keys).The agent protocol is so simple that one could write a basic SSH agent in a day or two. Most people use the ssh-agent that comes with OpenSSH, but there's a variety of open-source alternatives. SSH uses a Unix domain socket to talk to the agent via the SSH agent protocol. These keys may not even last the entire session a "rekey" event happens at regular intervals. Later in the handshake process, a set of new, ephemeral and symmetric keys are generated and used to encrypt the SSH session traffic. The server now has proof that the client is in possession of their private key.The server checks the signature using the client's public key.The client asks the SSH agent to sign the message and forwards the result back to the server.The server generates and sends a brief, random message, asking the client to sign it using the private key.The client presents a public key to the server.An SSH key pair is only used for authentication during the initial handshake.įor example, here's how a user's key is verified during the SSH handshake, from the server's perspective: When first learning about public and private SSH keys, it's natural to assume that SSH uses these key pairs to encrypt and decrypt traffic. But if the agent can only sign messages, how does SSH encrypt and decrypt traffic? Private keys stored in the agent can only be used for one purpose: signing a message. It doesn't allow your private keys to be exported.It doesn't write any key material to disk.The SSH agent keeps private keys safe because of what it doesn't do: It runs in the background on your system, separately from ssh, and it usually starts up the first time you run ssh after a reboot. It saves you from typing a passphrase every time you connect to a server. It holds your keys and certificates in memory, unencrypted, and ready for use by ssh. I'll help you reduce your risk when using agent forwarding, and I'll share an alternative to agent forwarding that you can use when accessing your internal hosts through bastions. I'll also describe agent forwarding and how it works. In this post, I'll explain what the agent is, how to use it, and how it works to keep your keys safe. The SSH agent is a central part of OpenSSH. Carl Tashian Follow Smallstep Introduction
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