SSH Basics

Secure Shell Protocol (SSH) is a cryptographic network tool. One of the most common usages is remote server access. For example, one can use SSH connecting local Git repository to Github, which negates the input of Github account password every time and is considered much safer.

Keygen

SSH as a command line tool is available natively on MacOS, Linux, and newer Windows. For these system, the terminal syntax should be similar if not the same. The idea of SSH is to use public and private key cryptographic method to safely establish connection between local and remote. We generate the keypair by

ssh-keygen -t ed25519 -C "your-comments"

Note that -t flag specifies which cryptographic algorithm one is using, in our case ed25519, and -C flag prompt a comment along with your key file. Why ed25519? Some argues that ed25519 is faster, safer, and more efficient than the famous RSA algorithm. I like its short and compact public key compared to its RSA counterpart.

Once type the command, you will be prompted to enter a file location and file name to save the keypair, typically in your home/usr/.ssh folder. Make sure you name it something sensible or go ahead with the default location. For demonstration, let's say you set

/home/usr/.ssh/id_ed25519

Then, you will be prompted to enter a passphrase. In the event someone has access to your drive, especially your keypair file, they must enter the passphrase in order to use your keypair. Thus, passphrase is your last defense. At this point, your keypair is generated, namely, two files id_ed25519 and id_ed25519.pub are generated in your .ssh folder. It is okay to share the .pub file since it is your public key, but never expose id_ed25519 (no suffix) to anyone.

ssh-agent

You have the key generated, then what? Who manages your key? ssh-agent! ssh-agent is a background process that handles the key, you must add your key to the agent for it to be usable. In certain operating systems, ssh-agent is a start-up process that automatically starts on boot, and sometimes it also automatically add your key file after generated. However, not all system has this feature, so should you want to start the process manually, type

eval "$(ssh-agent -s)"

Then, add your key by

ssh-add ~/.ssh/id_ed25519"

Note that you added your private key to ssh-agent rather than your public key. At this point, your key is ready to be used. For example, you can

cat ~/.ssh/id_ed25519.pub

then paste your public key to Github to enjoy password-less user experience while knowing it is safer than conventional password!