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Public key

What is private/public key?​

Creating a key pair​

cd ~/.ssh
ssh-keygen -t ed25519 -C "[email protected]"

Here's the descriptions of the options provided to the command.

  • -t: Type of key to create
    • By "type", it means cryptographic algorithms, such as ecdsa, ed25519, and rsa. As of 2024, ed25519 is the preferred cryptographic algorithm to generate keys to ensure security.
  • -C: Comment for the key
    • It is a common practice to put our emails here to identify our keys, but any texts could be written here.

Proceed by pushing the return key. Usually, you won't need to setup a passphrase. The final output would look something like this.

$ ssh-keygen -t ed25519 -C "[email protected]"
Generating public/private ed25519 key pair.
Enter file in which to save the key (/Users/john/.ssh/id_ed25519): 
Enter passphrase (empty for no passphrase): 
Enter same passphrase again: 
Your identification has been saved in /Users/john/.ssh/id_ed25519
Your public key has been saved in /Users/john/.ssh/id_ed25519.pub
The key fingerprint is:
SHA256:3MPdSlN9Pj+KKZZMtsK/f6O/5GWLi1m/HDRQNIF/D4M [email protected]
The key's randomart image is:
+--[ED25519 256]--+
|              o=.|
|             ....|
|             oo o|
|       . o .Eo+oo|
|        S + + .*+|
|         o o o. =|
|      . + o .o =.|
|       o *  OoB +|
|        +o+B+B+=.|
+----[SHA256]-----+

Let's check what's inside a public key.

$ cat ~/.ssh/id_ed25519.pub
ssh-ed25519 AAAAC3NzaC1lZDI1NTE5AAAAIDOYfZ33zaFZDws4FwLxNAxRqBykbGXM1H6QjglfJXPE [email protected]
info

As the name tells, public keys can be public. You can publish it on key servers, your websites, or email it to a friend. But NEVER share or publish the private key*, as will compromise the security.

Useful commands for private/public key pairs​

Change the SSH key comment

ssh-keygen -c -C "my new comment" -f ~/.ssh/id_ed25519

Change passphrase

ssh-keygen -f id_ed25519 -p

Regenerate public key from private key

ssh-keygen -y -f id_ed25519 > id_ed25519.pub

Check fingerprint of the public key

ssh-keygen -l -f id_ed25519.pub