Public key authentication is a way of logging into an SSH/SFTP account using a cryptographic key rather than a password.
If you use very strong SSH/SFTP passwords, your accounts are already safe from brute force attacks. However, using public key authentication provides many benefits when working with multiple developers. For example, with SSH keys you can
Keys come in pairs of a public key and a private key. Each key pair is unique, and the two keys work together.
These two keys have a very special and beautiful mathematical property: if you have the private key, you can prove you have it without showing what it is. It's like proving you know a password without having to show someone the password.
Public key authentication works like this:
You don't have to do the math or implement the key exchange yourself. The SSH server and client programs take care of this for you.
You should generate your key pair on your laptop, not on your server. All Mac and Linux systems include a command called ssh-keygen that will generate a new key pair.
If you're using Windows, you can generate the keys on your server. Just remember to copy your keys to your laptop and delete your private key from the server after you've generated it.
To generate an SSH key pair, run the command ssh-keygen.
It will look like this when you run it:
laptop1:~ yourname$ ssh-keygen Generating public/private rsa key pair.
You'll be prompted to choose the location to store the keys. The default location is good unless you already have a key. Press Enter to choose the default location.
Enter file in which to save the key (/Users/yourname/.ssh/id_rsa):
Next, you'll be asked to choose a password. Using a password means a password will be required to use the private key. It's a good idea to use a password on your private key.
Enter passphrase (empty for no passphrase): Enter same passphrase again:
After you choose a password, your public and private keys will be generated. There will be two different files. The one named id_rsa is your private key. The one named id_rsa.pub is your public key.
Your identification has been saved in /Users/yourname/.ssh/id_rsa. Your public key has been saved in /Users/yourname/.ssh/id_rsa.pub.
You'll also be shown a fingerprint and "visual fingerprint" of your key. You do not need to save these.
The key fingerprint is: d7:21:c7:d6:b8:3a:29:29:11:ae:6f:79:bc:67:63:53 yourname@laptop1 The key's randomart image is: +--[ RSA 2048]----+ | | | . o | | . . * . | | . . = o | | o S . o | | . . o oE | | . .oo +. | | .o.o.*. | | ....= o | +-----------------+
Now that you have an SSH key pair, you're ready to configure your app's system user so you can SSH or SFTP in using your private key.
To copy your public key to your server, run the following command. Be sure to replace "x.x.x.x" with your server's IP address and SYSUSER with the name of the the system user your app belongs to.
If you don't have the ssh-copy-id command (for example, if you are using Windows), you can instead SSH in to your server and manually create the .ssh/authorized_keys file so it contains your public key.
First, run the following commands to make create the file with the correct permissions.
(umask 077 && test -d ~/.ssh || mkdir ~/.ssh) (umask 077 && touch ~/.ssh/authorized_keys)
Next, edit the file .ssh/authorized_keys using your preferred editor. Copy and paste your id_rsa.pub file into the file.
You can now SSH or SFTP into your server using your private key. From the command line, you can use:
If you didn't create your key in the default location, you'll need to specify the location:
ssh -i ~/.ssh/custom_key_name SYSUSER@x.x.x.x
If you're using a Windows SSH client, such as PuTTy, look in the configuration settings to specify the path to your private key.
The .ssh/authorized_keys file you created above uses a very simple format: it can contain many keys as long as you put one key on each line in the file.
If you have multiple keys (for example, one on each of your laptops) or multiple developers you need to grant access to, just follow the same instructions above using ssh-copy-id or manually editing the file to paste in additional keys, one on each line.
When you're done, the .ssh/authorized_keys file will look something like this (don't copy this, use your own public keys):
ssh-rsa AAAAB3NzaC1yc2EAAAADAQABAAABAQDSkT3A1j89RT/540ghIMHXIVwNlAEM3WtmqVG7YN/wYwtsJ8iCszg4/lXQsfLFxYmEVe8L9atgtMGCi5QdYPl4X/c+5YxFfm88Yjfx+2xEgUdOr864eaI22yaNMQ0AlyilmK+PcSyxKP4dzkf6B5Nsw8lhfB5n9F5md6GHLLjOGuBbHYlesKJKnt2cMzzS90BdRk73qW6wJ+MCUWo+cyBFZVGOzrjJGEcHewOCbVs+IJWBFSi6w1enbKGc+RY9KrnzeDKWWqzYnNofiHGVFAuMxrmZOasqlTIKiC2UK3RmLxZicWiQmPnpnjJRo7pL0oYM9r/sIWzD6i2S9szDy6aZ mike@laptop1 ssh-rsa AAAAB3NzaC1yc2EAAAADAQABAAABAQCzlL9Wo8ywEFXSvMJ8FYmxP6HHHMDTyYAWwM3AOtsc96DcYVQIJ5VsydZf5/4NWuq55MqnzdnGB2IfjQvOrW4JEn0cI5UFTvAG4PkfYZb00Hbvwho8JsSAwChvWU6IuhgiiUBofKSMMifKg+pEJ0dLjks2GUcfxeBwbNnAgxsBvY6BCXRfezIddPlqyfWfnftqnafIFvuiRFB1DeeBr24kik/550MaieQpJ848+MgIeVCjko4NPPLssJ/1jhGEHOTlGJpWKGDqQK+QBaOQZh7JB7ehTK+pwIFHbUaeAkr66iVYJuC05iA7ot9FZX8XGkxgmhlnaFHNf0l8ynosanqt henry@laptop2
The following command will retrieve the public key from a private key:
ssh-keygen -y -f /path/to/your_private_key_file (eg. /root/.ssh/id_rsa or ~/.ssh/custom_key_name)
This can be useful, for example, if your server provider generated your SSH key for you and you were only able to download the private key portion of the key pair.
Note that you cannot retrieve the private key if you only have the public key.
The instructions in this article will create your server's .ssh directory and .ssh/authorized_keys file with the correct permissions. However, if you've created them yourself and need to fix permissions, you can run the following commands on your server while SSH'd in as your app's system user.
chmod 700 ~/.ssh chmod 600 ~/.ssh/authorized_keys
NOTE: When changing anything about the way SSH is accessed (ports, authentication methods, et cetera), it is very strongly recommended to leave an active root SSH session open until everything is working as intended. This ensures you have a way to revert changes in the event something goes wrong and logins are not working properly.
As an extra security precaution, once you have set up SSH keys, you may wish to disable password authentication entirely. This will mean no users will be able to log into SSH or SFTP without SSH keys. Anyone entering a password will receive a message like:
Permission denied (publickey,password).
No supported authentication methods available
Disabling password authentication is an excellent way to improve server security. Please see our guide here for the steps to accomplish this goal.
Then, test whether you're able to log in with a password by opening a new SSH or SFTP session to the server. Passwords should not be able to be used and, if everything has been done correctly, an error will be issued when someone tries to use a password. Unless this setting is changed back to allow password authentication, no users will be able to log in without an SSH key set up.