Fixing "Permission denied» with OpenSSH: Manually adding authorized_keys

To establish an SSH connection between your PC and Raspberry Pi 4 running Ubuntu, you may encounter a "Permission denied (publickey)" error when using client-side public key authentication. This article will guide you on how to resolve this issue by manually adding the authorized_keys file on your Raspberry Pi.

Prerequisites:

1. SSH client installed on your PC (e.g., PuTTY, OpenSSH)
2. SSH server installed and running on your Raspberry Pi (SSH is enabled by default on Ubuntu)

Step 1: Retrieve the client's public key

On your PC, locate the public key file (id_rsa.pub or id_ed25519.pub) in the .ssh directory of your user account.

$ cd ~/.ssh
$ cat id_rsa.pub

Step 2: Connect to your Raspberry Pi via SSH

Connect to your Raspberry Pi using SSH. If you haven't set up password authentication, you'll need to use a key-based authentication method.

$ ssh pi@raspberrypi

Replace pi with your Raspberry Pi's default username (ubuntu for Ubuntu) and raspberrypi with your Raspberry Pi's IP address.

Step 3: Create the authorized_keys file

Once connected, create the authorized_keys file in the .ssh directory on your Raspberry Pi.

$ sudo mkdir -p ~/.ssh
$ sudo touch ~/.ssh/authorized_keys
$ sudo chmod 600 ~/.ssh/authorized_keys

Step 4: Add the public key to the authorized_keys file

Copy the public key content from your PC and paste it into the authorized_keys file on your Raspberry Pi.

$ cat id_rsa.pub | sudo tee -a ~/.ssh/authorized_keys

Step 5: Test the SSH connection

Exit the SSH session on your Raspberry Pi and try to connect again from your PC. You should now be able to log in without encountering the "Permission denied (publickey)" error.

$ exit
$ ssh pi@raspberrypi

Summary:

1. Retrieve the client's public key.
2. Connect to your Raspberry Pi via SSH.
3. Create the authorized_keys file and set its permissions.
4. Add the public key to the authorized_keys file.
5. Test the SSH connection.

References:

1. SSH Authorized Keys
2. How to Copy the SSH Public Key to a Remote Server
3. Managing SSH Keys