After examining your configuration, the key issue lies in the filesystem structure. The error message explicitly states that /home/myuser/.ssh/authorized_keys is not a regular file - because in your setup, it's actually a directory.

Your current directory structure shows:

drwx------ 3 myuser myuser 4096 Aug 17 13:24 /home/myuser/.ssh
drwx------ 2 myuser myuser 4.0K Aug 17 13:23 /home/myuser/.ssh/authorized_keys
-rw------- 1 myuser myuser  396 Aug 17 13:17 /home/myuser/.ssh/authorized_keys/id_rsa.pub

This is incorrect. The authorized_keys should be a regular file, not a directory containing id_rsa.pub.

Here's how to fix it:

# First, remove the problematic directory
rm -rf ~/.ssh/authorized_keys

# Then recreate it as a file and set proper permissions
touch ~/.ssh/authorized_keys
chmod 600 ~/.ssh/authorized_keys

# Now add your public key directly to the file
cat ~/.ssh/id_rsa.pub >> ~/.ssh/authorized_keys

After fixing the file structure, verify all permissions are correct:

chmod 700 ~/.ssh
chmod 600 ~/.ssh/authorized_keys
chmod 600 ~/.ssh/id_rsa
chmod 644 ~/.ssh/id_rsa.pub

For future troubleshooting, you can use these commands:

# Server-side debugging
sudo tail -f /var/log/auth.log

# Client-side debugging (in PuTTY)
Event Log > SSH > Auth > Public key

If you need to convert keys between formats:

# Convert OpenSSH to PuTTY format
puttygen id_rsa -o id_rsa.ppk

# Convert PuTTY to OpenSSH format
puttygen id_rsa.ppk -O private-openssh -o id_rsa

For production environments, consider these additional measures:

# Disable password authentication
sudo nano /etc/ssh/sshd_config

Add/modify these lines:

PasswordAuthentication no
ChallengeResponseAuthentication no
UsePAM no

Then restart SSH:

sudo systemctl restart sshd

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When setting up SSH key authentication between Windows (PuTTY) and Debian, the critical error message authorized_keys is not a regular file indicates a filesystem permission/type problem. The SSH daemon expects ~/.ssh/authorized_keys to be a regular file, but in this case it's being treated as a directory.

The original poster's directory listing reveals the root cause:

drwx------ 3 myuser myuser 4096 Aug 17 13:24 /home/myuser/.ssh
drw------- 2 myuser myuser 4.0K Aug 17 13:23 /home/myuser/.ssh/authorized_keys
-rw------- 1 myuser myuser  396 Aug 17 13:17 /home/myuser/.ssh/authorized_keys/id_rsa.pub

Notice the directory flag (d) on authorized_keys. The public key is actually stored inside this directory as id_rsa.pub, which is incorrect.

The proper structure should be:

drwx------ 2 myuser myuser 4096 Aug 17 13:24 /home/myuser/.ssh
-rw------- 1 myuser myuser  396 Aug 17 13:17 /home/myuser/.ssh/authorized_keys

Here's how to correct the issue:

# Remove the incorrect directory
rm -rf ~/.ssh/authorized_keys

# Create the proper file and set permissions
touch ~/.ssh/authorized_keys
chmod 600 ~/.ssh/authorized_keys

# Add the public key (single line!)
cat ~/.ssh/id_rsa.pub >> ~/.ssh/authorized_keys

After making these changes:

1. Ensure the file exists: ls -la ~/.ssh/authorized_keys
2. Verify the content: cat ~/.ssh/authorized_keys
3. Check permissions: stat ~/.ssh/authorized_keys

For enterprise environments, consider these additional measures:

# Set strict home directory permissions
chmod 750 ~/
chmod 700 ~/.ssh

# Optional: Make authorized_keys immutable
chattr +i ~/.ssh/authorized_keys

When testing, run the SSH daemon in debug mode:

sudo /usr/sbin/sshd -d -p 2222

Then connect from PuTTY while watching server logs:

tail -f /var/log/auth.log

For Windows clients:

• Ensure Pageant is running with the correct .ppk key
• In PuTTY Configuration:
  • Connection > SSH > Auth: Enable "Allow agent forwarding"
  • Connection > Data: Set auto-login username

Watch out for:

• Windows line endings in authorized_keys
• Incorrect SELinux contexts (check with ls -Z)
• Missing newline at end of authorized_keys file
• Overly restrictive parent directory permissions