When setting up passwordless SSH between Cygwin (Windows) and Ubuntu, several technical nuances can break the authentication flow. Let's examine the critical components:

# Verify key permissions on Ubuntu server (critical!)
chmod 700 ~/.ssh
chmod 600 ~/.ssh/authorized_keys

While using IP addresses should work, hostname resolution issues sometimes interfere. Test both approaches:

ssh -v username@hostname.local
ssh -v username@192.168.1.101

The Windows environment adds unique requirements:

# Ensure proper home directory detection in Cygwin
export HOME=/cygdrive/c/Users/yourname
ssh-keygen -t rsa -b 4096 -C "cygwin-key"

The -vvv flag reveals authentication workflow details:

ssh -vvv username@192.168.1.101
# Look for these key messages:
# Offering public key: /home/user/.ssh/id_rsa
# Server accepts key: pkalg ssh-rsa
# Authentication succeeded (publickey)

Verify these Ubuntu SSH server settings in /etc/ssh/sshd_config:

PubkeyAuthentication yes
AuthorizedKeysFile .ssh/authorized_keys
PasswordAuthentication no  # For testing only

Windows line endings can corrupt keys. Convert using:

# On Cygwin:
dos2unix ~/.ssh/id_rsa.pub
# On Ubuntu after transfer:
sed -i 's/\r$//' ~/.ssh/authorized_keys

Instead of SCP, use ssh-copy-id for proper handling:

# From Cygwin:
ssh-copy-id -i ~/.ssh/id_rsa.pub username@192.168.1.101

Check these often-overlooked barriers:

# On Ubuntu:
sudo ufw allow ssh
sudo ausearch -m avc -ts recent  # For SELinux denials

When setting up SSH key authentication between Cygwin (Windows) and Ubuntu, several technical nuances often cause failures:

# Verify key permissions on Ubuntu server
chmod 700 ~/.ssh
chmod 600 ~/.ssh/authorized_keys

Cygwin requires special handling of SSH keys due to Windows filesystem permissions:

# Convert line endings if keys were edited in Windows
dos2unix ~/.ssh/authorized_keys

# Ensure proper ownership in Cygwin
chown $USER:$USER /home/$USER/.ssh -R

Enable verbose logging to identify the exact failure point:

ssh -vvv username@192.168.1.101

# Common output to look for:
# debug1: Offering public key: /home/user/.ssh/id_rsa RSA SHA256:...
# debug1: Authentications that can continue: publickey,password

Verify these critical settings in /etc/ssh/sshd_config:

PubkeyAuthentication yes
AuthorizedKeysFile .ssh/authorized_keys
PasswordAuthentication no  # Only after confirming key works

While IP addresses should work, hostnames provide better reliability:

# Add to ~/.ssh/config on Windows:
Host myserver
    HostName 192.168.1.101
    User username
    IdentityFile ~/.ssh/id_rsa

For modern systems, consider using ed25519 keys instead of RSA:

ssh-keygen -t ed25519 -C "cygwin@windows"