How to Fix SSHException: Incompatible ssh server (no acceptable ciphers) in Paramiko

SSHException: Incompatible ssh server (no acceptable ciphers) means cipher mismatch. Paramiko and the server don’t share ciphers. This tutorial explains how to solve it.

Understanding Cipher Mismatches

Ciphers encrypt communication between client and server. If no common cipher exists, the connection fails. This is a security feature.

Common Causes

Outdated server or client software is a main cause. Server misconfiguration can also cause this. This is more common with older servers.

Handling the SSHException

Use try-except blocks to catch the SSHException. This prevents your program from crashing. It allows for proper error handling.

import paramiko

try:
    ssh = paramiko.SSHClient()
    ssh.set_missing_host_key_policy(paramiko.AutoAddPolicy())
    ssh.connect("your_hostname", username="your_username", password="your_password")
    # ... your SSH operations
    ssh.close()
except paramiko.ssh_exception.SSHException as e:
    print(f"SSH Exception: {e}")
except Exception as e:
    print(f"Other error: {e}")

Specifying Ciphers in Paramiko (Less Secure)

You can specify ciphers in Paramiko (less secure). This forces Paramiko to use specific ciphers. Use this cautiously and only if necessary.

import paramiko

try:
    ssh = paramiko.SSHClient()
    ssh.set_missing_host_key_policy(paramiko.AutoAddPolicy())

    # Example (less secure, use with extreme caution):
    ssh.connect("your_hostname", username="your_username", password="your_password", ciphers=['aes128-cbc'])

    ssh.close()
except paramiko.ssh_exception.SSHException as e:
    print(f"SSH Exception: {e}")
except Exception as e:
    print(f"Other error: {e}")

Note: Using specific ciphers can weaken security. Only use this as a last resort. Prioritize updating server configurations.

Updating Server SSH Configuration (Recommended)

Updating the server’s SSH configuration is recommended. This allows the server to use modern ciphers. This is the most secure solution.

Edit the server’s sshd_config file. Add or modify the Ciphers line. Restart the SSH service after changes.

Example sshd_config (more secure):

Ciphers chacha20-poly1305@openssh.com,aes256-gcm@openssh.com,aes128-gcm@openssh.com,aes256-ctr,aes192-ctr,aes128-ctr

Updating Paramiko (If Possible)

Ensure you are using a recent Paramiko version. Newer versions support more ciphers. Updating can resolve compatibility issues.

By updating the server’s SSH configuration, you can resolve cipher mismatch issues securely. This ensures proper communication between Paramiko and the server. This is the best approach for long-term solutions.