Fixing Mm_answer_keyallowed Publickey Authentication Error with Ed25519 Key

If you encounter the error stating that an Ed25519 cryptographic method is disallowed when attempting to connect via SSH, the root cause is usually an incorrect configuration in the server’s SSH settings. Specifically, this often arises when the server does not permit certain algorithms for securing remote access. To resolve the issue, modify the sshd_config file located in /etc/ssh/sshd_config to allow the Ed25519 method.

Locate the HostKeyAlgorithms directive in the configuration file and ensure it includes the Ed25519 algorithm. If the line is missing or restricted, add or adjust it as follows:

HostKeyAlgorithms +ssh-ed25519

Next, restart the SSH service for changes to take effect:

sudo systemctl restart sshd

Once these modifications are made, try reconnecting to the server. This should resolve the issue and enable secure communication using Ed25519.

Common Causes of the “Ed25519 Key Not Allowed” Error in SSH

Ensure the server’s configuration permits the specific type of credentials being used. Often, this issue arises when the SSH daemon (sshd) does not have the proper settings in its configuration files to allow the method of identification you’re attempting to use.

Check if the sshd_config file includes PubkeyAcceptedAlgorithms or similar directives. If these parameters are misconfigured or absent, the system will reject certain signature algorithms. Update the configuration and restart the SSH service for changes to take effect.

Inspect the permissions on your authentication files. On the server, verify that both the ~/.ssh/authorized_keys file and the directory have correct file permissions. If this file is accessible by others, SSH will block access to these credentials.

Ensure the client and server are using compatible cryptographic algorithms. Some systems may not yet support modern encryption methods, leading to incompatibility errors. Check both client and server configurations for supported algorithms and update them if necessary.

If you’re using an outdated or incompatible SSH client, this may also trigger the error. Update your client to the latest version or switch to a client known to support the desired authentication methods.

Also, double-check that the correct public part of the credentials is placed in the server’s authorized_keys file. If the credentials are mismatched or corrupted, the server won’t be able to verify them, causing access failures.

How to Check If Ed25519 Authentication Is Supported on Your Server

To verify if Ed25519-based login is enabled, connect to your server and inspect the SSH configuration file. Run the following command to open the config:

sudo nano /etc/ssh/sshd_config

Look for the “HostKey” directive. If Ed25519 is listed here, it’s supported. Example:

HostKey /etc/ssh/ssh_host_ed25519_key

If the entry is missing, you’ll need to add the appropriate line or generate a new Ed25519 host key:

sudo ssh-keygen -t ed25519 -f /etc/ssh/ssh_host_ed25519_key

Next, confirm the SSH server version. Version 7.2 or higher typically supports Ed25519 keys. Use this command to check the version:

ssh -V

If the version is outdated, update it using your package manager:

sudo apt update && sudo apt upgrade openssh-server

Once updated, restart the SSH service to apply changes:

sudo systemctl restart ssh

Finally, attempt to log in with an Ed25519 private key. If successful, the server is configured properly. If not, verify that the client is configured with the correct private key and that there are no restrictions in the server’s config file blocking its use.

Configuring SSH to Accept Ed25519 for Secure Access

Edit the SSH configuration file located at /etc/ssh/sshd_config using a text editor like nano or vim:

sudo nano /etc/ssh/sshd_config

Locate and configure the following directive to enable the desired encryption method:

HostKey /etc/ssh/ssh_host_ed25519_key

If the file ssh_host_ed25519_key is missing, generate it using this command:

sudo ssh-keygen -t ed25519 -f /etc/ssh/ssh_host_ed25519_key

Next, confirm the PubkeyAcceptedAlgorithms directive exists in the file. If absent, add the line below to permit this algorithm:

PubkeyAcceptedAlgorithms +ssh-ed25519

Apply the changes by restarting the SSH service:

sudo systemctl restart sshd

To verify that the configuration is correct, attempt to connect from a client using the corresponding public encryption method.

If issues arise, review the system logs for additional information:

sudo journalctl -u sshd

Fixing Permissions and Configuration Issues for Ed25519 Keys

Ensure that the private file has proper permissions set. A common issue arises when the private file is too accessible, making it insecure. Set the permissions as follows:

If the file permissions are correct, check the server configuration file. Verify the settings in /etc/ssh/sshd_config. Ensure the line allowing the specified authentication method is uncommented and correctly configured:

Afterward, restart the SSH service for changes to take effect:

If the issue persists, verify the public part of the pair is correctly placed in the ~/.ssh/authorized_keys file on the server, ensuring no syntax errors are present. The file should have proper permissions as well:

Finally, check that the sshd_config file on the server doesn’t have any restrictive options like AuthorizedKeysFile pointing to an incorrect location, which could prevent proper access.

Diagnosing Key Mismatch Issues: Ed25519 vs RSA

Verify the algorithm compatibility between client and server. Ed25519 and RSA use different formats, and a mismatch will prevent proper verification. Ensure the server’s configuration supports both algorithms or the one you’re using.

• Check the server’s sshd_config file for accepted signature types, typically listed under PubkeyAcceptedAlgorithms or HostkeyAlgorithms. If Ed25519 is absent, add it or switch to an accepted format like RSA.
• Examine your SSH client’s configuration. It may need manual specification of the preferred format, such as using -oPubkeyAcceptedAlgorithms=+ssh-ed25519 for Ed25519 or -oPubkeyAcceptedAlgorithms=+ssh-rsa for RSA.

If switching between formats, regenerate the corresponding pair if necessary. RSA keys might need an upgrade to the newer 3072-bit or 4096-bit sizes due to security recommendations.

• Use ssh-keygen -t rsa -b 4096 for RSA keys of sufficient size.
• For Ed25519, use ssh-keygen -t ed25519 to ensure compatibility with modern clients and servers.

Test the connection with verbose output to diagnose any issues. Run ssh -v and review the logs for specific errors regarding key format issues. Look for messages such as key type not supported or key mismatch.