SSH authentication encryption selection

July 23, 2025

SSH has deprecated the arcfour-hmac encryption type for authentication. Upon logging into a server, the warning may emit:

Warning: encryption type arcfour-hmac used for authentication is deprecated and will be disabled

Check the SSH client configuration per-user file: “$HOME/.ssh/config”. There is also a system-wide SSH client configuration file that can be edited:

• Linux & macOS: /etc/ssh/ssh_config
• Windows: $Env:ProgramData/ssh/ssh_config

We have found this is often a server-side configuration issue, particularly when it only occurs with a specific server. The server may be using an older version of OpenSSH that still supports arcfour-hmac, or it may have been configured to allow this encryption type.

To check settings in the SSH client configuration file, look for lines that specify Ciphers or MACs. If arcfour-hmac is listed, it should be removed.