This is the corresponding client mod for use with the AuthMod Bukkit plugin. In conjunction with the Bukkit plugin, this allows users (particularly staff members) to authenticate themselves with an RSA certificate.
Install it as you would any other mod. It modifies NetClientHandler and Packet250CustomPayload, and adds two other classes.
Acquire an AuthMod key from your server owner, and put it directly in your .minecraft folder as "authmod.key".
You should be able to connect to the server as usual. If you want to connect from a different computer, you'll have to install AuthMod and copy your authmod.key to it as well.