from the documentation of Rex::Commands :

key_auth

If you want to use pubkey authentication, then you need to call key_au
+th.

 user "bob";
 private_key "/home/bob/.ssh/id_rsa"; # passphrase-less key
 public_key "/home/bob/.ssh/id_rsa.pub";

 key_auth;
[download]

perhaps you need this then?:

group "servers", "server_big" => { user => 'my_user', private_key => '
+/home/user/key.pem', key_auth => 1 };
[download]

or something to that effect.

bw, bliako