SSH

To set up passwordless SSH, there are three main things to do:

   * generate a pair of public/private keys on your local computer
   * copy the public key from the source computer to the target computer's authorized_keys file
   * check the permissions. 

You can repeat that transitively for "A->B->C". You can use the initial pair of keys everywhere.

See here for details:

http://www.stearns.org/doc/ssh-techniques.current.html