But undocumented system call exists and you could obtain the password for current user of local machine only. There were some old program that control access on MS-DOS computers. Usually, they used weak schemes and the password could be easily decrypted. Internet passwords 4.1. Yes, regardless OS being used.
Most providers require password in plain text.
You need to login to the computer and use any of wide-spread programs. It is possible but such a programs are not known. All these services in standard configuration require plain password from client. V. Strong and weak encryption software 5.1. But the possible weakness of RAR 2.x is it's own proprietary algorithm.