I have what I think is a security IT question (I'm more of a programmer myself).
Assuming I am running a dedicated machine (say a PC) for a single program, meaning that the only port visible and open to the outside is the port this program is listening on.
The program's code has no exploits, meaning no outside packet can cause the program to send information back or change anything on the machine's hard drive.
All communication to and from the machine are encrypted in PGP or AES+RSA in some other fashion, including all the supplementary conditions that are required by said algorithms (length of keys, padding scheme and whatnot).
What possibilities remain for an attacker to decrypt messages transmitted to and from the machine?
I add the single port and no exploits assumptions as I figure that given an ordinary PC runing some operating system with say 40 open ports this gives an attacker the option to infiltrate the machine via said ports and plant a keylogger or something similar.
The idea here is to create a maximal security two person chat system over open networks using for example a dedicated netbook over public wifi.