Method and system for secure communication between a public network and a local network

A telecommunication system comprising:

• a local area network and a wide area network suitable to exchange with each other data packets within respective communication sessions;
• a processing apparatus interposed between the local area network and the wide area network and configured to read a destination port value of packets directed from the wide area network to the local area network, the processing apparatus comprising:
• a network interface card for communicating with the local area network;
• a wide area network interface, for communicating with the wide area network;
• a central processing unit;
• a memory;
• and
• a session database associated with the processing apparatus for keeping a trace of said sessions, said sessions being identified in the session database by means of a session identifier;
• the processing apparatus configured to:
• open a communication session between the local area network and the wide area network;
• keep a trace of said session in a session database where said session is associated with a respective session identifier;
• send from the local area network to the wide area network at least one data packet having a source port value calculated as the sum of a fixed value and said session identifier;
• send from the wide area network to the local area network at least one data packet having a destination port value indicative of the session identifier;
• determine if said destination port value is higher than said fixed value;
• when said destination port value is higher than said fixed value:
• extract the session identifier from said destination port value by subtracting said fixed value from the destination port value, and
• use the extracted session identifier for directly accessing said session in the session database;
• and
• when said destination port value is lower than said fixed value:
• discard the data packet.

A network address translator configured to;

• change, by a processor, a source port of a first data packet transmitted within a communication session from a first source port value to a second source port value indicative of an identifier of said session, wherein the second source port value is the sum of a fixed value and said session identifier when the first data packet is being transmitted from a local area network to a wide area network;
• receive a second data packet, the second data packet being transmitted from the wide area network to the local area network, wherein the second data packet has a destination port value indicative of the session identifier;
• determining if said destination port value is higher than said fixed value;
• when said destination port value is higher than said fixed value:
• change the destination port value of the second data packet from the second source port value to the first source port value;
• and
• when said destination port value is lower than said fixed value, discard the second data packet.