Information Theory

Developed by Claude Shannon, information theory is mathematical theory that deals with the content, transmission, storage, and retrieval of information. Many concepts the relate to cryptography stem from information theory. Among these concepts are unconditional security, one-time pads, quantum key distribution, and computational security.

Unconditional security refers to a cryptographic system that cannot be broken. If somethings is unconditionally secure, then cryptanalysis would be impossible to perform and even an extensive brute force attack would not be able to determine which key is correct. The one-time pad is an example of a cryptosystem that is unconditionally secure. This system uses blocks of random data called pads. The pad has to be at least the length of the message and the data on the pad has to be truly random. When this is used, two pads are made. One pad is for the sender and the other is for the receiver. When the message is encrypted using the pad, the message is sent and that pad is destroyed to ensure it can only be used once. When the recipient receives the message, the second pad helps decipher the message and displays it for the recipient to view it.

A system being computationally secure means that the best algorithm for cracking it requires an unreasonable amount of resources and time. So technically it is possible to crack it, but it is not very feasible to attempt it. Most modern cryptosystems belong to this category and would take tens of thousands of years to crack, even with extensive computing resources. The security of cryptosystem is always measured using the current best-known algorithm.


Reference: http://dictionary.reference.com/browse/information+theory