Theoretical computer science
 Information theory talks about the notion of information. Originally it was developed for signal processing. It is concerned with how a message needs to be changed, so that it can be transmitted over a  channel, an how to undo the changes at the other end, so that the original message can be recovered. These processes are known as encoding and decoding. Information theory also talks about how much self-information there is in a message. This is the theoretical basis for data compression, cryptography and digital signatures.