Original: In cryptography , a Feistel cipher is a symmetric structure used in the construction of block ciphers , named after the German-born physicist and cryptographer Horst Feistel who did pioneering research while working for IBM ( USA ) ; it is also commonly known as a Feistel network .
Simplified: One advantage of this model is that the round function { rm F } does not have to be invertible , and can be very complex .