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: Decryption of a ciphertext ( R_n , L_n ) is accomplished by computing for i = n,n-1 , ldots ,1