A cryptographic system for encrypting a block of binary data under the control of a key consisting of a set of binary symbols. The cryptographic system is utilized within a data processing environment to ensure complete privacy of data and information that is stored or processed within a computing system. All authorized subscribers who are permitted access to data within the network are assigned a unique key consisting of a combination of binary symbols. The central processing unit within the computing network contains a complete listing of all distributed authorized subscriber keys. All communications transmitted from terminal input are encrypted into a block cipher by use of the cryptographic system operating under the control of the subscriber key which is inputed to the terminal device. At the receiving station or central processing unit, an identical subscriber key which is obtained from internal tables stored within the computing system is used to decipher all received ciphered communications.The cryptographic system develops a product cipher which is a combination of linear and nonlinear transformations of the clear message, the transformation being a function of the binary values that appear in the subscriber key. In addition to the transformation, the key controls various register substitutions and modulo-2 additions of partially ciphered data within the cryptographic system.