1. A device for ciphering a message block of data bits under control of a set of cipher key bits by a predetermined number of iteration operations, said cipher device comprising: first store means storing a first half of said message block of data bits, second store means storing a second half of said message block of data bits, control means including third store means storing said set of cipher key bits, and first linear transformation means connected to said third store means producing a permuted set of cipher key bits from said set of cipher key bits, expansion means connected to said first store means duplicating predetermined ones of the data bits of the first half of said message block to produce an expanded first half of said message block containing data bits equal in number to the number of cipher key bits in said permuted set of cipher key bits, means connected to said expansion means and said control means carrying out a substitution transformation function in accordance with the data bits of said expanded first half of said message block and the cipher key bits of said permuted set of cipher key bits to produce a substitution set of bits equal in number to the number of bits in the first half of said message block, second linear transformation means connected to said substitution transformation means producing a permuted substitution set of bits, the combined transformation performed by said substitution transformation means and said second linear transformation means resulting in a product block cipher of the first half of said message block, means connected to said second store means and said second linear transformation means modifying the data bits of the second half of said message block in accordance with the product block cipher of the first half of said message block to produce a set of bits representing a modified second half of said message block, means connected between said modifying means and said first store means to load said modified second half of said message block from said modifying means into said first store means, and means connected between said first store means and said second store means to load the first half of said message block from said first store means into said second store means concurrently with said modified second half of said message block being loaded into said first store means to complete a first iteration operation of said cipher device.