According to Computer Networking: a top down approach, Chapter 8 (6th edition), block ciphers involves breaking the input message into blocks of size k, where generally k = 64. However since k = 64 implies that we need to create a table of input size 2k or 264 in our case, therefore we simulate randomly permuted tables. To simulate, we use the following process. The function first breaks a 64-bit block into 8 chunks, with each chunk consisting of 8 bits. Each 8-bit chunk is processed by an 8-bit to 8-bit table, which is of manageable size. For example, the first chunk is processed by the table denoted by T 1 . Next, the 8 output chunks are reassembled into a 64-bit block. The positions of the 64 bits in the block are then scrambled (permuted) to produce a 64-bit output. This output is fed back to the 64-bit input, where another cycle begins. After n such cycles, the function provides a 64-bit block of ciphertext.
Here is the question: Why do we need to process the text through n such cycles? The book states that
The purpose of the rounds is to make each input bit affect most (if not all) of the final output bits. (If only one round were used, a given input bit would effect only 8 of the 64 output bits.)
The key for this block cipher algorithm would be the eight permutation tables (assuming the scramble function is publicly known).
*Personal Comment - However, if the input text has gone through even a single cycle still all the inputs would have been affected since each block went through a mapping based on the corresponding table Tj table. Hence we shouldn't be required to cycle it through n cycles.