I want to implement a pipelined architecture of AES128; and I couldn't figure out the number of the pipline stages. As I undestand, it is possible to make the piepline stages inside the round, or outside it or both. What I don't understand is if the I chose to make them inside a round, why the number of stages must be a divisor of number of rounds? I understand that it must be a divisor if the pipelined stages are between the rounds. Correct me if my understanding is wrong.
I think that since there is 4 operations inside every round then it is possible to make 4 stages? but what about the last round?
Here what I have read: A traditional methodology for design of high-performance implementations of secret-key block ciphers, operating in non-feedback cipher modes is shown in Fig. 21. The basic iterative architecture, shown in Fig. 21a is implemented first, and its speed and area determined. Based on these estimations, the number of rounds K that can be unrolled without exceeding the available circuit area is found. The number of unrolled rounds, K, must be a divisor of the total number of cipher rounds, #rounds. If the available circuit area is not large enough to fit all cipher rounds, architecture with partial outer-round pipelining, shown in Fig. 21b, is applied.