The internal structure of SHA-1 is shown in the figure below:
As we can see from the figures, the hash calculation can be partitioned into two parts.
- The pre-processing part pads or splits the input message which is comprised by a stream of 32-bit words into fixed sized blocks (512-bit for each).
- The digest part iteratively computes the hash values. Loop-carried dependency is enforced by the algorithm itself, thus this part cannot reach an initiation interval (II) = 1.
As the two parts can work independently, they are designed into parallel dataflow process, connected by streams (FIFOs).