Laboratory 4 (team) -- Hardware encryptor/decryptor

Worth: 20 points
Assigned: 30 January 2015
Due: 6 February 2015


In this lab, your team will design and implement a digital circuit to encrypt and decrypt a text message. You will build a datapath to perform the operations that encrypt/decrypt the message and a finite-state machine to control this datapath.

Max circuit

Before designing your circuit, make sure you understand the max circuit covered in lecture. Here is a schematic drawing of the datapath for the max circuit. The following are the Verilog files for that circuit; you may use any of them in your circuit:


The encryption algorithm you will implement is known as rot13. While rot13 is not strong enough for commercial use, it is useful for obscuring simple text.

rot13 translates each character of a message by "rotating" it 13 places in the alphabet. Thus, the letter a becomes n, b becomes o, c becomes p, and so on. Toward the end of the alphabet, the letters "wrap around" back to the beginning, so n becomes a, o becomes b, and so on. For example, the message "hello" would be encrypted as "uryyb". Note that you can reverse a rot13 transformation by re-applying rot13, so rot13 can be used to decrypt a message as well as to encrypt it.

Encryptor/decryptor circuit

You are to build a circuit that transforms a message using rot13. Assume the message is composed solely of lower-case letters, represented by their standard ASCII values (e.g., "a" is represented by 97, "b" by 98, and so on). The message is stored as an array in memory, starting at address 0. The end of the message is terminated by an entry with the value 0 (this is the same convention used by C to store strings).

When your circuit starts, it should transform all characters in memory, starting from address 0 and proceeding up to (but not including) the termination entry. Upon reset, your circuit should start back at address 0 and transform the message in memory again. Use ~KEY[0] to change the reset signal, just as in Lab 3.

Your circuit should be structured similarly to the max circuit. top.v should contain your module top, and control.v should contain your control unit, implemented as a finite-state machine. You will need other modules to complete your datapath; each of these modules should be contained in a separate file. Your circuit should display the current value of the 8-bit bus on HEX1 and HEX0.

Pre-lab assignment

Your team's pre-lab assignment is to design a datapath and control unit for the rot13 circuit and to write the Verilog for all components in your circuit (including the control unit). You should meet as a team at least once or twice before the lab to design a solution and review each other's parts. You should arrive at your lab section with a complete solution that you believe should work. The lab time is meant for testing, debugging, and demonstrating your circuit.

Here are some common errors to look out for:

In-lab demonstration

Demonstrate your hardware encryptor/decryptor to a lab instructor. Use Quartus' In-System Memory Content Editor to write a message to memory and verify the results of the transformation, then verify that the transformation is reversed when you reset the circuit and re-process the message.

To write data to memory with the memory editor, edit the memory contents shown on the main window, then right-click on this window and select Write Data to In-System Memory (screenshot). When downloading data to FPGA memory, you should stop the the circuit to prevent it from also accesssing FPGA memory. To do this, make sure the reset signal is on before downloading data to FPGA memory and leave the reset signal on until the download is done. Resetting the circuit in this way will also clear all registers (at the next positive clock edge), which will have the desired effect of re-starting the encryption/decryption algorithm at address 0.

After you've demonstrated your circuit to a lab instructor, one member of your team should submit the final version of the Verilog files used in your datapath. You need not submit Verilog files that are identical to the files in the max circuit.