====== Codebreaker ======
In this lab you will be given a secret message that has been encrypted with a secret key. You will create a circuit that tries every possible key until you find out which key successfully decrypts the message.
Your design will start searching for the key after the center button is pressed. The current key being tried is displayed on the LEDs, and your stopwatch from a previous lab is used to time how long it takes for you to find the correct key. Once you have discovered the secret message, you will display this secret message on the VGA monitor.
{{youtube>IgUdRXjBiwE?rel=0&noborder&560x315}}
The progression of the exercises is:
* Exercise 1: Set up your project, and draw a message of your choosing on the screen.
* Exercise 2 and 3: You will be given an encrypted message, and the decryption key. You will decrypt the message, and display the decrypted message on the screen.
* Exercise 4: You will be given an encrypted message, but not the decryption key. You will search through all keys to find the key that properly decrypts a given message, and then display the decoded message on the screen.
===== Learning Outcomes =====
* Implement a state machine using behavioral SystemVerilog
* Build a system in SystemVerilog that contains several interacting modules.
===== Preliminary =====
==== ASCII Character Values ====
In computer systems, we typically use an 8-bit integer value to represent different characters, including uppercase (A, B, C, ...) and lowercase (a, b, c, ...) alphabet characters, digits (0, 1, 2, ...), symbols ($, %, ^, ...) and special characters (space, tab, new line). Online you can easily find an [[http://www.asciitable.com/|ASCII Table]] that lists the integer values we use for each of these characters. Almost all computers and electronics you will encounter use these same standardized ASCII values. When you consult this table, be mindful that the values are typically listed in both decimal and hexadecimal, so be sure you are looking at the correct column!
*What is the hexadecimal ASCII value for character ''D''?
*What is the hexadecimal ASCII value for character ''d''?
*What character is represented by the ASCII value ''0x09''? (Enter it in LearningSuite as it appears in the linked ASCII table.)
==== Encryption ====
This lab is all about **encryption** and **decryption**, but it may be the first time you've learned about these things. At the most basic level, an encryption process takes two inputs: a message (we call **plaintext**) and a secret key (we call the **key**). It produces an encrypted message, called the **cyphertext**. Decryption is the reverse process. It takes the cyphertext and the same original key, and produces the original plaintext.
In this lab we will be using the [[https://en.wikipedia.org/wiki/RC4|RC4]] encryption algorithm. It's an old encryption algorithm, and don't worry, you don't need to understand how it works. What's important to know for our system is that we are going to use __messages that are 16 bytes long (128 bits), and a 24 bit key__.
* How many different key values can our system have?
As an example, we may have a 16 byte message, "WE LOVE ECEN 220", which we can assign to ''logic'' in SystemVerilog like this:
logic [127:0] plaintext;
assign plaintext = {8'h57, 8'h45, 8'h20, 8'h4C, 8'h4F, 8'h56, 8'h45, 8'h20, 8'h45, 8'h43, 8'h45, 8'h4E, 8'h20, 8'h32, 8'h32, 8'h30};
// The above is split up to show the individual bytes, we could also store it like this:
assign plaintext = 128'h5745204C4F5645204543454E20323230;
// Or like this:
assign plaintext = {"W", "E", " ", "L", "O", "V", "E", " ", "E", "C", "E", "N", " ", "2", "2", "0"};
// Or like this:
assign plaintext = {"WE LOVE ECEN 220"};
// All of the above assign the plaintext to an identical value
If we encrypt this message using the following key:
logic [23:0] key;
assign key = 24'h010203;
The resulting cyphertext will be equal to ''128'h5c057e9fd5458ec36d7ef9cc4d23ea3e''. If you look up these values in the ascii table you will see that the cyphertext is not a readable message. For example, the first byte, ''5c'' is for character ''\'', and some of the bytes (such as ''ea'') aren't even valid ascii character values.
If we decrypt this cyphertext with the same key we will get back the original message. If we use the wrong key, we will get another unreadable message
(If you're interested, you can try this out on [[https://cryptii.com/pipes/rc4-encryption]]. Just keep in mind you need to reverse the bytes of the key. If you choose ''key = 24'h010203;'', then enter ''03 02 01'' for the key on the website).
**In this lab you will be given the cyphertext, and then you need to try all of the keys until you find one that produces a readable message. Then you will have hacked the system and figured out the key!**
You might be worried that you will find a key that produces a readable message, but is not the correct key. Don't worry, the odds of this are extremely unlikely (and we've double checked that this won't happen with the encrypted messages we give you).
==== Modules ====
Once you've figured out the key and decrypted the secret message you will want to display it. Unfortunately there's no easy way to display messages like this on the board, so you are given some modules that can be used to display text on the monitor using the VGA connection.
You can download the following modules and add them to your Vivado project:
* The [[resources:rc4|decrypt_rc4 module]]. This module performs the decryption. You will provide a 128-bit message, a 24-bit key, and a start signal. It will takes some time to perform the decryption, and then it will raise its ''done'' output, and provide the decrypted message.
* The [[resources:vga_drawer|BitmapToVga module]]. This module draws pixels to the VGA monitor. You won't have to interact with this module directly in this lab.
* The BitmapToVga module has an internal bitmap image that it outputs it to the VGA monitor. How many pixels wide is the bitmap? How many pixels tall?
* In what corner of the screen is the origin (0,0)?
* The [[resources:char_drawer|CharDrawer]] module. You provide this module with a 128-bit ASCII message, a starting (x, y) coordinate, and a ''start'' signal. It will output (x,y) pixel values that when connected to the ''BitmapToVga'' module, will draw the message on the screen. You will be given a top-level that connects these two modules together, so you only need to worry about sending the correct inputs to the ''CharDrawer'' module.
/* For example, suppose we randomly choose some cyphertext in the hopes of decrypting it into a valid message. Since each byte has 37 valid characters, out of a possible 256, the odds that a specific key value will produce a valid 16-byte message is (37/256)16 = 3.63e-14. If we try all possible values for the 24-bit key, the odds become 224⋅(37/256)16, which is about 1 in 1.6 million. As you see, even for our small message and key size, it is very unlikely that we can find a valid message by luck. In the last exercise of the lab you will be given some cyphertext that //will// decrypt into a valid message, you just need to find the correct key.
*/
=====Exercises=====
==== Exercise #1 - Draw a message on the screen ====
The full system for this lab is shown below. Almost everything you design will be contained in the ''Codebreaker'' module that you will implement shortly. The top-level module, shown in the diagram below, is provided to you and you shouldn't need to modify it (except perhaps for your personal exploration).
{{:labs:codebreaker:codebreaker_diagram.png?1200|}}
=== Top-Level Module ===
The top-level module is ''codebreaker_top''. Download it here: {{ :labs:codebreaker:codebreaker_top.sv |}}
Look over the top-level module, and make sure you understand how it works. The module contains:
* A ''clk_generator'' instance, that generates a 25MHz clock needed by the VGA display.
* A ''BitmapToVga'' instance, that controls the VGA outputs, and has inputs that allow you to modify the pixel colors of the bitmap that is displayed over VGA.
* A ''CharDrawer'' instance, that is connected to the ''BitmapToVga'', that is used to draw messages to the bitmap, and thus the VGA display.
* ''SevenSegmentControl'' and ''stopwatch'' instances, that are configured like the Stopwatch lab.
* The ''Codebreaker'' instance.
**Complete each of the following steps**:
- [[tutorials:vivado_project_setup|Create a Vivado project]] (remember to run the commands to configure the error messages)
- Add the top-level module to your project.
- Create an appropriate constraints file for all of the top-level ports.
- Add all other necessary modules to your project. You will need to expand the modules in the Design Sources list to make sure you have included all necessary modules. For this lab, you only need to create the ''Codebreaker'' module (ports listed below). All other modules have been given to you, or were created in previous labs.
- In this first exercise, your Codebreaker module will be very simple:
- Drive the ''key_display'' and ''stopwatch_run'' outputs to 0.
- Drive the ''plaintext_to_draw'' output to a message of your choice to draw on the screen (the ''CharDrawer'' can only draw upper case letters, digits and spaces.
- Connect the ''draw_plaintext'' output to the ''start'' input.
^ Module Name = Codebreaker^^^^
^ Port Name ^ Direction ^ Width ^ Description ^
| clk | Input | 1 | 100 MHz Input Clock |
| reset | Input | 1 | Active-high reset|
| start | Input | 1 | Begin searching for the secret key |
| key_display | Output | 16 | Display portion of current key value being tested |
| stopwatch_run | Output | 1 | Active-high enable of stopwatch |
| draw_plaintext | Output | 1 | Raise this signal to tell the ''CharDrawer'' module to start drawing your message. |
| done_drawing_plaintext | Input | 1 | This input is high once the ''CharDrawer'' module is done drawing your message.|
| plaintext_to_draw | Output | 128 | The ASCII message to send to the ''CharDrawer''|
**Pass-off:** Generate the bitstream and program the board. You can use the **+** button on the monitors in the lab to switch them to the VGA input. Verify that your message is displayed after you press ''btnc''. You don't need to show it to the TAs.
----
==== Exercise #2 - Decrypt and Display a Message (SM design) ====
The next step of this lab is to actually decrypt and display a message. To do this, you will have to instance the ''decrypt_rc4'' module in your ''Codebreaker'' module. Before you start modifying your SystemVerilog, you will design the state machine that interacts with the decryption module.
This exercise is entirely done on paper. Design and draw a finite state machine that:
* Waits for the start button
* Decrypts the cyphertext and obtains the plaintext.
* Displays the plaintext message on the VGA display.
* Stays in a terminating state until reset.
Inputs to your state machine:
* ''Codebreaker'' input ''start''
* ''Codebreaker'' input ''done_drawing_plaintext''
* A new signal you create connected to the ''done'' output of the ''decrypt_rc4'' module.
Outputs of your state machine:
* A new signal you create that you should connect to the ''enable'' input of the ''decrypt_rc4'' module.
* ''draw_plaintext'' output of ''Codebreaker''.
Remember, the ''decrypt_rc4'' module takes some time to perform the decryption, so you will need to wait for the ''done'' output to be high before displaying the message on the display.
Tips:
* You can implement this in 4 states.
* It is good to choose meaningful state names (not S1, S2, etc.) This will help you reason about and debug your state machine.
**Pass-off**: Show the TA your state machine.
----
==== Exercise #3 - Decrypt and Display a Message ====
Implement your state machine in your ''Codebreaker'' module. Test your design with the following 128-bit cyphertext and 24-bit key. You should get a readable message on the display.
assign key = 24'h79726a;
assign cyphertext = 128'h93a931affae622e10a029bd3d4bd6ced;
Use simulation to debug your design when necessary.
For this exercise you can continue to connect 0 to the ''key_display'' and ''stopwatch_run'' outputs. Since your state machine is now assigning a value to ''draw_plaintext'', make sure you don't still have it connected to the ''start'' input.
**Pass-off:** Show the TA the decoded message being displayed on the monitor.
----
==== Exercise #4 - Brute-Force Key Search ====
{{ :labs:codebreaker:codebreaker_flow.png?nolink&600 |}}
The final object of the lab is to modify your state machine to complete the above flow diagram. This consists of a system that tries every possible key value until the input cyphertext is correctly decrypted.
**You know you have located the correct key when the produced plaintext only contains the characters A-Z, 0-9 or space.**
Changes from the last exercise:
* You will need to add a state that checks the decrypted message, and decides whether it is readable. If it is readable you can display the message and stop, otherwise, you should continue on to the next key.
* The **upper** 16 bits of the key should be output on the LEDs. This will help you see that your search is progressing.
* Your state machine should have a new output that is connected to the ''stopwatch_run'' output of ''Codebreaker''. Run the stopwatch after the user presses the button and stop once a valid plaintext message is found.
Checking that each of the 16 bytes in the plaintext is valid may require writing a very long logic expression. To save yourself some typing, we will give it to you:
// Check that each byte of the plaintext is A-Z,0-9 or space.
logic plaintext_is_ascii;
assign plaintext_is_ascii = ((plaintext[127:120] >= "A" && plaintext[127:120] <= "Z") || (plaintext[127:120] >= "0" && plaintext[127:120] <= "9") || (plaintext[127:120] == " ")) &&
((plaintext[119:112] >= "A" && plaintext[119:112] <= "Z") || (plaintext[119:112] >= "0" && plaintext[119:112] <= "9") || (plaintext[119:112] == " ")) &&
((plaintext[111:104] >= "A" && plaintext[111:104] <= "Z") || (plaintext[111:104] >= "0" && plaintext[111:104] <= "9") || (plaintext[111:104] == " ")) &&
((plaintext[103:96] >= "A" && plaintext[103:96] <= "Z") || (plaintext[103:96] >= "0" && plaintext[103:96] <= "9") || (plaintext[103:96] == " ")) &&
((plaintext[95:88] >= "A" && plaintext[95:88] <= "Z") || (plaintext[95:88] >= "0" && plaintext[95:88] <= "9") || (plaintext[95:88] == " ")) &&
((plaintext[87:80] >= "A" && plaintext[87:80] <= "Z") || (plaintext[87:80] >= "0" && plaintext[87:80] <= "9") || (plaintext[87:80] == " ")) &&
((plaintext[79:72] >= "A" && plaintext[79:72] <= "Z") || (plaintext[79:72] >= "0" && plaintext[79:72] <= "9") || (plaintext[79:72] == " ")) &&
((plaintext[71:64] >= "A" && plaintext[71:64] <= "Z") || (plaintext[71:64] >= "0" && plaintext[71:64] <= "9") || (plaintext[71:64] == " ")) &&
((plaintext[63:56] >= "A" && plaintext[63:56] <= "Z") || (plaintext[63:56] >= "0" && plaintext[63:56] <= "9") || (plaintext[63:56] == " ")) &&
((plaintext[55:48] >= "A" && plaintext[55:48] <= "Z") || (plaintext[55:48] >= "0" && plaintext[55:48] <= "9") || (plaintext[55:48] == " ")) &&
((plaintext[47:40] >= "A" && plaintext[47:40] <= "Z") || (plaintext[47:40] >= "0" && plaintext[47:40] <= "9") || (plaintext[47:40] == " ")) &&
((plaintext[39:32] >= "A" && plaintext[39:32] <= "Z") || (plaintext[39:32] >= "0" && plaintext[39:32] <= "9") || (plaintext[39:32] == " ")) &&
((plaintext[31:24] >= "A" && plaintext[31:24] <= "Z") || (plaintext[31:24] >= "0" && plaintext[31:24] <= "9") || (plaintext[31:24] == " ")) &&
((plaintext[23:16] >= "A" && plaintext[23:16] <= "Z") || (plaintext[23:16] >= "0" && plaintext[23:16] <= "9") || (plaintext[23:16] == " ")) &&
((plaintext[15:8] >= "A" && plaintext[15:8] <= "Z") || (plaintext[15:8] >= "0" && plaintext[15:8] <= "9") || (plaintext[15:8] == " ")) &&
((plaintext[7:0] >= "A" && plaintext[7:0] <= "Z") || (plaintext[7:0] >= "0" && plaintext[7:0] <= "9") || (plaintext[7:0] == " "));
Test your brute-force design on the cyphertext and key from the last exercise. Use simulation when necessary to help debug your design.
**Final Pass-Off:** Chose one of the cyphertexts below, decode the message, and display it on the VGA display.
assign cyphertext = 128'ha13a3ab3071897088f3233a58d6238bb;
assign cyphertext = 128'hb8935bbf5f819bcfec46da11d5393d4f;
assign cyphertext = 128'h396d6e70500754ff726bd5fb963998ce;
assign cyphertext = 128'h189f2800aac06ce4a74292bffe33fd2c;
assign cyphertext = 128'h19b39b044dc39c4e98f9dfb44a0b7c11;
/* "BRUTE FORCE RC4" key = 24'hAAAAAA;
"EZ KEY IS 000300" key = 24'h000300;
"THAT TOOK FORVER" key = 24'hFFFFFF;
" I PICKED NUM 4 " key = 24'h123456;
"YOU CRACK ME UP " key = 24'h6DB6DB;
*/
/*
assign cyphertext = 128'hca91b1577f34443894de1001885d6aa5;
assign cyphertext = 128'h57e967f1e86498a1eedc596a84f1fa26;
assign cyphertext = 128'h5b99cbef5dffe0f58c3e81df23ba858f;
assign cyphertext = 128'h77c58ceb8e5b342a583db6be53f8097c;
assign cyphertext = 128'hbd6a2012369d963f18802a8a70ca7ec7;
*/
/* Small keys for simulation:
"BRUTE FORCE RC4" key = 24'h000009;
"EZ KEY IS 000003" key = 24'h000003;
"THAT TOOK FORVER" key = 24'h000013;
" I PICKED NUM 4 " key = 24'h000004;
"YOU CRACK ME UP " key = 24'h000007;
*/
Submit your ''Codebreaker'' SystemVerilog module using the code submission on Learning Suite. (Make sure your SystemVerilog conforms to the lab SystemVerilog coding standards).
----
===== Personal Exploration =====
* Try creating your own encrypted message. You can use [[https://cryptii.com/pipes/rc4-encryption]]. Set the mode to encode, be sure your input string is 16 characters and your key is 3 bytes. Keep in mind, the byte order of the key is reversed; for example, if you choose a key of "AABBCC" on the website, the resulting key will be 24'hCCBBAA in the SystemVerilog implementation. The plaintext/cyphertext doesn't need to be re-ordered.
* Look through the ''CharDrawer'' SystemVerilog module and explore how it works.
* Look through the RC4 decryption module, and see how it implements the algorithm described on the RC4 Wikipedia page.
* Move the message to be drawn in a different location on the screen, and change the color.
* **Challenge:** Speed up the brute-force algorithm by using 2 or 4 simultaneous RC4 decryption instances, and have each of them try a different part of the key space. Stop as soon as any of the modules find the correct key.