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In this project you are required to implement an “encryption module”, in LC3 assembly, that allows users to encrypt their messages using a simple encryption algorithm (using a variation of the Caesar’s cipher algorithm and One Time Pads) that mimics the ARX (Add Rotate Xor) ciphers of today (though we will leave out the Rotate aspect in this project!). Applications where a sequence of digits/numbers, such as the social security number, passwords or credit card numbers, have to be encrypted are numerous – examples include online purchasing (such as Amazon), patient records in health care systems. In this assignment, you will develop a program to encrypt a string of at most 10 characters using an encryption algorithm described in this document. (The characters from the keyboard, in terms of their ASCII representation, are all printable characters with ASCII representation from x21, for the symbol “!”, to x7E which is the symbol ~). 1. Cryptography Background Cryptography, from the Greek word kryptos, meaning “hidden”, deals with the science of keeping information secret. The desire to do this has existed ever since humankind was first able to write. There are many ways to keep something secret. One way is to physically hide the document. Another way is to encrypt the text you wish to remain secret. Some people use this to keep others from understanding the contents of the files in their computer. Encrypting a file requires an input message, called the “plain text,” and an encryption algorithm. An encryption algorithm transforms “plain text” into “cipher text.” Just like a door needs a key to lock and unlock it, an encryption algorithm often requires a key to encrypt and decrypt a message. Just like a homeowner can selectively give the key to the front door to only those he wants to allow unaccompanied access, the author of a message can selectively give the encryption key to only those he wants to be able to read the message. In order for someone to read the encrypted message, they have to decrypt the cipher text, which usually requires the key. For example, suppose the plain text message is HELLO WORLD. An encryption algorithm consisting of nothing more than replacing each letter with the next letter in the alphabet would produce the cipher text IFMMP XPSME. If someone saw IFMMP XPSME, they would have no idea what it meant (unless, of course, they could break the encryption, or had the key to decrypt it.) The key to encrypt in this case is “pick the next letter in the alphabet,” and the decryption key is “pick the previous letter in the alphabet.” The decryption algorithm simply replaces each letter with the one before it, and presto: the plain text HELLO WORLD is produced. Encryption Algorithm In this programming project you will implement a simple ARX cipher (with the rotational component left out for simplicity; but you could think of it as adding a shift operation) encryption algorithm which we call “SAD” based on a composition of two methods – Caeser’s cipher and the Vigenere Cipher (both of which are described in detail at the end of this document). The goal is to encrypt a message, consisting of ASCII characters, using a key provided by the user. To decrypt the message correctly, the same key must be used by the decryption algorithm. 2. Project Requirements and Specifications Your encryption/decryption programs (in LC3 assembly) must meet the following requirements. Any deviation from this, without approval from the instructor, or the TAs will result in points being deducted. The program must use the SAD encryption algorithm described in this document. Specifications: Your program should prompt the user for three separate inputs from the keyboard, as follows: (0) When you start the program, it first prints out “Starting Privacy Module” and then goes to Step 1. (1)The prompt: ENTER: E TO ENCRYPT, D TO DECRYPT, X TO EXIT: o The user will type E or D or R or X. A real world program, with error checking for invalid inputs, should also detect any other character typed and respond with ? INVALID ENTRY. PLEASE TRY AGAIN. o Your program will accept that character, store it someplace (you should specify where in memory) and use it, as we shall see momentarily. o If the user types X then the program first erases the encrypted data (i.e., replace the contents in memory where the encrypted data was stored with zeros), and then exits and halts. (2) If either E or D is chosen in (1) then the program prints the prompts: ENTER KEY (Length 4, one non-numeric character followed by 3 digit number between 0 and 127). WHEN DONE PRESS ENTER (a) The user will type their key (note that the key in this case is a non-numeric character followed by a 3 digit number between 1 and 127). Again, we will assume the user can successfully hit digit keys on the keyboard. You can make any of the two assumptions: (1) user will enter return after they are done entering numeric number between 0 and 127 – for example A23 and enter return; or (2) user will always enter 3 digits after the non-numeric character – for example they will enter A023. To figure out which design is ‘easier’ take a look at the ASCII to Binary (and Binary to ASCII) subroutines from HW5 (and the Calculator example in the textbook). Question (during your design): what do you echo when the user types in the key? (b) Your program will accept this input, store it someplace, and use it to encrypt or decrypt the message. ? Important: You will need to figure out how to store and interpret this (max) length 4 key entered by the user. Read details of the encryption algorithm to determine what needs to be done. (c) If the input in Step 1 was D then it goes to Step (3). If the input in Step 1 was E then program goes to step (4). If the input in Step 1 was X then it goes to step (5). (3) If the input in (1) was D (decryption) then your program must decrypt the string (some ciphertext/message you stored) using the key entered in this step and print it out to the display. The cipher text string will be a 10 character cipher text (i.e., after encryption) stored starting at location with label MESSAGE – details follow in step (3). ? Note: to protect against losing the encrypted data stored starting at MESSAGE if a wrong key was entered, make sure you do not write into MESSAGE the string you are going to print. ? The decrypted message must be printed to the display. Note that if the user entered the wrong key, then it will print out a message (string of characters) that will not be equal to the original message. ? After decrypting the program returns to Step 1 to prompt for next user input. (4) If the input in (1) was E (i.e.,encryption) then after getting the key in Step (2), print the prompt: Enter input plain text of length at most 10. When done press a. The user will input a string/sequence of at most 10 characters (the ascii printable characters and do not include control characters) from the keyboard, terminating the message with the key. You can make the simplifying assumption of exactly 10 characters – for penalty of 2 points. b. Your program must store this ASCII string in memory location MESSAGE starting at x4000. It must then write the encrypted characters into the same location since you do not want plain text to persist. Question: – why ? c. Using the value of the key entered in Step(2), and the encryption algorithm provided, generate the cipher text and store the cipher text into memory starting at location labeled with MESSAGE (i.e., the label of the starting address of the location is MESSAGE). d. After encrypting, the program returns to Step 1 where it can prompt for next user input. Question: How would you change the program if instead of at most 10 message, you want to allow strings of any length ? (4) If the input in (1) was X (i.e., Exit) then the program erases the memory locations where the encrypted data was stored (erase is simply writing 0 into the locations) and exits and halts. 3. Hand-in Requirements – What you need to submit. You are required to hand-in two components (as a zip file): (1) your assembly code and (2) a report describing your implementation – this should include pseudo code. Your assembly code must be a text file – i.e., an .asm file. You must name the file as firstinitial-lastname.asm. For example, if your name is Jane Doe then the filename is JDoe.asm. The report is meant to assess the effectiveness of your design and we are looking (a) for an algorithm describing your implementation and (b) answers to the (underlined) questions in this document. You must document your code, and include a flow-chart describing your solution. Note that you will get a grade of zero if your program does not work and you do not submit a report describing your algorithm. 4. Grading Criteria ? You must submit code that assembles correctly without errors – if your code does not assemble then you get zero points. If the code crashes during testing of specifications, you cannot earn more than 50% of the points. ? You must document your code – you lose up to 10% for poor (or no) code documentation. ? The code must meet all the specifications (user interface, encryption and decryption algorithms). ? The encryption algorithms must be correctly implemented. If you implement part (subset) of the encryption algorithms correctly then you will receive partial credit (of up to 80%). ? Performance of the code – pay attention to minimizing redundant code. ? Error checking – you can get correctly working code without error checking (for invalid inputs) by assuming that the users enter valid input data. However, making this simplifying assumption means you will lose some points for not implementing error checking. (You can still get 90% without error checking if everything else works perfectly.) 5. Encryption Algorithms The encryption algorithm you will implement is a composition of two well known (and no longer used!) techniques: (1) f1 : Caeser’s cipher (generalized to a shift cipher), and (2) f2 : Vigenere Cipher – this implements a lightweight variation of the one time pad techniques. (For it to be in the ARX category, we need a third step of rotating the bits – this step is left out in this assignment but you are welcome to try adding this component for extra credit as a f3: shift k bits where 0

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