1 CSC 322 Systems Programming Fall 2019 Lab Assignment L1: Cipher-machine Due: Monday, September 23 1 Goal In the first lab, we will develop a system program (called cipher-machine) which can encrypt or decrypt a code using C language. It must be an errorless program, in which user repeatedly executes pre-defined commands and quits when he or she wants to exit. For C beginners, this project will be a good initiator to learn a new programming language. Students who already know C language will also have a good opportunity to warm up their programming abilities and get prepared for the rest of labs. 2 Introduction A. Basic Commands The system program provides three basic commands shown in Table 1. The program runs a related function or exits when a user enters a command as seen in Figure 1. Note that commands need parameters. Misspelled command should be handled as an exception, and the program must show an error message. Note that program must be in running and shows the next prompt. Table 1. Basic Commands Command Description encrypt Encrypt user’s input based on Caesar’s cipher. See the section II-B. decrypt Decrypt user’s input based on Caesar’s cipher. See the section II-B. encode Encode user’s input to get a binary code based on ASCII code. See the section II-C. (extra) decode Decode user’s binary input to get a character code. See the section II-C. (extra) exit Exit the program The program should run on Linux machine at cs.oswego.edu. Once it is compiled and executed, it draws user to its own computing environment by showing prompt following the format: your_loginID $. Figure 1 shows how the prompt looks when the logID is jwlee. When a command is correctly executed, output will be printed out immediately. The logID will be printed by using printf function. jwlee@altair~ > cc lab1-jwlee.c -o lab1 jwlee@altair~ > ./lab1 jwlee $ exot [Error] Please enter the command correctly! jwlee $ encrypt(“hello”) ebiil jwlee $ exit 2 jwlee@altair~ > Figure 1. Command Mode B. Caesar’s Cipher: encrypt/decrypt Caesar’s cipher is one of cryptography methods, which has a long history, back to Julius Caesar’s Roman Empire. He used this simple yet efficient method in sending and receiving private messages with his subordinates and political comrades. The method is based on a substitution cipher technique in which substitute an alphabet letter to a different alphabet letter by a certain rule. Look at the list of alphabets in the Figure 2. In this plaintext each character has a certain positional number, for example “A” has 1, meaning the first character. In the same manner, “B” has 2, “C” has 3, and so on. To get the ciphertext, all positional number is shifted right by a proposed number, 3 in the Figure 2. By the shifting rule, “A” is now in the 4th position, “B” is in the 5th position. Figure 2. Shifts in Caesar’s cipher method The shifting rule has issues when it shifts alphabets at the end. See Y and Z. After shifting 3, Y which was in 25th position should move to 28th position, but it is out of range of alphabet. Caesar solved it by modular operation, which means the positional number after shifting is modularized by the size of alphabets. For simplicity, let say A is in 0, B is in 1, and so on. Then the encryption formula E to get cipher text C from plain text P is below: C = E(k, P) = (P + k) (mod 26) In the formula k stands for the number of shifts and k is 3 in our example. Similarly, a cipher text can be decrypted by the following formula D: P = D(k, C) = (C – k) (mod 26) A cipher message can be decrypted by person who knows k. It makes the method secure. For example, the following sentence is encrypted to a cipher message, which will be sent: Actual message: SEE ME AFTER CLASS Sending message: VHH PH DIWHU FODVV Because you are given k above, you can understand the sending message, VHH PH DIWHU FODVV and will see me after class. But who don’t know it cannot see me, right? However, Caesar’s Cipher covers only 26 alphabet letters. Numbers cannot be encrypted. In this program, thus you extend it to cover characters in ASCII code1. Each ASCII character is encoded to an 8-bit number (7 bits are used in the traditional code). For example, “A” is 65, “B” is 66, the number “0” is 48, “1” is 49. Lowercases are also differentiated, for example, “a” is 97, “b” is 98. Moreover, it covers other special characters such as “!”, “+”, and so on. Because it encrypts more than 26 characters, it definitely enhances the secure level! A B C D E F … Y ZX Y Z A B C … V W… Y ZA B …plaintextciphertext3 In your program, you will encrypt a plaintext to get a ciphertext when k is 3. And you will decrypt a given ciphertext to get a plaintext. Your program will run as shown in Figure 3. Note that it must follow the correct syntax below: encrypt(plaintext) decrypt(ciphertext) Any typos in entering the command should be caught and returns an error message. It includes the format of command. jwlee $ encrypt(I have a key) L#kdyh#d#nh| jwlee $ encrypt(see me at 3) vhh#ph#dw#6 jwlee $ decrpyt(qr#l#fdq#phhw#dw#no i can meet at 9 jwlee $ encr(see you soon) [Error] Please enter the command correctly! jwlee $ decrypt “rndb” [Error] Please enter the command correctly! jwlee $ Figure 3. encrypt/decrypt example After the output, the system must print prompt so that user enters a next command. Are you ready? JRRG OXFN!! C. [Extra work] Encoding/Decoding: encode/decode It is not mandatory but optional, which means those who develop this command will earn extra credits up to 6 points per command. The commands are executed when you want to encode a text to a binary code and decode a binary code to a readable text. The encoding and decoding are commonly used in editing video or audio data. The idea is to convert data (of image, video, audio, etc.) to a format which can be read and managed by machine. Since the machine is a passive device which can use two characters – 0 and 1, the format is usually written in the binary code. There is variation for the format, but you implement a simple encoder/decoder, which convert a character into its ASCII code’s binary code. You can use the ASCII code in the last page. See the example of encode and decode commands in Figure 4. The text “key” has three ASCII codes of 107, 101, and 121. The encoder converts them into a binary number (01101011, 01100101, 01111001) and concatenate them. Note that the traditional ASCII code has 7 bits code, but you will consider 8-bit code which enables 8 bits Unicode. Thus, after convert 107 to 1101011, and make it 01101011 by putting 0 in the beginning. jwlee $ encode(key) 011010110110010101111001 jwlee $ encode(7-eleven) 0011011100101101011001010110110001100101011101100110010101101110 jwlee $ encod(location) [Error] Please enter the command correctly! 4 jwlee $ decode “011100” [Error] Please enter the command correctly! jwlee $ decode(here) [Error] Please enter the command correctly! jwlee $ Figure 4. bin2dec example The commands have the following syntax: encode(text) decode(binary code) You should check misspelled commands and incorrect syntax. And you should check binary code input for decode command, since it decodes “binary code” to a text. Look at the last case of decode(here) in the Error! Reference source not found.. It is incorrect, because “here” is not a binary code. After the output, the system must print prompt so that user enters a next command. Each command is worth 6 points, thus those who develop both commands will get 12 points. 3 Hints in Implementation A. Reading user input There are several ways to get your input: scanf, sscanf, gets, fgets, and so on. But the best way to handle exceptions such as non-numerical inputs and excess of inputs is to get a string and parse it into required forms. For needs, the good choice is fgets but not limited to use other functions in this lab. Find full descriptions of the functions using man. B. Parsing user input Once user enters a command, the system must parse it to recognize the type of command and input. Tokenize
r enables to parse commands and return the information needed for further processing. String library provides necessary functions such as strtok. Find full descriptions of the functions using man. 4 Requirements A. Developing environment The program should be implemented in C only. The program in another language will not be graded. And the program should be executable in CS Linux machine. The program which cannot be compiled or executed in the CS Linux machine will be considered as incomplete program and will lose most points. B. Handling exceptions The program must detect errors while running. The errors may occur when user violates the syntax of the command or enters incorrect inputs (characters, more than required inputs, or see more in the section II). When the program detects any error cases, it should handle 5 properly by giving some error messages to the user, and still be runnable. The program, which fails to detect errors and to properly handle them, will lose points. C. Readability of source code The source code should be easy to read with good indentation and proper amount of comments. D. Creating a readme file The program should be submitted along with a readme file which has information such as your ID, name, etc. Students who work on the extra work should notify to instructor through the readme file. It also may provide a special instruction to run this program if exists. The source file and readme file should be named following the format below. Those who are not following the naming rule will also be taken off penalty points. lab1-your_loginID.c lab1-your_loginID_readme.txt E. Submitting by due The program should be submitted to Blackboard within the two weeks after the project is posted. Due is September 23rd. All submission by 11:59 PM on that day will be accepted without any penalty. On the due date, Blackboard may be suffering of too much network traffics and be unstable. There is no excuse about the issue, therefore you are strongly recommended to submit earlier than the due date. 5 Grading A. Grading criteria The lab is assigned 30 points, which is 15% of the final grade. It will be graded by evaluating the requirement. Any missing and unsatisfiable criteria will take off points. The tentative and brief criteria are below. • Compilation: 5 points • Execution: 20 points • Error detection & others: 5 points The extra work will give you up to 12 bonus points, which are worth of 40%. B. Late penalty Late submission will take off 10% per day per week after due date. Thus, submission after 10 weeks will not be accepted in any circumstances. 6 Academic Integrity Any dishonest behaviors will not be tolerated in this class. Any form of plagiarism and cheating will be dealt with according to the guidelines on the Academic Integrity Policy on-6 line at http://www.oswego.edu/integrity. For more information about university policies, see the following online catalog at: http://catalog.oswego.edu/content.php?catoid=2&navoid=47#stat_inte_inte Student who is against the honor code will not have any credits in this project. 1 ASCII code with binary (not covering UNICODE)
