Table of Contents

Corning Community College

CSCS1320 C/C++ Programming

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Project: Data Obfuscation - SECRET AGENT MESSAGES (sam0)

Objective

To explore the implementation of caesar cipher encoding and decoding programs.

Background

A cipher is defined as “a secret or disguised way of writing; a code”, and (as a verb): to put (a message) into secret writing; encode.

In the realm of secrecy (think elementary school secret agent), obfuscation is key. If we can remove direct accessibility to the message (encode), yet still preserve its intent, it can be transmit to a recipient who has the ability to retrieve the message (decode).

The caesar cipher (or shift cipher) is a relatively simple cipher, where each letter of the alphabet is shifted by a fixed amount, enabling a once legible message to appear unrecognizeable (at least directly).

Further background information can be found here:

There are two processes related to a lossless obfuscation of data:

Program

You are to implement one program, which contains two fundamental modes of operation:

The key should be a signed char, allowing for a cipher shift of -128 to +127 (your shift can be left or right, depending on the sign of the number).

The encode and decode functionality will be located in functions you declare and define, which your program's main() function calls upon determining the specified mode of operation.

Operating mode will be determined by the program's name:

Your program needs to:

Your program should be a “one shot”. It should only perform its intended operation and exit. No prompting for encode/decode, no “do you want to go again”… just a read from input, process, output, and exit with appropriate return value. They should conform to the execution examples found in this project.

When compiling, an additional constraint is added: compile with the -Wall flag.

Sample execution: encode

Via positive command-line key:

lab46:~/src/cprog/sam0$ ./encode 2
"hello"!!
"jgnnq"!!
^D
lab46:~/src/cprog/sam0$ 

(NOTE: ^D indicated the CTRL-d sequence, which generates an EOF).

Via negative command-line key:

lab46:~/src/cprog/sam0$ ./encode -3
hello, there...
ebiil, qebob...
^D
lab46:~/src/cprog/sam0$ 

Without command-line nor cipher.key file:

lab46:~/src/cprog/sam0$ ./encode
ERROR: key not found
lab46:~/src/cprog/sam0$ 

With 4 in the cipher.key file:

lab46:~/src/cprog/sam0$ echo "4" > cipher.key
lab46:~/src/cprog/sam0$ ./encode
hello there
lipps xlivi
^D
lab46:~/src/cprog/sam0$ 

Same thing, but saving the encoded text to a file:

lab46:~/src/cprog/sam0$ echo "4" > cipher.key
lab46:~/src/cprog/sam0$ ./encode <<< "hello there" > code.out
lab46:~/src/cprog/sam0$ 

With 4 in the cipher.key file, decoding previous message (redirected to code.out):

lab46:~/src/cprog/sam0$ ./decode < code.out
hello there
^D
lab46:~/src/cprog/sam0$ 

Note that when you decode, you should get the original message before it was encoded.

Via positive command-line key, decoding:

lab46:~/src/cprog/sam0$ ./decode 2
"jgnnq"!! 
"hello"!!
^D
lab46:~/src/cprog/sam0$ 

You can also save typing, by providing your input via a here string (also a nice way to check for EOF):

lab46:~/src/cprog/sam0$ ./decode 2 <<< "jgnnq." 
hello.
lab46:~/src/cprog/sam0$ 

Submission

To successfully complete this project, the following criteria must be met:

To submit this program to me using the submit tool, run the following command at your lab46 prompt:

$ submit cprog sam0 sam0.c
Submitting cprog project "sam0":
    -> sam0.c(OK)

SUCCESSFULLY SUBMITTED

You should get some sort of confirmation indicating successful submission if all went according to plan. If not, check for typos and or locational mismatches.