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

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:

• https://en.wikipedia.org/wiki/Caesar_cipher

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

• encoding - taking the readable text and making it obfuscated, according to a set process
• decoding - taking the obfuscated text and making it readable, by reversing the process

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

• encode functionality: takes plain text message as input (via stdin), along with a cipher key, and outputs the encoded (obfuscated) result
  • key should be provided via command-line argument (or, if present, a file in the current directory called 'cipher.key')
• decode functionality: takes the encoded (obfuscated) message (via stdin), along with a cipher key, and outputs the decoded (readable, plain text) result
  • key should be provided via command-line argument (or, if present, a file in the current directory called 'cipher.key')

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:

• Naming your program encode will invoke the encoding functionality.
• Naming your program decode will invoke the decoding functionality.
• Mode auto-detection should work regardless of any prefixing path information (./decode, /home/$USER/src/cprog/sam0/encode, etc.)
  • you may want to explore the strtok(3) function, or write your own.

Your program needs to:

• load the provided input (via stdin) into an array for processing (you may want to check for an EOF character to terminate input)
  • assume a maximum input size of 4096 bytes
  • use the fgetc(3) function instead of fscanf(3) for reading input from STDIN (do NOT use any scanf() or gets() related function, ONLY fgetc(3)).
• obtain the signed char key value from the command-line, if present, or read from the 'cipher.key' file
  • if neither command-line nor key file are available, display error and exit
• appropriately “shift” the alphabet according to the key (magnitude and direction and mode), storing the results in an array
  • display the resultant string to STDOUT
  • the cipher should work only on upper and lowercase letters of the alphabet. Any punctuation, number, whitespace, or other symbol should remain intact (knowing your ASCII table would be helpful).
  • the encoding and decoding functionality needs to occur within declared/defined functions you create that are called from main() to perform the intended operation.
    • As they will be performing operations on your array filled with the input data, you also need to make productive use of loops.
    • all data needed by the function needs to be passed in by parameter! No global variables.
    • The functions should return an integer containing the exact count of non-letters encountered (and therefore left unmodified from input to output). Your main() function needs to return this value when complete.

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.

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$ 

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

• Code must compile cleanly (no warnings or errors)
  • Use the -Wall flag when compiling.
• Output must be correct, and resemble the form given in the sample output above.
• Code must be nicely and consistently indented (you may use the indent tool)
• Code must utilize the algorithm/approach presented above
• Code must be commented
  • have a properly filled-out comment banner at the top
  • have at least 20% of your program consist of //-style descriptive comments
• Track/version the source code in a repository
• Submit a copy of your source code to me using the submit tool.

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.