Assignment #5 (PA5) Steganography

CS 241L - Data Organization

Programming Assignment #5 (PA5)
Steganography
Total points: 100

In this assignment, you will write a more complex C program that includes processing input, using
control structures, and bitwise operations.
1 Assignment:
Encryption is the science of hiding information. Encryption allows Alice and Bob to communicate
without their adversary Eve (the eavesdropper) reading and understanding the “secret message”. For
example, ROT13 will transform the secret message “helloworld” into “uryybjbeyq”. Although the
message is secret, Eve is still able to observe that communication is taking place between Alice and
Bob, and this information alone can be very valuable.
Steganography, in contrast to encryption, is the science of preventing others from learning information
by hiding that information in “plain sight”. The secret message is not scrambled, or encrypted, but
rather it is mixed in with other non-secret information in such a way that, although it is easy to read,
people are not even aware that the secret message is there. In this scenario, Eve does not even realize
that communication is occurring between Alice and Bob.
Log into the cs machines. Create a directory for this assignment using the following one-line input
to the Unix shell.
$ cd cs241; mkdir prog05; chmod 700 prog05; cd prog05
The input for your program will be a text file containing a large amount of English. Typically,
an English sentence ends with a period (aka, dot). Many years ago, when people used mechanical
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typewriters, the proper form was to place one space between words in a sentence, but two spaces after
the period at the end of the sentence. This rule is no longer taught, since word processors automatically
control spacing.
Your program must extract the “secret message” from the input file. The message is hidden inside
the file using the following scheme. The message is hidden in binary notation, as a sequence of 0’s and
1’s. Each block of 8-bits is the ASCII encoding of a single character in the “secret message”. Your
program will scan the input, and for each period (aka, dot) encountered, your program will count the
number of spaces (ASCII 0x20) that immediately follow the dot. If the number of spaces is 0, then your
program will ignore that dot completely. If the number of spaces is 1, then that corresponds to a single
0 bit of the secret message. If the number of spaces is 2, then that corresponds to a single 1 bit of the
secret message. Finally, if the number of spaces is 3 or more, then that indicates that there are no more
characters in the secret message.
Your program must scan the input file, and output to stdout the plain text secret message. If your
program successfully completes its task, then the program should return an exit status of 0 to the
operating system. If the number of bits in the message is not a multiple of 8 (8 bits for each ASCII
character), then your program should return an exit status of 1. If the input file does not contain the
“message over” signal, i.e., a dot followed by 3 or more spaces, then your program should return an exit
status of 2.
I have provided several input files from which you should be able to extract the “secret message”.
I have also provided input files where the “secret message” is incorrectly encoded, i.e., your program
should return a non-zero exit status. You can write your program foo.c to read input from stdin (e.g.,
using getchar()), and then test your program on the given files using input redirection, as follows:
$ gcc -std=c99 -pedantic -Wall foo.c; ./a.out < goodInputFile
Your program should output the “secret message” to stdout.
This program is an example of “stream processing”. You will read the input file, character by
character, in a single pass. You must not use arrays of any kind, since there is no need to do so,
and using arrays is simply inefficient (i.e., overkill). I strongly suggest that before writing a single line
of code, not even the include or the main signature, you think of the algorithm you will use to process
each character and in particular every dot and spaces after the dot. Understanding the problem clearly
and even running a simple example by hand, will save you time at debugging. How are you going to
store each bit of each character of the secret message?
NOTE : The program should not print any characters until it has the complete secret message. So
if that message does have a number of bits that is a multiple of 8, you may print the secret message,
otherwise the program should not print anything but provide the non-zero error code. You may store
the sequence of chars in a string literal that you build as you go along.
You should be sure to include THE NAME OF THE AUTHOR OF THE PROGRAM in a
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comment at the top of your source code file, for this and all other assignments in this course.
Your source code must use proper style, that is variables should be well named (name is not too
short, not too long, and is meaningful), and bodies of loops, if’s, etc.. should be properly indented.
Refer to the coding style file for this class, published on Canvas under Coding Standards in a file named:
cs241 codingStandards2020.pdf.
Create a .c file for this assignment and name it using your last name and the initial of your first
name, like this: lastName initialFirstName secret.c
Submit this file for grading to Canvas in the place of this assignment. If you consulted some external
sources to produce your program you must cite those sources in a separate document; a README file,
which is a plain text file to explain aspects about your program that would be too lengthy to put as
comments in the code, is an appropriate way to do so. This is a typical practice, to accompany your
code with a README file that gives important additional information. Identify your README file
with your name inside and the name of the assignment.
2 What to submit:
Submit a C file to Canvas with name lastName InitialFirstName DNA.c and possibly a README
file.
3 Grading rubric:
If any of your C programs do not compile with the -std=c99 -pedantic -Wall options without errors
or warnings the points given for the assignment will be zero. Otherwise the following rubric will be
used:
+ 10 pts: Your C files follows the class coding style.
+ 40 pts: Your lastName InitialFirstName DNA.c file passes a diff test with the good input
files provided in the assignment and outputs the correct exit code.
+ 40 pts: Your lastName InitialFirstName DNA.c file passes a diff test with the bad input files
provided in the assignment and outputs the correct exit code.
+ 10 pts: Your lastName InitialFirstName DNA.c file passes a diff test with local input files
available to the grader and outputs the correct exit code.
Exception: if your program generates warnings but no errors at compile time, your submission might
get some partial credit, but warnings are important to avoid since some of them lead to execution errors.
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