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ASSIGNMENT 2
Cryptography
COMP-202
You must do this assignment individually and, unless otherwise specified, you should follow all the instructions. Graders have the discretion to deduct up to 15% of the value of this assignment for deviations from
the general instructions and regulations.
Before starting this assignment, you need to download the files web2.txt and SentenceChecker.java from
mycourses. These files should be placed in the same folder as the file you will create called Cryptography.java
If you are working using Dr. Java, you need to open SentenceChecker.java at the same time. However,
when you click compile and then run, you must ensure that you run your file Cryptography.java by leaving
this window open.
Part 1: 0 points
Part 2, Question 1: 15 points
Part 2, Question 2: 10 points
Part 2, Question 3: 20 points
Part 2, Question 4: 15 points
Part 2, Question 5: 10 points
Part 2, Question 6: 30 points
100 points total
Free pass usage
If you wish to use your “free pass” (see the course webpage for more details), you are required to email the
TA who is marking your assignment before the deadline. Please see the table below, which is referenced by
the last name of the student, to determine your TA. Note that this is also the person whom you should see
if you have questions or other complains about your marks.
1
TA Name Email Address Student Last Name Range
Xiaozhong Chen [email protected] A-BAN
Stephanie Laflamme [email protected] BAO-BUDD
Teng Long [email protected] BUDE-CHR
Andrew Holliday [email protected] CHS-DOW
Carlos Gonzalez Oliver [email protected] DOX-GAG
Hua Qun (Robin) Yan [email protected] GAH – HARB
Feras Abu Talib ta [email protected] HARC – JON
David Bourque [email protected] JOO – KOR
Paul Husek [email protected] KOS – LI, H
Babak Samari [email protected] LI, I – MA
Seyyed Mozafari [email protected] MAC – MUR
Egor Katkov [email protected] MUS – PARE
Chenghui Zhou [email protected] PARF – RENT
Jonathan Campbell [email protected] RENU – SHIM
Neeth Kunnath [email protected] SHIN – TANG
Ayush Jain [email protected]@mail.mcgill.ca TANN – WANG, Q
Tzu-Yang (Ben) Yu [email protected] WANG, T – YANI
Mohammad Patoary [email protected] YAY- Z (end)
Part 1 (0 points): Warm-up
Do NOT submit this part, as it will not be graded. However, doing these exercises might help you to do the
second part of the assignment, which will be graded. If you have difficulties with the questions of Part 1, then
we suggest that you consult the TAs during their office hours; they can help you and work with you through
the warm-up questions.
Warm-up Question 1 (0 points)
Write a method printTypeOfChar. This method should take as input a char and print the type of
character. If the char is upper case, your method should print UPPERCASE. If the char is lower case,
your method should print LOWERCASE. If the char is any other symbol, your method should print SYMBOL.
Note that it is not necessary to make 52 different cases if you consider the order of the Unicode chart
and the fact that you can use the < and operators on them.
Warm-up Question 2 (0 points)
Write a method countUppercase. Your method should take as input a String and return an int
representing the number of upper case letters in the String. Now add a main method where you use
the Scanner class to read a String from the user and call your method, outputting the returned result.
Warm-up Question 3 (0 points)
Write a method toUpperCase which takes as input a String and returns another String which is
the original String but with all lower case letters converted to upper case. Any letters that are not
lower case should remain the same. Do NOT use any methods in the Character class or the
.toUpperCase() method in the String class
For example the input String of “Hi!” should become “HI!”
Warm-up Question 4 (0 points)
Write a method reverseString which takes as input a String and returns the string in reverse order.
For example if the input String is “Comp 202 is awesome” the result should be “emosewa si 202 pmoC”
Hint: Use a new String called reverse and initially store into it the empty String. Then read the
input String in reverse by using the method .charAt(int i) to get a specific element.
Warm-up Question 5 (0 points)
A prime number is a positive number whose only even divisors are 1 and itself. Write a method isPrime
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that takes as input an integer n and returns a boolean representing whether n is prime or not. Make
sure to handled cases where the integer is negative. (Your method should return false in these cases.)
Warm-up Question 6 (0 points)
Write a method firstPrimeNumbers which takes as input an int n and returns an int[] . The int[]
should contain the first n prime numbers.
Warm-up Question 7 (0 points)
Practice using the utility code we have provided by calling the method countEnglishWords() from the
SentenceChecker class. To ensure things work, put all files (web2.txt, SentenceChecker.java, and your
new class) into the same directory. You can then leave both SentenceChecker.java and your new class
open at the same time in Dr. Java.
The method countEnglishWords takes as input a String and returns how many English words are part
of it, using a list of words provided in the file web2.txt. Try calling this method to check how many
English words are in the sentence The laydee hit teh man wit the bay bee.
Warm-up Question 8 (0 points)
Practice using the Random class: Question four requires using the Random class. In this warmup question,
you will generate random numbers.
Write a method that takes as input an int n and an int max and returns an int[]. The array you
return should be an array of size n and filled with n random numbers between 0 and max, counting 0,
but not counting max. Assume max is a positive number.
See the instructions within question 4 for help using Random class.
Warm-up Question 9 (0 points)
Same question as before except your method should now take as input three values: int n , int min,
and int max. Instead of ranging from 0 to max, you should range from min to max.
Again, you may assume both min and max are positive numbers as well as assume that min is less than
max.
Warm-up Question 10 (0 points)
x is a factor of y if y is a multiple of x. Write a method calculateFactors. The method should take
as input an int n and return an int[] containing all the factors of the number n
Warm-up Question 11 (0 points)
Write a method that takes as input an array of Strings and prints all of the Strings on separate lines.
Warm-up Question 12 (0 points)
Create a file called Counting.java, and in this file, declare a class called Counting. This class should
ask the user when the computer should stop counting.
When should I stop counting to?
10 <—- User typed this
I am counting until 10: 1 2 3 4 5 6 7 8 9 10
Warm-up Question 13 (0 points)
For this question you have to generalize the last question. The user will give you the number they want
the computer to count up to and the step by which it will do so.
When should I stop counting to?
25 <—-
Which step should I use?
3 <—-
I am counting to 25 with a step of 3:
1 4 7 10 13 16 19 22 25
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Warm-up Question 14 (0 points)
This program should ask the user how big she wants a square to appear. Using two loops, you should
be able to describe the outline of a square like the following.
How big do you want your square to be?
10 <—- User typed this
##########
# #
# #
# #
# #
# #
# #
# #
# #
##########
N.B. It is normal that it does not output a perfect square as the width and the length of the characters
are not equal.
How would you extend your program to output a rectangle with width and length specified by the user?
Part 2
The questions in this part of the assignment will be graded. All of your code in this assignment should go
into a class Cryptography.
Beginning in this assignment, a 75% non-compilation penalty will apply. This means that if your code does
not compile, you will receive a maximum of 25% for the question. If you are having trouble getting your
code to compile, please contact your instructor or one of the TAs or consult each other on the discussion
boards.
For this assignment, you will be writing several methods to help you practice using arrays. As
such you are not allowed to use any methods from the Java library class Arrays.
Encrypting a String With a Shift Schema
Question 1: Encrypting a String with a Caesar shift (15 points)
A simple way to encrypt things is by shifting all of the letters by a fixed amount n. This is known as a
Caesar shift. The letters are “cycled” and all characters other than letters (e.g. numbers, punctuation,
etc. remain the same).
For example, when employing a Caesar shift of 3 on the String go Rangers! you would get jr Udqjhuv!.
Note that the letter ‘j’ comes three letters after the letter ‘g’ in the alphabet. The letter ‘U’ comes three
letters after the letter ‘R’, and is upper-case, because the letter ‘R’ in this example is also upper case.
We consider the alphabet of upper-case letters to be a loop (a shift of 3 applied to the letter ‘Z’ would
give ‘C’), and similarly with lower-case letters (a shift of 3 applied to the letter ‘z’ would give ‘c’).
Julius Caesar famously used this method of encoding messages. At the time, very few people were
literate and those that could read simply assumed it was a foreign language! If interested, you can read
more about it at https://en.wikipedia.org/wiki/Caesar_cipher#History_and_usage
Write a method caesarEncrypt which takes as input a String originalMessage and an int shift
and returns the String created by applying a shift of size shift to the original message. Note that your
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method should work with all non-negative integers. That is, you can assume that the input will never
be smaller than zero. In the case that shift is greater than 25, you should cycle over the alphabet an
additional time. For example the letter a shifted by 30 is the same as the letter a shifted by 4, which
would be e.
Two useful methods that you can use are charAt() and length() from the String class. In order to use
them, write the name of the String type variable, then a dot, and then the method name followed by
arguments. For example, if you have:
String s = ‘‘hello’’
then
s.charAt(x)
will return the x
th character in s. The counting starts from 0. So for example:
System.out.println(s.charAt(0))
will print the letter h.
System.out.println(s.charAt(3))
will print the letter l.
System.out.println(s.charAt(5))
will give you a run time error because the String s does not have a character at index 5.
In addition, you can obtain the length (number of characters) of a String by using the method .length().
To do this, for example, write
int length = s.length()
In order to add a number to a character and have the result be a character, you will have to be careful
with type-casting. For example:
char d = (char)(‘A’ + 3)
will will store the unicode for the letter ‘D’ in the variable d. Clarification: The casting must be
done after the addition in order for the types to work properly.
For this question, do not use any methods from the Java Character class. Additionally you
may not use any methods from the String class other than those listed above (charAt and
length).
Question 2: Decrypting a String (10 points)
Write a method caesarDecrypt which takes as input a String encoded and an int shift and returns
the String created by applying a reverse shift of shift to the original message.
caesarEncrypt() is the inverse of caesarDecrypt(). This means that for any String s,
caesarDecrypt(caesarEncrypt(s)) is the same as s
Hint: Can you use the method encrypt() that you wrote in the previous question to help for this part?
Question 3: Cracking the code! (20 points)
Write a method crackCipher() which takes as input a String encoded and an int numberLetters.
It should then call the method caesarDecrypt() with all different shifts from 0…numberLetters − 1.
After getting the result from this call, it should call the method countEnglishWords() given in the
file SentenceChecker. This method takes as input a String and outputs the number of English words
it contains. The method provided to you will take care of things such as removing punctuation. This
means, for example, that he!!!!!!llo will count as one English word and hello, world 🙂 counts
as two English words.
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Your method should return a String representing the message decryption with the most English words.
In the case of a tie, you can pick whichever one you like.
To make this work correctly, you will have to make sure that you put the file web2.txt as well as
SentenceChecker.java in the same directory as your code. Both of these files can be found on MyCourses. To compile your code from the terminal, you should type javac SentenceChecker.java
Cryptography.java. See the instructions in the warm up question if you are having trouble using this
method.
Generating a better code using permutations
Caesar’s method of encryption is clearly insecure with computers. Now we will generate a more robust
code, that is much more difficult to crack.
The first step is to shuffle letters in an array.
Question 4: Shuffling (15 points)
Write a method shuffle that takes an array of characters as input and returns void. This method
should shuffle the elements in the array (remember that arrays are reference types, so any changes you
make to the contents of the array in the method will apply in other methods as well). If you are having
confusion over this, please read the class notes and examples or see a TA or instructor.
One way (and the way you will use on this assignment) to shuffle the characters of the array is by
repeatedly choosing two random elements (more on this below) and swapping them with each other. For
an array of length n, you should use n
4
random swaps.
To generate the two random numbers you will use Java’s Random class. First, import the class, which
is part of java.util.
Then, to create a variable of type Random (a random number generator) you need to write the following
line of code. To generate good random numbers, it is absolutely critical that the below line
only be executed one time throughout your method. Otherwise, you will always generate
the exact same number.
Random generator = new Random(12345);
Note that the number 12345 is not a suggestion. We require that all students use the same seed in order
to simplify the marking process. The seed determines the order of the pseudo-random numbers. You
will notice that if you run your code repeatedly, you will obtain the same sequence of numbers every
time. While this is sub-optimal for generating true random numbers, it is very helpful in writing code
with reproducible behaviour, and in grading assignments. 🙂
Once you have created the Random object, you should use the random number generator to generate a
random number between 0 (inclusive) and n (exclusive) with the method generator.nextInt(n);.
Question 5: Generating a Random Mapping (10 points)
Still in the same class, write a method generatePermutation() which takes no input and outputs a
random char[] . The char[] should be created and filled according to the following rules.
1. It should have size of exactly 26
2. It should contain each upper case English letter once and only once
Hint: You should use the shuffle() method that you wrote previously in order to do this.
Question 6: Encrypting Using a Random Permutation (30 points)
Write a method permuteEncrypt() which takes as input a String input and returns a String which
represents an encoded String. You should do this by first calling the method you wrote in the previous
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section generatePermutation(). Then, for every letter, you can figure out the mapping of the i
th letter,
by figuring out the value of the i
th element in the array.
For example, the letter A is the 0th letter in the alphabet. The character to which A maps is stored in
the 0th element of the returned random char array.
Note that A is considered the 0th letter, B the 1st letter, etc until Z, which is the 25th letter.
Note that the generatePermutation method you wrote returns an array with only uppercase letters
in it. Your method must work on lowercase letters, but it is allowed that the lowercase letters become
uppercase. Any thing that is not a letter to begin with should keep the same value.
If you encrypt this way, it is extremely difficult to guess the code. Spiritual Growth Question (not for
credit): How many possible encryptions are there in the first method of encoding using shifts? How
many possible encryptions are there in the random permutation scheme?
What To Submit
You have to submit one zip file with all your files in it to MyCourses under Assignment 1. If you do not
know how to zip files, please ask any search engine or friends. Google might be your best friend with this,
and a lot of different little problems as well.
These files should all be inside your zip.
Cryptography.java
confession.txt – You should write in this file any information that you think is useful for the
TA to mark the assignment. This should include things you were not sure of as well as parts of
your code that you don’t think it will work. Of course, like a confession, this will draw the TA’s
attention to the part of your code that doesn’t work, but he/she will probably be more lenient
than if he/she has to spend a lot of time looking for your error. It demonstrates that even though
you couldn’t solve the problem, you understand roughly what is going on.
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