repetitions-vigenerecypher/README-en.md

Repetition Structures - Vigenere Cipher

One of the advantages of using computer programs is that we can easily implement repetitive tasks. Structures such as the for, while, and do-while allow us to repeat a block of instructions as many times as needed. In this lab experience you will use for loops to complete a simple ciphering application.

Objectives:

1. Apply repetition structures to cipher a plain-text message.
2. Practice character arithmetic.

Pre-Lab:

Before coming to the lab session you should have:

1. Reviewed basic concepts related to repetition structures.

2. Reviewed the string class in C++. In particular, how to create string objects and the length, toupper and push_back methods. Also review the isalpha function and character arithmetic.

3. Watched Khan Academy’s “Ceasar Cypher” video at https://www.youtube.com/watch?v=sMOZf4GN3oc .

4. Watched Khan Academy’s “Vigenere Cypher” video at https://www.youtube.com/watch?v=9zASwVoshiM .

5. Studied the concepts and instructions for the laboratory session.

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Criptography

Cryptography is the area of knowledge that studies the theory and methods that are used for protecting information so that non-authorized persons cannot understand it. A cryptographic system is a system that transforms a cleartext message (a message that is understandable to humans) to a cyphered text (text that is unintelligible to unauthorized persons). Authorized persons can decypher the cyphered text to obtain the original cleartext.

The Ceasar Cypher

The Caesar Cypher is a substitution encryption technique that is said to have been used by Julius Caesar (100 BC - 44 BC), the Roman political and military leader, to communicate with his generals. To encrypt a message using the Caesar Cipher each letter of the cleartext message is substituted by the letter found a given number of positions ahead in the alphabet. You may think of this as a shift of the letter in the alphabet. Figure 1 illustrates a shift of 3 spaces within the alphabet. For instance, letter ‘B’ would be substituted by letter ‘E’.

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Figure 1. Caesar Cypher with shift of three.

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Example 1. With a shift of three, the word “ARROZ” is ciphered as “DUURC”. Observe that, with the shift of 3, letter ‘Z’ is ciphered as letter ‘C’. Any shift that takes us beyond letter ‘Z’ starts again at letter ‘A’. This is called cyclic shift. Figure 2 illustrates a circular shift of eight positions.

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Figure 2. A Caesar cipher disk showing a shift of 8 positions.

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Using the modulo operator

Modular addition is essential for implementing ciphering systems in programming. Let’s say that you map every letter of the (English) alphabet to a number between 0 to 25 (‘A’ is 0, ‘B’ is 1, .., ‘Z’ is 25). To Caesar cipher a letter we convert the letter to its corresponding number in the range [0,25], then add the displacement. We then apply the modulo 26 operation to the total. The letter that corresponds to the result after the modulo is the ciphered letter. The effect of the circular shift is achieved by using the modulo 26 operation because it converts the result of the addition to an integer in the range [0,25]. For instance, if we were to shift the letter ‘Z’ by three positions, this would be performed by adding ( 25 + 3 ) % 26 which equals 2, whose corresponding letter is ‘C’.

To convert an uppercase letter to a number in the [0,25] range we can apply our knowledge of the ASCII code. The code for the uppercase letters is in the range 65,90. Thus, to convert an uppercase case to a number in the range [0,25], a simple subtraction by 65 does the trick (i.e. ’A’ - 65 = 0, ’Z’ - 65 = 25). Observe that to go from the [0,25] range to the ASCII code of its corresponding uppercase character we simply add 65 to the number. For instance, number 3 corresponds to the letter whose ASCII code is $3 + 65 = 68$, i.e. letter ‘D’.

Figure 3 shows the pseudocode of an algorithm for the Caesar cipher. Each letter ‘c’ in the cleartext message is converted to a number in the range 0,25. The displacement d is then added to the number (modulo 26) . Lastly, the result of the modular addition is converted back to its corresponding letter by adding the ASCII code of ‘A’.

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    Input: msg: a string of the cleartext message, d: the displacement
    Output: Caesar ciphered message 

    1. cypheredText = ""
    2. for each character c in msg:
           c = ascii(c) - ascii('A')  # map c from [‘A’,’Z’] to [0,25]
           c = ( c + d ) % 26         # circular shift c by d positions
           c = ascii(c) + ascii('A')  # map c from [0,25] to [‘A’,’Z’]
           cypheredText.append(c) 
    3. return cypheredText 

Figure 3. Pseudocode for a Caesar cipher algorithm.

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The Caesar cipher is considered a weak encryption mechanism because it can easily be deciphered by using frequency analysis on the ciphered message. For example, we can use the fact that letter ‘e’ is the most frequent letter in most texts. If we find the most frequent letter in a ciphered text it very probably corresponds to the letter that was substituted by ‘e’. With this information we can compute the displacement that was used and decipher the rest of the message.

The Vigenere Cypher

A main weakness of the Caesar cipher is that every letter in the cleartext message is shifted by the same number of positions. The Vignere cipher is a somewhat stronger encryption method because the shift used on each letter is not constant. Whereas the Caesar cipher receives as input a cleartext message and a displacement, the Vignere receives the cleartext message and keyword. For now, lets assume that the keyword and the cleartext message have the same length. The Vignere cipher uses the keyword to determine the shift that will be applied to each letter of the cleartext message, i.e. the first letter of the keyword determines the shift number for the first letter of the message and so forth.

Example 2. Suppose that the cleartext is “PET” and the keyword is “BED”. Each letter in the keyword determines the shift amount of the corresponding letter in the cleartext: letter ‘A’ specifies a shift of 0, ‘B’ is a shift of 1, and so on. Thus, the ‘B’ in the keyword “BED” states that we shall shift the first letter of the cleartext by 1, the ‘E’ states that we will shift second the letter by 4 and the ‘D’ states that we will shift the last letter by 3.

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cleartext	P	E	T
keyword	B	E	D
-----------------	---	---	---
ciphered text	Q	I	X

Figure 4. Letter ‘P’ in the cleartext is shifted by 1 to ‘Q’ as indicated by the corresponding letter in the keyword (B). Letter ‘E’ in the cleartext is shifted by 4 to ‘I’ as indicated by the corresponding letter in the keyword (E). Letter ‘T’ in the cleartext is shifted by 3 to ‘X’ as indicated by the corresponding letter in the keyword (D).

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Figure 5 shows a table that can be used to determine the Vigenere-ciphered letter, given the cleartext and keyword letter. Notice that each row contains the entire alphabet shifted by the amount specified by the letter at the beginning of the row.

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Figure 5. Table for the Vigenere-cipher. The shaded row and column illustrate how to cipher the letter ‘R’ with key letter ‘M’.

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If the keyword is shorter than the cleartext, the Vigenere cipher simply repeats the keyword as many times as needed to account for all the letters of the cleartext. Figure 6, illustrates the keyword and cleartext pairing for an example.

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cleartext	P	R	O	G	R	A	M		T	H	I	S
keyword	S	H	O	R	T	S	H	O	R	T	S	H
-----------------	---	---	---	---	---	---	---	---	---	---	---	---
ciphered text	H	Y	C	X	K	S	T		K	A	A	Z

Figure 6. Alignment of a cleartext with a shorter keyword and the resulting cipher.

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Functions that will be used in this laboratory experience:

The program that you will be modifying in today’s experience uses the following methods of the string class:

• length: Returns the length of a string object, i.e. the number of characters in the string (including invisible characters such as the space and end-line). To invoke the length method of string object write .length() after the object’s name, e.g. msg.length().

• push_back(char c): adds the character passed as argument to the end of the string. For example, the instruction msg.push_back(‘a’) adds character a to the end of an object called msg.

We will also use the following functions:

• char toupper(char c): given a character as an argument, this function returns the character in uppercase. For example, toupper(‘b’) returns B.

• int isalpha(char c): Given a character as an argument, this function returns a non-zero value when the character is a letter. Otherwise it returns 0. As you know, C++ interprets non-zero values as true and zero as false. Thus, for example, the call isalpha(‘3’) returns false. The call returns isalpha(‘f’) returns true.

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!INCLUDE “../../eip-diagnostic/vigenere/en/diag-vigenere-01.html”

!INCLUDE “../../eip-diagnostic/vigenere/en/diag-vigenere-02.html”

!INCLUDE “../../eip-diagnostic/vigenere/en/diag-vigenere-03.html”

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Laboratory session:

You will be completing an application to cipher a message using the Vigenere technique. To simplify coding, the keyword and cleartext must consist exclusively of letters. Furthermore, your program must change both the cleartext and keyword to uppercase before ciphering.

Exercise 1: Keyword and cleartext of the same length

In this exercise you will implement a function that given a cleartext and keyword of equal length returns the cipher message.

Instructions

1. Load the Qt project called VigenereCypher by double-clicking on the VigenereCypher.pro file in the Documents/eip/Repetitions-VigenereCypher folder of your computer. Alternatively you may clone the git repository http://bitbucket.org/eip-uprrp/repetitions-vigenerecypher to download the Repetitions-VigenereCypher folder to your computer.

2. You will be writing your code in the cypher.cpp file. In this file, the cypher function receives a message and a keyword of equal length and that consist exclusively of letters, and returns the Vigenere-ciphered message. Your task is to finish the implementation of the cypher function.

   Using the methods and functions discussed earlier, your implementation must verify if the message and keyword consist exclusively of letters and are of equal length. If that is not the case, the ciphered message must be (literally) "MENSAJE O CLAVE INVALIDO". Remember that your program must change both the cleartext and keyword to upper-case letters before ciphering.

3. After you implement the cypher function, go to the main function and uncomment the line that invokes test_cypher1. The test_cypher1 function is a unit test for the cypher function. It calls the cypher function with various arguments to verify if it returns correct results. If any of the results is incorrect the program stops and reports the test case that failed. You will not see the application’s graphical user interface until the unit test passes all validations. Once you see the graphical user interface you may continue onto the next part of this lab experience.

Exercise 2: Keyword and cleartext of arbitrary lengths

In this exercise you will modify the code for the cypher function from Exercise 1 so that the application can now cipher a message using a keyword of arbitrary length.

Instructions

1. Modify the implementation of the cypher function so that it can cipher a message with a keyword of any (non-zero) length. For this exercise the message may contain any character (including non alphabetical characters). The keyword must consist exclusively of letters.

   Whenever the character in the cleartext is not a letter it will not be ciphered, as seen in Figure 7. If any character in the keyword is not a letter, the ciphered message will be (literally) ”CLAVE INVALIDA”.

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cleartext	P	R	@	G	R	*	M		T	8	I	S
keyword	S	H	O	R	T	S	H	O	R	T	S	H
-----------------	---	---	---	---	---	---	---	---	---	---	---	---
ciphered text	H	Y	@	X	K	*	T		K	8	A	Z

**Figure 7.** Example Vignere cipher of the cleartext `“PR@GR*M T8IS”` using the keyword `SHORT”`. 

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1. After you implement the cypher function, modify the main function so that the line that invokes test_cypher2 is uncommented, and the line that invokes test_cyper1 is commented. Once again, you will not see the app’s graphical user interface until the test_cypher2 verifications.

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Deliverables

Use “Deliverables” in Moodle to upload the cypher.cpp file that contains the cypher function that you created in Exercise 2. Remember to use good programming techniques, include the names of the programmers involved, and to document your program.

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References

http://www.nctm.org/uploadedImages/Classroom_Resources/Lesson_Plans/