melekncci / Text-Encryption-and-Decryption

Text-Encryption-and-Decryption

Introduction In cryptography, Caesar's cipher is one of the simplest and most widely known encryption techniques. The action of a Caesar cipher is to replace each letter within a text with a different letter, using a fixed number of places down the alphabet [1]. For example, to encrypt a given text, we can use a left shift of three characters, so that (for example) each occurrence of the letter E in the text becomes B in the encrypted text. This transformation can be represented by aligning two alphabets : the original plain alphabet, and the cipher alphabet. The cipher alphabet is the plain alphabet shifted left by some number of positions n. This number of positions (n=3 in the previous example) plays the role of an encryption/decryption key. Plain Alphabet : ABCDEFGHIJKLMNOPQRSTUVWXYZ Cipher Alphabet : XYZABCDEFGHIJKLMNOPQRSTUVW To encrypt a message using a given key (e.g. n=3), a person looks up each letter of the message in the Plain Alphabet and writes down the corresponding letter in the Cipher Alphabet. Example: Clear Message: « THE QUICK BROWN FOX JUMPS OVER THE LAZY DOG » Encrypted message: « QEB NRFZH YOLTK CLU GRJMP LSBO QEB IXWV ALD » To decrypt the encrypted message, we repeat the same process in reverse (i.e., with a right shift of 3 positions). NOTE: some built-in Python functions that may be useful for this mini-project are ord(…) and chr(…). Check out, for example, the following links (or other links) to learn about these built-in functions:

• https://www.w3schools.com/python/ref_func_ord.asp
• https://www.w3schools.com/python/ref_func_chr.asp
• https://www.geeksforgeeks.org/ord-function-python/ Project Description

1. Encryption function First, you are asked to define a function encrypt(msg, n) that takes as arguments a text msg (as a string) and a key n (as an integer number). This function is supposed to shift all the characters in the text message to the left by n positions, and return a new encrypted message. If msg contains any characters that are not part of the alphabet (such as spaces etc.), then you can keep them unchanged (i.e. no need to encrypt them). Once your function encrypt(message, n) is defined, you can test it on some text (e.g. "THE QUICK BROWN FOX JUMPS OVER THE LAZY DOG") and check if it returns the expected encrypted text (e.g. "QEB NRFZH YOLTK CLU GRJMP LSBO QEB IXWV ALD" if you chose n = 3).
2. Decryption function Second, you are asked to define a function decrypt(encrypted_msg, n) that takes as arguments an encrypted text encrypted_msg (as a string) and a key n (as an integer number). This function is supposed to decrypt the text message by shifting all the characters of the encrypted message to the right by n positions, and return the decrypted message. If encrypted_msg contains any characters that are not part of the alphabet (such as spaces etc), then you can keep them unchanged (i.e. no need to decrypt them). Once your function decrypt(encrypted_msg, n) is defined, you can test it on some text (e.g. "QEB NRFZH YOLTK CLU GRJMP LSBO QEB IXWV AL") and check if it returns the correct decrypted text (e.g. "THE QUICK BROWN FOX JUMPS OVER THE LAZY DOG" if you chose n = 3).
3. Reading / Writing to text a file In this part, you are asked to write a code to read each line from a text file named clearText.txt, encrypt the line by calling the encrypt function that you defined previously, then write (append) the encrypted line to another file encryptedText.txt. To read from an existing file in Python you first need to open the file in read mode using the function open("clearText.txt", "r") (note the second argument "r" which stands for "read mode") and then read the lines from the file by calling the function readlines() as follows: f1 = open("clearText.txt", "r") lines = f1.readlines() The variable lines consists of a list of strings (corresponding to lines). So, you can iterate over each line in lines using a for loop. To append an encrypted line to a file (i.e. write the line without overwriting the previous content), you can open the file in append mode and then write to the file by calling the function write(..) as follows: f2 = open("encryptedText.txt", "a") lines = f2.write(encrypted_line)
4. Letter frequency analysis The encryption method that you implemented previously is not very secure. It is possible to find the original text from the encrypted text by performing a process called frequency analysis. The reason why this kind of encryption is not secure, is because each letter in the original text is always encrypted the same way. For example, the most common letter in the English language is "E" and it might always be encrypted as "B" for example, so if a hacker finds that "B" is the most common letter in the encrypted text then he can assume it represents "E". From this, the hacker can deduce that the original text has been encrypted by shifting letters to the left by 3 positions. Now, that he knows the key (i.e. 3), he can easily decrypt the text. This process can be carried out for all letters, and it is called frequency analysis. A first step to perform frequency analysis is to count the frequency (or number of occurrence) of each letter in the encrypted text. In this part of the project, you are asked to write a function that computes the number of occurrences of each letter of the alphabet in the encrypted text. The output should correspond to something like the following (of course, the numbers will depend on the text that you use) :

A occurs 29 times in the encrypted text

B occurs 15 times in the encrypted text

C occurs 18 times in the encrypted text

... etc ...

Z occurs 7 times in the encrypted text

[1] Caesar_cipher, https://en.wikipedia.org/wiki/Caesar_cipher