# Cryptography with Python ## History of Cryptography

The word “Krypto” means “hidden” and “Graphene” means “writing”. So, the word means “hidden writing”. The art of Cryptography can be traced back to as old as the Roman and Egyptian civilizations. The Egyptians would use hieroglyph codes to send secret messages on behalf of their kings. During World War, Morse Code ciphers were used by Germany and US to send their secret messages.

## What is Cryptography?

Cryptography is a technique of securing information and communications through the use of codes so that only those people for whom the information is intended can understand it and process it. Thus, preventing unauthorized access to information. One of the techniques to encrypt the text is to replace the original letters with letters using some set pattern which as a result encrypts the text.

Coding enhances problem-solving skills while making our life easier. At Skoolofcode, students use Python as a programming language for cryptography, making things easier for those who wanted to hide the original text. It was not only less time-consuming but also more efficient. Cryptography with Python is a simple code and you can easily do it with your kids. Let’s see how it’s done.

## Algorithm for Cryptography with Python

Build the program where user-entered text is encrypted with python.

1. Make a list of all the alphabet.
2. Create a function that takes the text and a number as a parameter.
3. Move through each element.
4. If it’s a space add it to the new list as it is.
5. Take out the position of the character it should replace with.
6. Join.
7. Display the encrypted text.

## Code for Cryptography with Python

Now that we understand the algorithm. Let’s code to encrypt text with python.

Let’s make a list of all letters.

``````letters = ['a', 'b', 'c', 'd', 'e', 'f', 'g', 'h', 'i', 'j', 'k', 'l', 'm', 'n', 'o', 'p', 'q', 'r', 's', 't', 'u', 'v', 'w', 'x', 'y', 'z']

``````

Create a variable with space

``playing = ''``

It’s now time to define the function with parameters

``````def encrypt(str, n):

``````

Create a new empty list inside the function.

result = [ ]

To move through the text element by element, we would use for loop.

``for x in str:``

We would now need to check if the element is space. If it is a space then append the result list as it is.

``````if x == " ":
converted = " "
result.append(converted)
``````

else we shall calculate the position of the character that we should replace with the original text and then append the result list. index() is an inbuilt function in Python, which searches for a given element from the start of the list and returns the lowest index where the element appears. For example, there is a list of fruits= [“banana”, “strawberry”, ”cherry]. We want to know the position of the cherry. In this case we can write: y=fruits. index(“cherry”). If I print the value of y here I would get 2. Which is the index value of cherry.

We would use this function and pass the element in x and look for the position in the letters list, that is letters. index(x). We are further adding the number entered by the user to this index. Therefore, we get the index of the character that should replace the element. `(letters.index(x) + n). Now, this would work fine until the number is so added that the index goes out of the range. Say for example the original text had the element x and the number added is 3. The original index of x is 24 when 3 is added it makes it 27. Index 27 is not in the letters list and would give us an error. Also, we want that in such a case x should be replaced by a. Thus, we would use modules and save it as the converted variable.`

``converted = (letters.index(x) + n) % 26``

Next, is to append the result list using the new index. Code shall be-

``````else:
converted = (letters.index(x) + n) % 26
result.append(letters[converted])
``````

Picking up letters of the mentioned positions from the result list to the final text and printing it.

``````final = ''.join(result)
print(final)
``````

Now, that we have created the function. We will ask for the input of the text that we should encrypt with python. Also, ask for the number of the shift in letters that the user wants, and lastly, call the function using the text and number entered.

``````str=input("Enter the string to be encrypted")
n=int(input("Enter the key "))
encrypt(str, n)
``````

Say for example; The user enters the text – “hide me from others” and I want the text to be encrypted such that h is replaced by k and so on. The user would therefore enter 3 for the number. The text now becomes “klgh ph iurp rwkhuv”.

Similarly, we can create a decrypt function that can be used by the user at the other end to get the original text back. For that, the only change we would do is subtract the number from the index this time to get the index of the original character. We will have to create the final encrypted text as global so that we can later pass the text to the decrypt function.

## CODE

``````letters=[‘a’,’b’,’c’,’d’,’e’,’f’,’g’,’h’,’i’,’j’,’k’,’l’,’m’,’n’,’o’,’p’,’q’,’r’,’s’,’t’,’u’,’v’,’w’,’x’,’y’,’z’]
playing = ’ ’
final = ‘ ‘
org = ‘ ‘
def encrypt(str,n):
result = []
for x in str:
if x = = “ “:
converted = ‘  ‘
result.append(converted)
else:
converted = (letters.index(x) +n) %26
result.append(letters[converted])
final = ‘ ‘.join(result)
print(final)
return final

def decrypt(str,n):
back = []
for x in str:
if x = = “ “:
original = ‘  ‘
back.append(original)
else:
original = (letters.index(x) -n) %26
back.append(letters[original])
org = ‘ ‘.join(result)
print(org)

str=input(“enter the string to be encrypted”)
n=int(input(“enter the key”))
final=encrypt(str,n)
print(“let’s decrypt the text”)
decrypt(final,n)
``````

## CONCLUSION

The simple algorithm shown above can be modified to make the code difficult to crack. The ciphers used today, are a combination of mathematical rules and hash functions. So, to introduce Cryptography to kids, it’s good to start with simple algorithms.  You are all set now to code and encrypt text with python. Happy Coding !!

Are you seeking introductory or advanced online python for kids coding classes? At SkoolofCode, we offer project-based learning modules where students use live code to find logical and inventive solutions to problems. So, why wait and Book a FREE trial class today.

By – Divya Dalal, an educator working with SkoolofCode taking Scratch and Python classes. She has done MCA and Masters in Technology in Software Engineering