A Simple Encryption And Decryption Algorithm In Python

Background

The idea of the code was inspired by a Google Code Jam competition question in 2019 called “Cryptopangrams”. Using the feature of prime numbers to encode and decode the text. The idea is intriguing and I decided to write the encryption and decryption algorithms by myself. You are welcome to leave a comment if you think there is anything could be improved.

Brief Explanation

The main idea is to map the prime number to the ASCII table. Each prime number represents a character. According to the feature that the if you factorise the product of two prime numbers, it can only be the two prime number themselves, which means the there is only one result. Then we can chain up all the text by multiply the numbers next to each other. Finally, a sequence of numbers is the encoded ciphertext.

Python3

encoder

import random
#encryption the code according to this random number
ran = random.randint(6666,9999)

#Find all the prime numbers less than or equal to the input number
#INPUT:    maximum -- a random integer
#OUTPUT:   primes -- the largest 200 primes less than or equal to the input
def findPrime(maximum):
    primes = []
    count = 1
    #find primes from the largest to the smallest
    for i in range(maximum+1, 2, -1):
        isPrime = True
        for j in range(2, i//2+1):
            if i%j == 0:
                isPrime = False
                break
        if isPrime:
            primes.append(i)
            if(count == 128):
                return primes
            count += 1
    return primes

keys = findPrime(ran)

#generate mapping for ascii table
d = {}
#mapping
for i in range(128):
    d.update({i : keys[i]})

#Decode the encrypted text
#INPUT:    plainText -- the decrypted text
#OUTPUT:   ciphertext -- the encrypted text
def encode(plainText):
    if plainText == "":
        return ""
    #convert string to a list of chars
    charlist = list(plainText)
    #the first number is the largest key's square
    ciphertext = str(keys[0] ** 2)
    #the second number is the square of the key of the first letter
    ciphertext = ciphertext + " " + str(d.get(ord(charlist[0])) ** 2)

    for i in range(len(charlist)-1):
        ciphertext = ciphertext + " " + str(d.get(ord(charlist[i])) * d.get(ord(charlist[i+1])))
    return ciphertext

#read file
print("Which text file do you wannt to encrypt?")
print("TIP: File type must be in .txt format")
choose = True
while(choose == True):
    try:
        filename = input()
        with open(filename, 'r') as rfile:
            data = rfile.read()
            rfile.close()
            choose = False
    except IOError:
        print("Read File error, Please retry\n")

#write file
print("\nPlease type output encrypted file name:")
print("TIP: Please include \".txt\" at the end")
choose = True
while(choose == True):
    try:
        filename = input()
        with open(filename, 'w+') as rfile:
            rfile.write(encode(str(data)))
            print("data has been successfully encoded")
            choose = False
    except IOError:
        print("Write File error, Please retry\n")

decoder

import sys
#Find all the prime numbers less than or equal to the input number
#INPUT:    maximum -- a random integer
#OUTPUT:   primes -- the largest 200 primes less than or equal to the input
def findPrime(maximum):
    primes = []
    count = 1
    #find primes from the largest to the smallest
    for i in range(maximum+1, 2, -1):
        isPrime = True
        for j in range(2, i//2+1):
            if i%j == 0:
                isPrime = False
                break
        if isPrime:
            primes.append(i)
            if(count == 128):
                return primes
            count += 1
    return primes

#Decode the encrypted text
#INPUT:    ciphertext -- the encrypted text
#OUTPUT:   plainText -- the decrypted text
def decode(ciphertext):
    if ciphertext == "":
        return ""
    plainText = ""
    splited_text = ciphertext.split(" ")
    try:
        map_key = int(int(splited_text[0]) ** 0.5)
    except Exception as Exc:
        print("Error: Wrong file content, cannot be decoded!!")
        print("Terminating program!! and destory file content!!")
        sys.exit()
    keys = findPrime(map_key)
    try:
        firstLetter_key = int(int(splited_text[1]) ** 0.5)
    except Exception as Exc:
        print("Error: Wrong file content, cannot be decoded!!")
        print("Terminating program!! and destory file content!!")
        sys.exit()
    #generate mapping for ascii table
    d = {}
    #mapping
    for i in range(128):
        d.update({keys[i] : i})

    plainText = plainText + str(chr(d.get(firstLetter_key)))
    currentLetter_key = firstLetter_key
    for i in range(2, len(splited_text)):
        try:
            nextletter_key = int(splited_text[i]) // currentLetter_key
        except Exception as Exc:
            print("Error: Wrong file content, cannot be decoded!!")
            print("Terminating program!! and destory file content!!")
            sys.exit()
        plainText = plainText + str(chr(d.get(nextletter_key)))
        currentLetter_key = nextletter_key
    return plainText

#read file
print("Which text file do you wannt to decrypt?")
print("TIP: File type must be in .txt format")
choose = True
while(choose == True):
    try:
        filename = input()
        with open(filename, 'r') as rfile:
            data = rfile.read()
            rfile.close()
            choose = False
    except IOError:
        print("Read File error, Please retry\n")

#write file
print("\nPlease type output decrypted file name:")
print("TIP: Please include \".txt\" at the end")
choose = True
while(choose == True):
    try:
        filename = input()
        with open(filename, 'w+') as rfile:
            rfile.write(decode(data))
            print("data has been successfully decoded")
            choose = False
    except IOError:
        print("Write File error, Please retry\n")

Run Example

The text file to be encoded.
poem

Run command “python3 encoder.py”

commandline1

ciphertext:

ciphertext

Note

The decoding process is the similar to the encoding process.
A File can be encoded multiply times (of course, if you want to decode it, need to decode it the same times as you encode it)
If the file to be decoded is in the wrong format (cannot be decoded), the original content will be deleted!!! (not very friendly!)

Download

Download encoder.py Download decoder.py