Files
004_comission/banson_hker/deliver/shift_cipher_encrypter.py
louiscklaw 72bacdd6b5 update,
2025-01-31 19:28:21 +08:00

197 lines
6.0 KiB
Python

#!/usr/bin/env python
import os,sys
ORD_a = ord('a') # 97
ORD_A = ord('A') # 65
def shift_cipher_encrypt(plaintext, key):
# apply encryption to text with given key
encrypted_message = ''
for char in plaintext:
if char.isalpha():
ascii_offset = ORD_A if char.isupper() else ORD_a # Determine ASCII offset based on uppercase or lowercase letter
# the comment shown below are the pseudo code, it demonstrate the ideas only
# let say the input is 'the' // without quote
# find distance of target character with reference to A or a
# i.e. t - a = 19 , h - a = 7 , e - a = 4
distance = ord(char) - ascii_offset
# [19,7,4] + [8,8,8] (key) = [27, 15, 12]
# Shift the character by adding the key and taking modulo 26 to wrap around
# [27,15,12] % [26,26,26] = [1,15,12] // get modules
shifted_distance = (distance + key) % 26
# [1,15,12] + [97,97,97] = [98,112,109]
# chr(98) , chr(112) , chr(109) = 'bpm'
shifted_char = chr(shifted_distance + ascii_offset)
# so: the -> bpm
encrypted_message += shifted_char
else:
# consider integer case, retain
encrypted_message += char
return encrypted_message
def shift_cipher_decrypt(ciphertext, key):
plaintext = ""
for char in ciphertext:
if char.isalpha():
ascii_offset = ORD_a if char.islower() else ORD_A # Determine ASCII offset based on lowercase or uppercase letter
# Calculate the distance of the target character from a or A
distance = ord(char) - ascii_offset
# apply shift, get the remainder of 26
shifted_distance = (distance - key) % 26
# Convert back to ASCII
decrypted_char = chr(shifted_distance + ascii_offset)
plaintext += decrypted_char
else:
# If it is not an alphabetic character, retain as is.
plaintext += char
return plaintext
def count_letter_e(txt_in):
# reserved function for demonstration purpose
occurence = 0
for char in txt_in:
if char.isalpha():
if char.lower() == 'e':
occurence += 1
return occurence
def count_most_occurrence_letter(txt_in):
# letter e, as stated have the most occurrence in the message by statistics.
# as 'Shift Cipher' is a encryption by letter shifting, the letters have good chance
# to have the most occurrence too in the encrypted text.
output = [0] * 26 # bucket for 26 letters
for char in txt_in:
if char.isalpha():
output[ord(char.lower()) - ORD_a] += 1
# output contains the statistics of paragraph letter by letter
return output
def find_max_occurrence(char_occurrences):
# get the letter of the most occurrences. i.e. m
# by subtract between this letter to e, k can be guess
# find max occurrence and its index
max_idx = char_occurrences.index(max(char_occurrences))
# subtract it with index of e -> 4
return max_idx - 4
def encrypt_file(file_path, key=8):
# open a file and apply encryption
output_file = file_path.replace('.txt','_e.txt')
# convert it to integer
key = int(key)
# open source file (plaintext)
with open(file_path,'r') as fi:
temp = ''.join(fi.readlines())
# open target file (encrypted text)
with open(output_file,'r+') as fo:
fo.truncate(0)
fo.writelines([shift_cipher_encrypt(temp, key)])
print(f'encryption done and file saved to {output_file}')
return
def decrypt_file(file_path):
# will open an encrypted file and decrypt it by a guessed key
with open(file_path,'r') as fi:
# beginning of the process
# read file and join the lines all
lines = fi.readlines()
e_temp = ''.join(lines)
characters_distribution = count_most_occurrence_letter(e_temp)
print('')
print('distribution of letters in encrypted text (case insensitive, from a to z)')
print(characters_distribution)
print('')
guess_k = find_max_occurrence(characters_distribution)
print(f'guessed k: {guess_k}')
print('')
print('decrypted text:')
decrypted_text = shift_cipher_decrypt(e_temp, guess_k)
print(decrypted_text)
while True:
# show menu
print()
print("1. Encrypt File")
print("2. Decrypt File")
print("q. quit")
print()
option = input("Select an option (1/2/q): ")
if option == "1":
# run if user want to encrypt file
# check if user entered a file
user_not_enter_file = True
while user_not_enter_file:
file_path = input("Enter the path of the file to encrypt: ")
if len(file_path) > 0:
if os.path.exists(file_path):
user_not_enter_file = False
else:
print('sorry but the file not exist')
else:
print('please enter a file path')
# check if user entered a key
user_not_enter_key = True
while user_not_enter_key:
key = input("Enter the key(k) to encrypt: ")
if (len(key) > 0):
user_not_enter_key = False
else:
print('please enter a key(k)')
if os.path.exists(file_path):
encrypt_file(file_path, key)
print('encryption done')
else:
print("File does not exist.")
elif option == "2":
# run if user want to decrypt file
file_path = input("Enter the path of the file to decrypt: ")
if os.path.exists(file_path):
decrypt_file(file_path)
print('decryption done')
else:
print("File does not exist.")
elif option.lower() == "q":
print('quitting bye ...')
break
else:
print('')
print('ERROR !')
print('please enter either [1/2/q]')
input("press a key to continue ...")
print('')
print("Exiting...")