block cipher q15 school use only

blockcipher_q15_schooluseonly.py

# block_cipher (q15 of SCHOOL USE ONLY mock exam)
message_to_send = "DigitalSolutions"

##########################################
# 1. create (random) block encryption key:
import random
key_letters = []
key_numbers = []
#to create a random key:
for counter in range(len(message_to_send)):
    number = random.randint(1,26)
    key_numbers.append(number)
print("Random key as numbers:", key_numbers)

#... or to use the keys from mock exam, calculated from the number of
#character ordinals shifted from the decrypted to the encrypted msg:
key_numbers = [4,4,4,9,4,3,8,2,6,11,14,3,1,1,8,5] #from exam question
for index, number in enumerate(key_numbers):
    if message_to_send[index].isupper(): #deal with case
        key_letters.append(chr( (ord("A")-1) + number))
    else:
        key_letters.append(chr( (ord("a")-1) + number))
print("Exam key as numbers:", key_numbers)
print("Key as letters:", key_letters) #not necessary, but easily found

########################################
# 2. encrypt text:
encrypted_letters = []
for index, letter in enumerate(message_to_send):
    if letter.isupper():
        temp = ( ord(letter) - ord("A") + key_numbers[index] ) % 26
        encrypted_letters.append( chr( temp + ord("A") ) )
    else:
        temp = ( ord(letter) - ord("a") + key_numbers[index] ) % 26
        encrypted_letters.append( chr( temp + ord("a") ) )
cipher_text = "".join(encrypted_letters)
print("Ciphertext:", cipher_text)

########################################
# 3. decrypt text:
decrypted_letters = []
for index, letter in enumerate(list(cipher_text)):
    if letter.isupper():
        temp = ( ord(letter) - ord("A") - key_numbers[index] ) % 26
        decrypted_letters.append( chr( temp + ord("A") ) )
    else:
        temp = ( ord(letter) - ord("a") - key_numbers[index] ) % 26
        decrypted_letters.append( chr( temp + ord("a") ) )
decrypted_message = "".join(decrypted_letters)
print("Plaintext:", decrypted_message)

########################################
# Strengths -
# 1) there is an almost infinite number of possible keys because
# of the randomness of each selected shift
# 2) there is no proven method to crack this method since it is
# based on randomness
#
# Weaknesses - 
# 3) it is symmetric in nature and therefore the sender and
# receiver must have a copy of the key
# 4) the key has to be the exact same length or more of the message

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