Question

1. Using Python, Implement a decryption function: Implement a function named decrypt that opens a file named input_file and decrypts its content using the opposite of the dictionary created using a key (which is a parameter of the function) and stores the decrypted file to a file named output_file. The input and output file names will be positional parameters. The key parameter should be a keyword-only optional parameter where the default value is “IT RAINS A LOT IN MILWAUKEE IN THE WINTER, BUT SOMETIMES IT SNOWS." This function should create the substition dictionary using the create_substition_dic() function and create the opposite of the substitution dictionary using the opposite() function then use this opposite dictionary to generate the decrypted text.

For instance, for a file where the content is ONKKB123 the output file content will be HELLO123 (using the default key).

Answer 1

thanks for the question, here is the complete python program. Let me know in case you have any doubts or question.

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import string
# function that returns the dictionary given a sentence
def substitution_code(sentence):
    alphabets_only=''
    for letter in sentence.upper():
        if letter in string.ascii_uppercase:
            alphabets_only+=letter
    for uppercase_ascii in range(65,91):
        alphabets_only+=chr(uppercase_ascii)
    unique_letter_list=[]
    key=''
    for letter in alphabets_only:
        if letter not in unique_letter_list:
            key+=letter
            unique_letter_list.append(letter)

    substitution_dict={}
    for i in range(0,26):
        substitution_dict[chr(i+65)]=key[i]
    return substitution_dict

# function reverses the key value to value key pair
def reverse_dict(key_dict):
    reverse_dictionary={}
    for key,value in key_dict.items():
        reverse_dictionary[value]=key
    return reverse_dictionary


# function takes in inputfile, outputfile and key_dictionary
# reads from the encrypted file and writes to the output file using the key dictionary
def decrypt(input_file_name,output_file_name,key_dict):

    reverse_dictionary=reverse_dict(key_dict)
    with open(input_file_name,'r') as infile:
        with open(output_file_name,'w') as outfile:
            for line in infile.readlines():
                write_line=''
                for letter in line.upper():
                    if reverse_dictionary.get(letter) is None:
                        write_line+=letter
                    else:write_line+=reverse_dictionary[letter]
                print(write_line)
                outfile.write(write_line)

def main():
    key_dict=substitution_code('IT RAINS A LOT IN MILWAUKEE IN THE WINTER, BUT SOMETIMES IT SNOWS.')
    input_file_name='D:\\plain_text.txt' # encrypted file that needs to decrypted
    output_file_name='D:\\decrypt.txt' # output file which contain the original text
    decrypt(input_file_name,output_file_name,key_dict)
main()

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