Beta Decay Caesar

# Changes from version 6
#
# - More changes making the program more user friendly

# Import random and time module
import random, time

# offset() function
def offset(key):
    # Set the offset to 0
    offset = 0

    # Iterate through the key
    for i in range(8):
        # Get ASCII code of key[i]
        code = ord(key[i])

        # Add ASCII code of key[i] to offset
        offset += code

    # Divide offset by eight, round it down and subtract 32
    offset = int(offset/8) - 32

    # Return offset
    return offset

# encrypt() function
def encrypt(word):
    # Generate key
    key = ""

    for i in range(8):
        # Generate random number
        rnum = random.randint(33,126)

        # Convert rnum to character
        char = chr(rnum)

        # Add to key
        key += char

    # Output the key
    print("\nThe eight character key is: ", key)
    print("Write this down for when you decrypt the message")

    # Wait for three seconds
    time.sleep(3)

    # Call offset()
    oset = offset(key)

    # Encrypt message
    encrypted = ""

    for i in range(len(word)):
        # Check if word[i] is a space
        if word[i] == " ":
            # Leave space unchanged
            encrypted += " "

        else:
            # word[i] is not a space
            # Get ASCII code of word[i]
            letter_code = ord(word[i])

            # Add offset to letter_code
            letter_code += oset
            # Is letter_code greater than 126?
            if letter_code > 126:
                # Take 94 off letter_code
                letter_code -= 94
            else:
                # If not, do nothing
                pass

            # Convert letter_code to a character
            letter = chr(letter_code)

            # Add letter to encrypted
            encrypted += letter

    # Return encrypted message and the key
    return encrypted, key

def decrypt(file_name, key):
    # Open and read file
    file = open(file_name, "r")
    word = file.read()
    file.close()

    # Call offset()
    oset = offset(key)

    # Decrypt message
    decrypted = ""

    for i in range(len(word)):
        # Check if word[i] is a space
        if word[i] == " ":
            # Leave space unchanged
            decrypted += " "

        else:
            # word[i] is not a space
            # Get ASCII code of word[i]
            letter_code = ord(word[i])

            # Subtract offset from letter_code
            letter_code -= oset

            # Is letter_code less than 3?
            if letter_code < 33:
                # Add 94 to letter_code
                letter_code += 94

            # Convert letter_code to a character
            letter = chr(letter_code)

            # Add letter to decrypted
            decrypted += letter

    # Return decrypted message
    return decrypted

# Start

# Get user choice
choice = input("Do you wish to 'encrypt' or 'decrypt' the message or 'quit' the program? ").lower()

if choice[0] == "e":
    # User chose to encrypt the message
    # Get the filename of the message to be encrypted
    file_name = input("\nEnter the name of the file containing the message to be encrypted. ")

    # Check if user has added .txt or not
    if ".txt" != file_name[-4:]:
        # The last four characters in file_name != ".txt"
        # Add ".txt" to file_name
        file_name += ".txt"

    # Open and read file
    file = open(file_name, "r")
    word = file.read()
    file.close()

    file_length = len(word)

    if file_length > 500:
        print("File is too long (greater than 500 characters)")
        print("\nPlease reduce the length of the file or split the file into")
        print("multiple files before trying again.")

        input("\nPress enter to exit")
        exit()

    # Check if user wishes to use extra encryption
    extra = input("Do you wish to use 'extra encryption'? (y/n) ").lower()

    if extra == "y":
        # Loop through word, stripping the spaces
        nspaces = ''

        for i in range(len(word)):
            # Check if word[i] is not a space
            if word[i] != " ":
                # Add word[i] to nspaces
                nspaces += word[i]

        # Empty word
        word = ''

        # Loop through nspaces, splitting it up into five letter words

        for i in range(len(nspaces)):
            # Add nspaces[i] to word
            word += nspaces[i]

            # Is i a multiple of 5?
            if (i+1)%5 == 0:
                # Add a space
                word += " "

        # Call the encrypt() function
        encrypted, key = encrypt(word)
    else:
        # Call the encrypt() function
        encrypted, key = encrypt(word)

    # Print the encrypted word
    print("\nEncrypted message")
    print("=================")
    print(encrypted)

    # Get the name of the file to which the ciphertext should be saved
    file_name = input("\nEnter the name of the file where you wish to save the encrypted word. ")

    # Check if user has added .txt or not
    if ".txt" != file_name[-4:]:
        # The last four characters in file_name != ".txt"
        # Add ".txt" to file_name
        file_name += ".txt"

    # Open file for writing to
    file = open(file_name, "w")

    # Write the ciphertext to the file
    file.write(encrypted)

    file.close()

    # Check if file has saved correctly
    # Run decrypt function on contents of file
    savecheck = decrypt(file_name, key)

    # Check if savecheck is equal to word
    if savecheck == word:
        # Encryption works, save successful
        print("File has saved successfully")
    else:
        # Either encryption does not work or write to file was unsuccessful
        print("File save was unsuccessful. Please run the program again.")

elif choice[0] == "d":
    # User chose to decrypt the message
    # Get the filename of the message to be decrypted
    file_name = input("\nEnter the name of the file containing the message to be decrypted. ")

    # Check if user has added .txt or not
    if ".txt" != file_name[-4:]:
        # The last four characters in file_name != ".txt"
        # Add ".txt" to file_name
        file_name += ".txt"

    # Get key from user
    key = input("\nEnter the 8 character key used to encrypt the message. ")
    # Call the decrypt() function
    decrypted = decrypt(file_name, key)

    # Output the decrypted message
    print("\nDecrypted message")
    print("=================")
    print(decrypted)

# Otherwise, user chose to exit
# Get user to press enter before exiting
input("\n\nPress enter to exit ")

# End