Encryption algorithm for an app I have created an encryption algorithm for this app I am currently in the process of developing. I was wondering if my code looks "sharp", concise, and most importantly, succinct. I was also wondering if it is secure. 

I know that 99% of the time, encryption algorithms that are invented are not secure, however I was wondering if this algorithm is in the 1% of the safe ones.

#Key Size: 128 bytes [recommended]

import random
import os
import time
import hashlib

def generate_key(key_size):
    key = os.urandom(key_size)
    return key;

def encrypt(_key, message):
    letter_ascii = []
    letter_ascii_final = []
    ascii_key_value = ''.join(str(ord(x)) for x in str(_key))
    for letter in message:
        letter_ascii.append(ord(letter))
    for _letter in letter_ascii:
        added = int(ascii_key_value) + int(_letter)
        letter_ascii_final.append(str(added))
    return letter_ascii_final;

def decrypt(_key, message):
    message_repieced = []
    ascii_key_value = ''.join(str(ord(x)) for x in str(_key))
    for item in message:
        _item = chr(int(item)-int(ascii_key_value))
        message_repieced.append(str(_item))
    return ''.join(message_repieced);
No. This is just a worse Caeser cypher.

steps to break:


Convert each string to an int in the output 
Subtract everything from the min output value - ord('A')
Everything is now between 0 and 26
If the message is readable, you are done (this will be true if the message contains an 'a')
Otherwise, add 1 to the each character
After repeating this between 1 and 24 times, the message will be decrypted.