Architecture-cw1

demo Monday 4 Nov
demo Monday 4 Nov
name cw1
three command line arguments
	- int 1st: 0 = encryption, 1 = decryption 
	- string 2nd: first private key
	- string 3nd: second private key
	./cw1 0 keystring1 keystring2

# mod is the operation of finding the remainder when dividing 2 numbers
passed to the program using standard input
1. preprocessing
	1.1 ensure 2 keys' length are co-prime(GCD of keystring1 keystring2 = 1)
		(if not)error message: Key lengths are not co-prime. ->> exit
	1.2 upper case to lower case
	1.3 strip all non-English alphabet characters and whitespaces
2. encryption
	n = key1 + key2
	t = n - 4
	(x - key1 - key2 + 4) mod 26
	= (x - (key1 + key2) + 4) mod 26
	= (x - n + 4) mod 26
	= (x - (n - 4)) mod 26
	= (x - t) mod 26

3. decryption
	(x + key1 + key2 + 22) mod 26
	= (x + key1 + key2 + (26 - 4)) mod 26
	= (x + key1 + key2 - 4) mod 26
	= (x + n - 4) mod 26
	= (x + t) mod 26