Web servers and security:
	I haven't taught CS475 for a while
		Fall 23 I think
	It's not on the menu for Spring

So let's talk about how to secure a server in this class

Encryption basics:
	We won't dwell on the historical ciphers too much
	Basic ideas:
		substitution
		keys
		permutation
		blocks
		modes of operation
		differential cryptanalysis
	We want to *completely* change the output given:
		A 1-bit difference in the input block
		A 1-bit difference in the key
	That might require a number of rounds

We can invent one easy enough
	We'll make sure to include all the ideas above
	Must be reversable
		xor is easy to reverse
		addition and subtraction are ok too
		multiplication could cause loss of overflow
			Not a problem if we overflow by 1 bit with addition
		division can lose the reminder
	An idea:  apply a complicated operation to the key
		Then xor the result with our block
	AES operating modes

xor, undo, and three-padlock exchange