Review:
		hash		- anything in, small thing out; fingerprint
		signature		- hash encrypted by one person and decryptable by all
		key exchange	- share some random numbers to create a shared secret
		encrypt		- give shared secret, communicate soe no one can listen in

	Additional:
		certificate
		salt + pepper
	
	Applications:
		HTTPS
		"login with"
		telephone
		email
		blockchain