Say I want to write an encryption algorithm to communicate between me and my friend for this private use. How is that bad?
E.g. I can take the word 'Hello'Hello and encrypt it with a simple algorithm i.e. take– for example – take each letter and multiply its value by 2 and add 13 so that it becomes completely unreadable.
Is writing a private encryption that stays between two people discouraged or not? What's wrong with it?