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I'll probably take it on the chops for this but here's your encryptor/decryptor, tested and working in 53 lines...
(This is your one and only freebie... from here follow the 4 steps: Analyse -> Plan...

OK... What you need to do is step away from your keyboard for a bit and actually think about what you are being asked to do...

Think about it step by step... don't think about code or writing...