One time pads. Key distribution is done out of band. For eg: n-th character of prime numbered paragraphs in the book Bleak House by Dickens is the key. True t

One time pads. Key distribution is done out of band. For eg: n-th character of prime numbered paragraphs in the book Bleak House by Dickens is the key. True to its name, never reuse the one-time-pad between messages.