A puzzle, spies ... and a beheading (2)

Breaking the system

Elinor has of course been getting to the post first, steaming open the envelopes, getting the key and notebook, reading the message (and for the last one adding her own note). She then seals them back in the envelopes and leaves them for Amy. A similar thing happened to betray Mary Queen of Scots to her cousin Queen Elizabeth I. Read on.

Is there a better way? Emma suggests a solution using the notebooks and keys in a similar way, but in which no keys are posted anywhere. To prove her method works, she sends a secret message to Amy, that Elinor fails to read. How does she do it? See if you can work it out before reading on...and what is the link to computer science?