It's a secret (8)

(Continuation from Friday)
Our substitution cipher is an example of what is called a monoalphabetic cipher - which means that the same plaintext letter will always result in the same ciphertext letter - and so will be easily solvable by frequency analysis.  When Mary, Queen of Scots was being held prisoner in the Tower of London she managed to send and receive letters from her supporters outside written in what was believed to be an uncrackable cipher - but although rather than letters, this cipher used esoteric symbols somewhat like Egyptian hieroglyphics which the users believed would make it secure, it was essentially monoalphabetic - the same plaintext letter would always produce the same symbol and so what she was writing and what was being written to her was being easily decrypted by the government of the day, and eventually resulted in her execution.  So the holy grail of cryptographers at that time was to come up with some method which would destroy this inevitable link between plaintext and ciphertext.  And how they went about this is for next time.