Posted by Dave W on May 06, 2001 at 09:15:58 from 194.222.52.34:
The idea of secure communications, by which I mean only the intended recipient being able to receive or understand the message, occurs in quite a few of the books. In Swallowdale: the message hidden inside the arrow shaft; in Winter Holiday: the visual signalling scheme with signal boards (requiring knowledge of the code book to decode the meaning); in Pigeon Post : the pigeons carrying messages only to the home base; in PM the letter-box in the wall (behind a loose stone).
To me, the more advanced forms are given in Missee Lee, with Captain Flint's SOS hidden inside the bag of Parrot Food (the message itself having similarity to the stick-men semaphore message occuring in a Sherlock Holmes story: The Adventure of the Dancing Men).
Plus of course, the very elegant scheme using two-tone whistling:
In ML Chapter 7, the scheme is first described "... the man was fingering a queer instrument like two bamboo flutes joined together...the man put the instrument to his lips and began whistling on it, a queer high pitched, ear splitting whistling on two notes.
'He's signalling', said Roger.
And then later in Chapter 13, Miss Lee explains. "Miss Lee smiled proudly. 'When I was in England I was a Girl Guide,' she said. She tapped with her fingers, giving the call up sign in Morse. ' No teleglath in Chinese. so I made a signal code for Thlee Islands men... We can talk from Dlagon Island to Tiger or Turtle and no one knows what is said, only the whistlers... I taught twelve men English letters so they could whistle messages..."
This seems to suggest that rather than the original message in Chinese being translated into an English Text at the sending end, and then back into Chinese at the receiving end, it seems more likely that the English letters are the basis for a corresponding message in Chinese in their code book, exactly as the symbols in Winter Holiday corresponded to a message known only to the holders of the WH code book.
This would allow the security to be maintained in the long term by changing the code book every so often, without having to change the signalling scheme. This also offers the security advantage that the whistlers themselves could be ignorant of the message meaning and therefore not be in a postion to give anything away, although according to Miss Lee the whistlers did know the meaning.
Apart from the similarity with the "touch-tone" buttons on modern telephones which generate two or more tones for each button, the ML whistling scheme also shows some similarities with that developed later (probably in the 1950s) for secure Radio Communications of diplomatic messages, devised by the Diplomatic Wireless Service of the British Foreign Office. Owing to its characteristic sound it was known as "Piccollo".
A short description can be found at this web site here