In BBC’s Sherlock, Season 1, Episode 2 “The Blind Banker”, the Black Lotus crime syndicate uses ancient Chinese number characters to refer to pages and words in a commonly known book (which was currently the A-Z London Guide).
What class of cryptographic functions would this most closely fall under? Is it symmetric/private key encryption? Or asymmetric/public key encryption? Or something else?
My understanding is that the key is the common book, and the Chinese characters are the ciphertext of the original words. Hence this would make it symmetric encryption? But at the same time, the book is publicly accessible so it's not strictly a secret/private key.