Welcome to a world of secrecy, spies and subterfuge. Ciphers and codes have been around since the Romans, and are one of the oldest forms of secret communication.
Cryptography is the science of encoding and decoding secret messages. It is all around us in everyday life, when we send emails to our friends and colleagues, or when we buy something over the internet using a credit card. It is constantly evolving as cryptographers endeavour to break existing codes and create new unbreakable codes.
Although we tend to use the words code and cipher interchangeabley, they actually have quite distinct meanings. Both are a cryptopgraphic system where the original message (the plaintext) is arbitrarily transformed according to a predetermined system.
Typically, a code is an encryption system at the level of words or phrases. So, each word or phrase is assigned a symbol to represent it. So, you might have a codebook like this:
our message, ‘The Prime Minister of England will be assassinated at midnight by a spy’ might be encoded as ‘The green cow will read the book at the supermarket’
On the other hand, in a cipher, the encryption is done at the level of the letter, making it far more difficult to crack, and therefore more secure.
Although modern cryptographic developments such as PGP and RSA are extremely secure, we all know that they are hiding secret information, and so are vulnerable to attack. Steganography (literally ‘covered writing’)is the art and science of hiding messages within seemingly trivial places. This is the stegotext. The ancient Greek historian Herodotus writes of slaves sent as messengers who had their heads tattooed with secret information warning the Greeks of Persian invasion plans. The slaves’ hair would then grow back on the long journey and the message was only revealed when their heads were shaved!! The Greeks also carved messages into wooden tablets and then covered them with a thin layer of wax to make them look as if they were new and unused tablets.
Nowadays, it works by replacing bits of unused data in computer files with the secret information, often securely encrypted beforehand. Messages can even be encrypted into images, so you could be sending secret information embedded in a picture of you on holiday!
In World War II, the Germans invented the use of microdots to send information securely. These tiny dots were the size of a full stop and contained vital information undetectable to the human eye, such as a meeting time, or an address. Being so small they often went unnoticed by the inspectors and could only be read by the intended recipient with the use of a microscope. British intelligence often referred to microdots as ‘duff’, because the dots would be spread throughout letters like raisins in the English steamed pudding Plum Duff!!
Velvalee Dickinson, a Japanese spy in New York during World War II was a dealer in dolls. She disguised her secret messages in details of doll shipments and repairs, but in fact was alluding to movements of the US fleet. Her letters were interecepted and translated by the FBI and she was caught and imprisoned.