where only a few chosen words
are significant. This could consist of the first letter after every comma, the fourth letter in every seventh word in a series of paragraphs, or the next to last word after the definite article
in each section. All of the surrounding words and letters are meaningless null
characters, whose purpose is to make the openly seen text appear natural to the enemy.
Often considered to be a code or cipher, null ciphers really transcend both categories. It is make up of a chosen series of letters and combinations thereof, like a cipher, but they are neither transposed nor substituted. When decoded, these letters combine to form the full message. The use more resembles a code function than a cipher, though. Either way, it is definitely a form of steganography.
Because the volume of mail was so large, 20th century wartime postal censors were presented with the hard task of trying to catch suspicious wording that could contain a null cipher. The presence of contrived or stilted phrasing was quite often the best clue, and placing suspicious words in an adjacent order often yielded some results.
Here is an example, taken from Codes, Ciphers, & Other Cryptic & Clandestine Communication, by Fred Wrixon:
Inspector number five
detaches the new
forms found with
this shipment. Stop.
Acknowledge earliest opportunity
the first new
contract you receive
from their courier
The plaintext order, strike now, becomes clear in a pattern using the third letter of every third word.
Let's take a look at a real world example, though. Here's a dispatch the Germans tried to hide inside of a press cable during World War I:
President's embargo ruling should have immediate notice.
Grave situation affecting international law.
Statement foreshadows ruin of many neutrals.
Yellow journals unifying national excitement immensely.
Taking the first letter from each word, the message becomes more clean:
Pershing sails from N.Y. June 1.
Even Arthur Conan Doyle employed a null cipher in his story "The Gloria Scott", Sherlock Holmes' first case ever. Victor Trevor, a colllege buddy of Holmes' asks for his help after his father suffers a fatal stroke, presumably due to receivng a strange and disturbing note:
The supply of game for London is going steadily up.
Head keep Hudson, we believe, has been now told
to receive all orders for fly paper and for
preservations of your hen-pheasant's life.
After only a couple of tries, old reliable Holmes came up with our solution: The game is up. Hudson has told all. Fly for your life. This decoding led to his first success.
Today, null ciphers are not really used as a trusted military form of secure communication, but they're definitely still popular among your local junior high school students.