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Hidden in Plain Sight

A cipher makes a message unreadable. This makes it invisible. The proper word is steganography, and its oldest rule is that nobody looks for a letter they do not know is there.

A perfectly ordinary letter

Monday was wet again, so we stayed inside.

Everyone is well, and the dog has stopped barking.

Emma says thank you for the book you sent.

Tomorrow we are going to see grandma.

Mum has been baking, which never happens.

Eric is being Eric, as always.

Anyway, I should stop and do my homework.

Tell your brother I said hello.

Next week is half term, which is something.

Oh, and it snowed for about four minutes.

Our cat sat in the snow and was not impressed.

Night now, write back soon.

 

Nothing to break

There is no cipher here. Every word is ordinary and every sentence is true. The message is in where the letters sit, and a person who does not know to look down the left-hand edge will read the whole thing and find nothing.

That is the whole idea of this section. A cipher makes a message unreadable, and announces that a secret exists. Hiding makes the secret invisible, so nobody starts looking.

The grille

Same trick, different tool. Cut windows in a card, lay it over an ordinary letter, and only the words that matter show through. Cardano invented it in the 1550s and spies were still using it four hundred years later.

 

Why it beats a cipher. The letter is not gibberish, so nobody suspects it. It can be posted, read aloud, left on a table.

Why it is risky. Both of you need the same card, and if it is found in your pocket, the letters you have already sent become readable too.

Build a grille
Invisible inkLemon juice, a warm lamp, and a message that was always there Micro-writingLetters too small to notice without a lens Hidden lettersA message written between the lines of an ordinary one AcrosticsRead the first letter of every line and the poem talks back Null ciphersReal words carrying a message in every third letter Hidden compartmentsThe message travels inside something dull
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Hidden writing

Invisible inkLemon juice, a warm lamp, and a message that was always there Micro-writingLetters too small to notice without a lens Hidden lettersA message written between the lines of an ordinary one AcrosticsRead the first letter of every line and the poem talks back Null ciphersReal words carrying a message in every third letter

Physical concealment

Hidden compartmentsThe message travels inside something dull Messages in objectsA hollow heel, a book spine, an egg shell MicrodotsA whole page shrunk to the size of a full stop The ThingA carved wooden seal hung in the American embassy in Moscow for seven years. No battery, no wires, nothing to find

Visual steganography

Anamorphic writingNonsense until you look from the right angle Mirror messagesWritten backwards, read in a mirror Optical illusionsWhat you see depends on what you are looking for Pictures inside picturesOne image hiding in the grain of another

Grilles and masks

Cardan grilleA card with windows: the letter is dull, the windows are not Cut-out masksLay it over the page and only the message shows

Text steganography

First-letter messagesThe message hides down the left-hand edge Word-position messagesOnly every fifth word counts Spacing patternsWide and narrow gaps, saying nothing to the eye TypographyTwo typefaces, one carrying a hidden binary

Image and sound

Hidden pixelsChange the last dot of colour and nobody sees it Least significant bitThe quietest part of a number, holding a message Colour-channel hidingA message living only in the blue
Hidden audioSound under sound, below what you notice
Spectrogram messagesA picture drawn in noise, seen only when you graph it

Digital steganography

Image filesThe oldest trick on the internet
Audio and video filesBigger files, more places to hide
MetadataThe notes a file keeps about itself File structuresSpace between the parts, where nothing is meant to be

Why hide instead of scramble?

Cipher text announces itself. A page of letters in groups of four tells anyone who finds it that you have a secret, and that is often all they need to know. A dull letter about the weather tells them nothing. The two together are stronger than either: hide the message, and encipher it in case it is found.

The ciphers Build a grille

Read more elsewhere

Longer, harder and written for grown-ups. Worth a look when this page runs out.

Steganography