SECRET WRITING

SECRET WRITING

Got a message that only one person must see? Need to pass along a secret that must not, at any cost, fall into the wrong hands? Over the centuries, people have developed many ways to keep writing secret. Cryptography involves changing letters and words to a code or cipher. With steganography, you hide the message altogether.

CODES AND CIPHERS
In everyday speech, code and cipher are often used to mean the same thing, but, to the cryptographer, these two terms have different meanings. A code is the substitution of one word for another. So, if you were talking about oranges, you could call them elephants. Armies and police often use codes—giving operations particular codenames. A cipher is when you substitute letters in the original text with other letters, numbers, or symbols.
Cryptography  is the practice and study of techniques for secure communication in the presence of third parties (called adversaries). More generally, it is about constructing and analyzing protocols that overcome the influence of adversaries and which are related to various aspects in information security such as data confidentiality, data integrity, and authentication. Modern cryptography intersects the disciplines of mathematics, computer science, and electrical engineering. Applications of cryptography include ATM cards, computer passwords, and electronic commerce.
The earliest forms of secret writing required little more than local pen and paper analogs, as most people could not read. More literacy, or literate opponents, required actual cryptography. The main classical cipher types are transposition ciphers, which rearrange the order of letters in a message (e.g., 'hello world' becomes 'ehlol owrdl' in a trivially simple rearrangement scheme), and substitution ciphers, which systematically replace letters or groups of letters with other letters or groups of letters (e.g., 'fly at once' becomes 'gmz bu podf' by replacing each letter with the one following it in the Latin alphabet). Simple versions of either have never offered much confidentiality from enterprising opponents. An early substitution cipher was the Caesar cipher, in which each letter in the plaintext was replaced by a letter some fixed number of positions further down the alphabet. Suetonius reports that Julius Caesar used it with a shift of three to communicate with his generals. Atbash is an example of an early Hebrew cipher. The earliest known use of cryptography is some carved ciphertext on stone in Egypt (ca 1900 BCE), but this may have been done for the amusement of literate observers rather than as a way of concealing information. Cryptography is recommended in the Kama Sutra (ca 400 BCE) as a way for lovers to communicate without inconvenient discovery.

This famous cipher is called the Caesar Shift. It is easy to crack—you simply experiment with all the possible shifts until you hit on a sensible message.

MODERN CIPHERS
Today, powerful computers are used to create complicated ciphers which are very hard to break—but, of course, computer hackers also have powerful machines to help them break these codes.

SCYTALE

THE ANCIENT GREEK INSTRUMENT CALLED A SCYTALE USED A LONG STRIP OF LEATHER ROLLED AROUND A ROD. THE STRIP HAD AN UNBROKEN SERIES OF LETTERS ON IT, SO THE HIDDEN MESSAGE COULD ONLY BE READ WHEN WRAPPED AROUND A ROD OF THE SAME SIZE.


HIDDEN HANDWRITING

The simplest way to keep a message secret—with or without putting it in code—is to hide it. The science of hiding messages is called steganography.

INVISIBLE INK
An easy way to hide a message is to write in invisible ink—or ink that only becomes visible when you know how to reveal it. There are lots of different kinds of invisible ink, but the simplest ones can be found in a kitchen cupboard. Milk, lemon juice, sugar or honey dissolved in water, and vinegar or wine can all be used to make hidden messages. When applied to white paper and left to dry, they will all be invisible, but the message will show up brown when the paper is heated against a radiator or under a cool iron. Many other foods can also be used as invisible ink and are revealed using a chemical reaction. A mesage written in vinegar, for example, becomes visible when red cabbage water is painted over the top, and messages written with starch will turn blue in iodine. Some special pens use ink that is only visible under ultraviolet (UV) light. Security marker pens, used to mark people’s possessions in case of theft, work like this.

HAIR PIECE
In the 5th century BCE, Greek general Histiaeus tattooed his battle plans onto a slave’s shaven head. He waited until the slave’s hair had grown back before sending him off with the message—which the recipient could reveal by a haircut.
In 480 BCE, Greek general Demaratus sent a hidden warning to Greece that the king of Persia was about to invade. At the time, messages were written on wax tablets with wooden bases. Demaratus simply scratched his message onto the wood and set fresh wax on top with a false message written on it. To see the real message, the recipient simply had to melt the wax.