CRYPTOGRAPHY - PowerPoint PPT Presentation

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Greece was organized into small, disunited, independent city-states. ... The apparently blank tablets got to Greece, where they realized (how? ...

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Lecture 1

3 week summer course

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Why we need secure means of communication?

  • Government: diplomacy is sometimes better done quietly.

  • Military: strategies often rely on the element of surprise.

  • Business: competitors will win if they know your secrets.

  • Love letters . . . Secret affairs . . .

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History of secret writing I

  • Herodotus chronicled the conflicts between Greece and Persia in the fifth century (499-472 BCE).

  • Greece was organized into small, disunited, independent city-states. Persia was a large empire (and growing) ruled by Darius, and later his son Xerxes.

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History of secret writing I

  • The Persian rulers had a long history of feuds with Athens and Sparta. Any minor problem could spark a major war.

  • When Xerxes built a new capital for his kingdom (Persepolis), other countries sent tributes and gifts, but Athens and Sparta did not. Xerxes was upset by this lack of respect, and began mobilizing forces to attack the Greek city-states.

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History of secret writing I

  • The Persians spent 5 years building up their forces. This was one of the largest fighting forces in history. In 480 BCE they were ready to attack.

  • There was a Greek exile, Demaratus, who lived in the Persian city of Susa and saw the forces being built up for war with Greece. He still felt a loyalty to his homeland, and decided to send a message to warn the Greeks of the impending attack. But how?!

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History of secret writing I

  • He scraped the wax off a pair of wooden folding tablets, wrote on the wood underneath, and covered over the tablets with wax. The apparently blank tablets got to Greece, where they realized (how?) that there may be a secret message in it, and found it. Now the Persians lost the element of surprise – and the war.

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  • This is hidden writing or steganography. Histaeus, ruler of Miletus, wanted to send a message to his friend Aristagorus, urging revolt against the Persians. Histaeus shaved the head of his most trusted slave, then tattooed a message on the slave's scalp. After the hair grew back, the slave was sent to Aristagorus with the message safely hidden.


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  • Other examples include

    • The ancient Chinese would write messages on fine silk, roll it into a tiny ball, coat it with wax and swallow it . . .

    • Secret ink: in the 1st century, Pliny the Elder explained how the “milk” of the thithymallus plant becomes transparent after drying, but reappears upon heating. (Many organic fluids behave this way, e.g. urine)

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In the 16th century, the Italian scientist Giovanni Porta described how to conceal a message in a hardboiled egg by making ink from alum and vinegar and writing on the shell. The solution penetrates the shell, leaving its mark only on the egg underneath!

During WWII, German agents in Latin America would photographically shrink down a page of text to a little dot, and hide it on top of a period or dotted I on a page. A tip-off allowed American agents to find this in 1941.

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Invisible Ink

  • During the American revolution, both sides made extensive use of chemical inks that required special developers to detect, though the British had discovered the American formula by 1777. Throughout World War II, the two sides raced to create new secret inks and to find developers for the ink of the enemy. In the end, though, the volume of communications rendered invisible ink impractical.

  • From

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20th Century Steganography

With the advent of photography, microfilm was created as a way to store a large amount of information in a very small space. In both world wars, the Germans used "microdots" to hide information, a technique which J. Edgar Hoover called "the enemy's masterpiece of espionage." A secret message was photographed, reduced to the size of a printed period, then pasted into an innocuous cover message, magazine, or newspaper. The Americans caught on only when tipped by a double agent: "Watch out for the dots -- lots and lots of little dots."


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21st Century Steganography

  • Modern updates to these ideas use computers to make the hidden message even less noticeable. For example, laser printers can adjust spacing of lines and characters by less than 1/300th of an inch. To hide a zero, leave a standard space, and to hide a one leave 1/300th of an inch more than usual. Varying the spacing over an entire document can hide a short binary message that is undetectable by the human eye. Even better, this sort of trick stands up well to repeated photocopying


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  • The modern version of Trithemius' scheme is undoubtedly SpamMimic. This simple system hides a short text message in a letter that looks exactly like spam, which is as ubiquitous on the Internet today as innocent prayers were in the 16th century. SpamMimic uses a "grammar" to make the messages. For example, a simple sentence in English is constructed with a subject, verb, and object, in that order. Given lists of 26 subjects, 26 verbs, and 26 objects, we could construct a three word sentence that encodes a three letter message. If you carefully prescribe a set of rules, you can make a grammar that describes spam.


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White noise messages

  • The key innovation in recent years was to choose an innocent looking cover that contains plenty of random information, called white noise.

  • The secret message replaces the white noise, and if done properly it will appear to be as random as the noise was.

  • The most popular methods use digitized photographs and video also harbor plenty of white noise:

  • A digitized photograph is stored as an array of colored dots, called pixels. Each pixel typically has three numbers associated with it, one each for red, green, and blue intensities, and these values often range from 0-255.

  • Each number is stored as eight bits (zeros and ones), with a one worth 128 in the most significant bit (on the left), then 64, 32, 16, 8, 4, 2, and a one in the least significant bit (on the right) worth just 1.


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  • A difference of one or two in the intensities is imperceptible, and, in fact, a digitized picture can still look good if the least significant four bits of intensity are altered -- a change of up to 16 in the color's value. This gives plenty of space to hide a secret message.

  • Text is usually stored with 8 bits per letter, so we could hide 1.5 letters in each pixel of the cover photo. A 640x480 pixel image, the size of a small computer monitor, can hold over 400,000 characters. That's a whole novel hidden in one modest photo!

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  • Hiding a secret photo in a cover picture is even easier:

  • Line them up, pixel by pixel.

  • Take the important four bits of each color value for each pixel in the secret photo (the left ones).

  • Replace the unimportant four bits in the cover photo (the right ones).

    To an untrained eye you're sending a completely innocuous picture!


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Websites of interest

Look at the steganography software at

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Invisible inks

  • lemon juice, milk, vinegar and onion juice all work as secret inks that can be revealed under heat.

  • Baking soda can be used, and then painted over with purple grape juice to reveal the color

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Steganography suffers from one problem: if it is uncovered all is lost.

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If we hide the message but then make it difficult to read if found, we have an added level of security.

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The aim is to hide the meaning of the message rather than its presence. This can be done by scrambling the letters around.

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