CRYPTOGRAPHY

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CRYPTOGRAPHY

• 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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Steganography

• 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.
• From http//www.strangehorizons.com/2001/20011008/
  steganography.shtml

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Steganography

• 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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Steganography

• 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 http//www.strangehorizons.com/2001/20011008/
  steganography.shtml

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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."
• From http//www.strangehorizons.com/2001/20011008/
  steganography.shtml

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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
• From http//www.strangehorizons.com/2001/20011008/
  steganography.shtm

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Spam-onography

• 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.
• From http//www.strangehorizons.com/2001/20011008/
  steganography.shtml

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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.
• From http//www.strangehorizons.com/2001/20011008/
  steganography.shtml

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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!
• From http//www.strangehorizons.com/2001/20011008/
  steganography.shtml

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Websites of interest

• Look at the steganography software at
• http//wwwrn.inf.tu-dresden.de/westfeld/f5.html
• http//www.stegoarchive.com/

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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

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

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Cryptography

• 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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Cryptography

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