The Evolution of Secret Writing 楊子陞 B 李柏勳 B 馬天祥 B 陳孟彙 B

What is Steganography ? Derived from the Greek words stegano, meaning “covered,” and graphcin, meaning “to write.”

Various forms of Steganography The ancient Chinese wrote messages on fine silk, which was then scrunched into a tiny ball and covered in wax. The Italian scientist Giovanni Porta described how to conceal a message within a hard-boiled egg.

Various forms of Steganography The thithymallus plant could be used as an invisible ink. Many organic fluids behave in a similar way, because they are rich in carbon and therefore char easily. Modern spies who have run out of standard issue invisible ink to improvise by using their own urine.

How can secret writing affect history ? secret writing saved Greece from being conquered. Be used in the war

What is Cryptography Greek word “kryptos” “hidden” Not to hide the message

Two kinds of cryptography: Transposition the letters of the message are simply rearranged Substitution give each alphabet which we know a new meaning

Transposition A word is rearranged randomly by us. For example, a word “are” which has 6 different way to be rearranged, e.g., are, aer, rea, rae, ear, era. The drawback of this way is that too many different ways we can’t understand it’s meaning immediately.

Another way is using the “rail fence” transposition For example: Life science is the main trend of the world L f s i n e s h m i t e d f h w r d i e c e c I t e a n r n o t e o l our cipher Lfsineshmitedfhwrdiececiteanrnote ol

Picture the scytale is a wooden staff around which a strip of leather or parchment is wound.

Substitution To pair letters of the alphabet at random, and then substitute each letter in the original massage with its partner. If we apply the principle to the roman alphabet, we can pair letters as follows: A D H I K M O R S U W Y Z V X B G J C Q L N E F P T

Another form of substitution is often called the Caesar shift cipher. Plain alphabet: a b c d e f g h I j k l m n o p q r s t u v w x y z Cipher alphabet: DEFGZJKLMNOPQRSTUVWXYZABC The drawback of Caesar shift cipher is that someone only try 25times can find its cipher

Another way is rearrange alphabets randomly, and then there are 400,000,000,000,000,000,000,000, 000 kinds of differences.

The other way is using a keyword or keyphrase to process, and the left alphabets are processing in turn as normal. Plain alphabet: a b c d e f g h I j k l m n o p q r s t u v w x y z Cipher alphabet: J U L I S CAERTVWXYZBDFGHKMNOPQ L I FESCNOPQRSTUVWXYZABDGHJK

Sender receive Key key algorithm ciphertext algorithm Plaintext plaintext

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