## Mercurial Essays

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# Cryptography Is The Science Of Encoding A Message Into A Form That Is

unreadable and making sure only the proper people are capable of decoding the message back into its original form. This is usually done by using an encryption algorithm and a decryption algorithm (these two are often the same) and very often a secret key. Some of the early cryptographic systems did not use a key but instead kept the algorithm itself secret. The message sender uses the encryption algorithm and the key to encode the message, and then sends it to the receiver. The receiver then uses the decryption algorithm and the key to turn back the encrypted message into its original form and read it.

If the message is intercepted others, they will only have unreadable data and will have gained nothing, unless they can figure out the decryption algorithm and obtain the key. This is why the key is never to be sent with the message, and has to be kept secret at all cost. If the key is compromised, the sender and the encrypted data is no longer safe. The sender and the receiver then usually agree on a new key to prevent any further damage.

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In ancient Greece, around 550 Bc, messages were sent encoded to generals and could only be decoded using special staff keys. The key actually consisted of a physical object, which was applied on the message to get the decrypted version of it. In 50 Bc., one of the most simple cryptographic algorithms ever used was the one called the Caesar cipher, that was used by Julius Caesar to send messages to his generals. It consisted simply of switching each letter with the letter that was 3 letters further down the alphabet.

For example Stephen would become Vwhskhq. To decrypt the message, the receivers would simply subtract 3 letters from each letter. This algorithm was later improved and called ROT13, where the letters could be shifted to any number between 1 and 25, and the number of letters shifted was the secret key. This very simple algorithm has been used on Usenet successfully to prevent people from inadvertently reading materials they might find offensive.

Monoalphabetic substitution is another simple step away from the ROT13 algorithm. In this algorithm, each letter correspond to another letter but in no particular order. For example a = d, d = x, f = e, etc for all 26 letters. This made it much harder to break but also made fairly big keys that couldn’t be memorized, since they consisted of 26 pairs of letters.

In France during 1585, members of the king’s court liked to send romantic or gossip messages to each other and encrypt them for safety, which becomes almost a necessity. Blaise de Vigenere came up with a poly-alpabetic substitution known as the Vigenere cipher. Basically, the algorithm would encrypt messages several letters at a time instead of letter by letter. For example ab = fh, th = sq. To simplify the huge keys it would require, the key was broken into a table and a key, the table was fairly big but the key was small enough to be memorized, and the table was useless without the key. This cipher wasn’t totally safe but no totally sure method to break it was developed before early in the 20th century.

During World War I, American troops used Native Americans to send messages over the radio, which could only be understood by other native Indians, and almost nobody in Germany could understand it. Also in World War I, the Playfair algorithm was developed by the Allies, the key, like in the Vigenere cipher, is based on a little table and a short keyword, which were both changed periodically. The rules used with the table were much more complex and made it fairly safe.

In World War II, however, the Germans gave up on abstract algorithms and came up with a physical encrypting/decrypting machine called the Enigma. It had different wheels of different sizes which were to be tuned differently depending on the date, the different turnings were listed in a little booklet that came with the machine. It wasn’t broken before the Allies finally managed to capture enough pieces of

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