As you Asking for the Notes on the of the Cryptography and Security of the of the Computer Science Engineering of the Cochin University of Science And Technology the Not es on Cryptography is given below


TYPES OF CRYPTOGRAPHY
There are many types of cryptography, including codes, steganography (hidden or
secret writing), and ciphers. Codes rely on codebooks. Steganography relies on different ways to hide or disguise writing. Ciphers include both computer-generated ciphers and those created by encryption methods.

The different types of ciphers depend on alphabetical, numerical, computerbased, or other scrambling methods.

Codes and Codebooks
A well-constructed code can represent phrases and entire sentences with symbols,
such as five-letter groups, and is often used more for economy than for secrecy. A properly constructed code can give a high degree of security, but the difficulty of printing and distributing codebooks—books of known codes—under conditions of absolute secrecy limits their use to places in which the books can be effectively guarded. In addition, the more a codebook is used,

the less secure it becomes. Imagine a codebook with two columns. In the first column is a list of all the words that a military commander could possibly need to use to communicate. For example, it contains all the possible geographic areas in a region, all possible times, and all military terms.

In the other column is a list of plain words. To create a coded message, the encoder writes down the actual message. He then substitutes words in the codebook by finding matches in the second column for the words in the message and using the new words instead.

For example, suppose the message is Attack the hill at dawnand the codebook contains the following word pairs: attack =
bear, the = juice, hill = orange, at = calendar, and dawn = open. The encoded message would read Bear juice orange calendar open.


If the coded message fell into enemy hands, the enemy would know it wasin code,
but without the codebook the enemy would have no way to decrypt the message. Codebooks lose


some of their value over time, however. For example, if the coded message fellinto enemy hands and the next day the hill was attacked at dawn, the enemy could link the event to the coded message. If another message containing the word orangewere captured, and the following day,
something else happened on the hill, the enemy could assume that orange = hill is in the codebook.

Over time, the enemy could put together more and more code word pairs, and eventually crack the code. For this reason, it is common to change codes often.


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Cochin University of Science And Technology
Address: South Kalamassery, Kalamassery, Kochi, Kerala 682022
Phone: 0484 257 7550

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