Computer systems security

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4.1 Classical Encipher Techniques
For the purpose of sending secret messages there was introduced encryption. As encryption get developed few technique were standardized. They are;
Substitution cipher
Transposition cipher
Product cipher

4.1.1 Substitution cipher
Replacing each element of the plaintext into another element is called as substitution cipher.
The most famouse is the Ceaser cipher which was invented by Julius Ceaser. Each letter is replaced by the letter three positions further down the alphabet. (Lai, 2010)

Plan text: ceaser cipher technique
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 albhabet: d e f g h i j k l m n o p q r s t u v w x y z a b c
Chiper Text: fhdvhu flskhu whfkqltxh

In using Ceaser Cipher, was easily predicted by having only a small piece of the cipher text. So that Ceaser had to protect even the cipher text. It was kept easy way so that any one can quickly send a meesage in no time. Making the cipher text harder the algorithm had to be taken with the messanger for decryption so there was a big risk in loosing one messager will make all the cipher text to get deciphered by the enimies. So Ceaser made his algorithm easy and simple. (Pfleeger & Pfleeger, 2007)

4.1.2 Transposition cipher
Rearranging the order of elements in the plaintext is known as Transposition cipher. (Lai, 2010)
Columnar transposition is way of arranging plaintext charactors in cloumn wise.
Here is a sample paintext which been put into 4 cloumns. On creation of 4 column they were created raw by raw. When reading it, was read column wise so that its hard to pridict while the number of the columns are unknown. Also to make it more harder the way of reading was chaged rather than reading from first to ...

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...0: Generated keys
Encryption
A user will be selecting a text file to be encrypted. For the completion user will have to select the key pair, private key of the user and public key of the receiver who will decrypt this message. Figure 21, 22 and 23 are evidence of process.

Figure 21: Encryption process

Figure 22: Test text file used for encryption

Figure 23: Encryption output

Decryption
Same as for encryption, decryption process will also need of the private and public keys of both users (private of the receiver who is going to decrypt and public of sender who encrypted the message). The encrypted message will be also located by the user. After providing a location to save the decrypted text, the process will create the file at that location in name of Decipher_text.txt as mention above.

Figure 24: Decryption process

Figure 25: Deciphered text file

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