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# Computer Systems Security Essay

2231 words - 9 pages

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 last, it was read in a ramdom way. (Pfleeger & Pfleeger, 2007)

Plaintext: transposition cipher
In 4 columns: t r a n
s p o s
i t i o
n c i
p h e r
Cipher text: tsinprpt haoicensoir
ramdom order: aoicetsinpnsoirrpt h
(Column order is 3,1,4,2)

Column transposition does no other than character rearrangment so it depends on the length of the plaintext to be cipherd.
It is also not able to be decipher until the cipher text is fully received as all the character in the first column or the ramdomaised first column needed to be fulfilled. So the length of the plaintext affect here too. This also so need a cosiderable time period to decipher as for the delay time of the cipher to be fulfill.
This technique not effective on long messages, its been avoided amoung the most as the time is more important. (Pfleeger & Pfleeger, 2007)

4.1.3 Product cipher
This uses multiple stages of substitutions and transpositions. (Lai, 2010).
Any combination of two techniques or algorithms for the perpose of encryption is known as Product cipher. (Pfleeger & Pfleeger, 2007)

Cipher text = E2 (E1 (P, k1 ), k2 )

Plaintext: product cipher
Substitution method: surgxfw flskhu
(According to Ceaser...

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