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Symmetric Key Cryptography Techniques

Symmetric Key Cryptography Technique
  1. Substitution Technique
  2. Transposition Technique

Substitution Technique :-
1. Caesar cipher - It was proposed by Julius Caesar. In this, the characters of a plain text message are replaced by other characters, numbers or symbols. It is a special case of substitution techniques wherein each alphabet in a message is replaced by an alphabet three place down the line , that is-  +3   for example- HELLO  can be written as  KHOOR   
2. Modified Version of Caesar Cipher - This technique is modified version of old Caesar Cipher. In spite of using +3, we use +K like an alphabet A in plain text would not necessarily be replaced by D [which is +3]. It can be replaced by any alphabet , that is - by E, or by F, or by G etc.Once replacement scheme is decided, it would be constant and will be used for all other alphabets in that message +K value should be, 1<=K<=26 .  for example-   +K value is 5                HELLO  can be written as  MJQQT     
3. Mono-alphabetic cipher - In Mono-alphabetic cipher techniques, each symbol uses different K value.                           
for example- H(+3)   E(+4)  L(+7)   L(+7)  O(+5)    can be written as  K   I   S   S   T          
4. Homo-phonic Substitution  Cipher - It is very similar to Mono-alphabetic cipher. The major difference is, in case of mono-alphabetic replacing alphabet is fixed   (for example, replace A with D, B with E etc.) example-     H - {Q,T,Z}, E -  {A,C,F}, L -  {R,T,Z}, O -  {M,T,P},  H  E  L  L  O  can be written as  Q  A  R  T  M. If alphabet is repeat than we use second alphabet in the set and so on.
5. Poly-gram Substitution Cipher -  In this technique, rather than replacing one plain text alphabet with one cipher text alphabet at a time, a block of alphabets is replaced with another block.       
for example- H E L L O could be replaced by   Y U Q Q W, but H E L L could be replaced by a totally different cipher text block T E U I.
6. Poly-alphabetic Substitution Cipher - This cipher uses multiple one-character keys. Each of the keys encrypts one plain text character. The first key encrypt the first plain text, second encrypts second plain text. In this key size is unlimited it's not limited up to 26 bits.for example-     key  is   12597  HELLOWORLD                H(+1) E(+2) L(+5) L(+9) O(+7) W(+1) O(+2) R(+5) L(+7) D(+9)   can be written as I G Q U V X Q W U K    
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