cryptography basics part iv

COMPOSITION OF CIPHERS

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In previous articles of cryptography we learnt how to encrypt a text using block substituted ciphers and transposition ciphers.these are good ciphers to a noob but these ciphers are cakewalk for a man having appreciable knowledge of cryptography.so let us make our ciphers even harder to crack using composition of ciphers.
so what really is composition of ciphers?? it is nothing but making a harder cipher using ciphers we already know.Double columnar cipher which we learnt in previous section is an example of this approach.
First let us apply this for block substituted ciphers.we learnt monosubstituted cipher and polysubstituted cipher.let us try to combine both the ciphers.
let us take the example "WELCOME TO FREE LEARN HACKING"

1. write the text in the blocks of length 5 "WELCO METOF REELE ARNHA CKING "
2. first applying key for polysubstituted cipher take key as replace first alphabet by its first successor,second alphabet by its second successor,third alphabet by its third successor,fourth alphabet by its fourth successor,fifth alphabet by its fifth successor
   our encrypted text will become "XGOGT NGWSK SGHPJ BTQLE DMLRL"
3. Now apply monosubstituted cipher ,deciding our key as replace each alphabet with their second successor like A from C
   now our text will become "YHPHU OHYTL JHIQK CURMF ENMSM"

This our new secret code and believe on me it is better than previous one.

Now let us experiment for route ciphers and columnar transposition cipher.we will be first performing route cipher and then columnar transposition

1. let us take the example "WELCOME TO FREE LEARN HACKING"
2. write the text in the blocks of length 5 "WELCO METOF REELE ARNHA CKING "
3. now we will apply route cipher therefore write in form according to the figure given below
   
4. now the encrypted text becomes WMRAC KINGA EFOCL EEERN HLOTE
5. now applying transposition cipher we get "WMRAC EEERN KINGA HLOTE EFOCL"
our new crypted text.

hey isnt it great forming powerful ciphers from existing ciphers we already know. we can also make composition of transposition cipher and substituted cipher, any permutation in your mind could result in a new cipher.

cryptography basic

CRYPTOGRAPHY BASICS

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What is cryptography?
cryptography is art of changing any simpler text to a secret code.so how are hacking and cryptography related?hacking is simply gaining access to any system,while the work of cryptography is to encode a simpler text to secret code to provide confidentiality and information security.having knowledge of cryptography helps to make our system more secure.

cryptography is study dealing with mathematics and computers both.so to achieve command on it we have to learn some basic mathematics.
lets catchup some information regarding functions..

what is functions?
let us take Y as a function of X
Domain:the values which is given as input
Range:the values which came as output
So elements in X constitutes domain,and elements in Y which are also called as image constitute range
it is a relation satisfying the below conditions
1.Each element of X should have one image in Y
2.No element in X could have two or more corresponding values in Y
3.Y could have one or more than one values of X
4.It is not mandatory that each element in Y should have corresponding X.

Bijective funtions:functions in which each element in X has only one image in Y and no element in Y should be empty

Inverse function:If Y is made to be codomain then X will automatically become image of Y if the function is bijective function, otherwise inverse of a function is not possible

Trapdoor one way functions:They are the most important type of functions for cryptography.By definition one way function is that function whose inverse is hard to find unless some very essential information is given which is called trapdoor.so for now only we will keep ourselves reserve with this definition only and will exploit trapdoor functions in our next topic