Cryptography (lecture22)

Cryptography is a technique of converting message
to non-understandable form. Now here you must think why we need to do
so? Let me explain, I write a message and I want to send it to Naila
but I don’t want to share this message with Asma and Umeed or with
other people. So now I want a way to convey my message to Naila
secretly, and I wish that if somebody except Naila got my message then
he/she will still be unable to understand that message.
History of
cryptography is thousands years old but the purpose of the cryptography
was always same and that is securing information from unauthorized
people.
Techniques of cryptography:
I don’t know how many
techniques are using here for cryptography but in our it430 course only
two techniques are mentioned, I try my best to explain them briefly.
Substitution:
It is the technique in which we replace characters for unscramble our message.
Hi, Naila I am OK.
That’s my message for Naila now I encrypt it using substitution method to change it in non-understandable form.
For example I changed it as:
Lm, Rempe M eq SO.
Do
you understand the above line? Here what did I do I replace every
character with 4th character of ascending order, Like A to E, Because
B-1, C-2 ,D-3,E-4 so I changed A to E now here what is key for naila I
just tell her 4 so 4 is a key which naila use to decrypt my message in
understandable form.
This is manually procedure which I did here
just for explaining you what is basically cryptography. But these days
procedure of cryptography is bit changed hehehe
Transposition:
The 2nd technique in our course is Transposition, what we do in this technique.
H i N a i l a
I a m o k
I write my message in row wise and now I will rewrite it with column wise.
H I I a N m a o i l k a
In
this way I just tell naila 27 this will be key for naila but if Asma
got my message she will never be able to understand my message.
Here what is the meaning of 27, 7 characters in 1st row and here are 2 rows.
Now what naila will do?
H I I a N m a o i k l a
H I N a I l a
I a m o k
She starts to write 2 rows and putting characters in 1st and 2nd row 1 by 1 n in this way she unscramble my msg n understand it.

Types of cryptography:
Here are two types of cryptography
Symmetric Cryptography
In symmetric cryptography same keys are used for encryption and decryption.
Asymmetric or Public Key Cryptography
In this type a pair of public and private keys is used for encryption and decryption.



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