**C**onsider the today’s scenario of Internet where , there are more than 2 billion Internet users throughout the Globe. People interacting with each other, exchanging mails, purchasing items online and many other activities are being done over Internet.

**Intruder.**

Suppose there are 2 persons , James and Steve. They want to communicate over Internet. Lets consider a scenario, what can be there basic needs when they communicate. First of all, they both surely wanted that the data they are exchanging, must not be read by any third person, it should be between them only. Secondly, while transmitting the data, the message should not be deleted, or modified. It should reach the destination in its original form. Thirdly, they both wants to verify that the person they are communicating , is the legal person, with whom they wants to communicate. Steve wants to make sure that the person on other side is James only, and James wants to make sure, that he is communicating with Steve only.

Taking these 3 scenarios, we can say that there are some basic needs of the communicating parties. These are :

### 1. Confidentiality :

### 2. Integrity :

### 3. Authentication :

### 4. Organisation Security:

Now lets have a look at what an Intruder can do and how it can harm the network. Suppose James and Steve are communicating and they want to ensure Confidentiality, Data Integrity and Authentication. Now, what different things an Intruder can do.

**i) ** Record the messages on the channel.

**ii)**Delete or Modify the original message during its transmission.

**iii)** Impersonating himself as someone else.

If proper measures are not taken, then an Intruder can attack in a numerous number of ways. For example, if not properly encrypted, an Intruder can steal your username and Password. He can do Denial of Service ( DoS ) attack by overloading the network resources and disabling other Network users to communicate. There are various other attacks also. We will discuss each and there measure in detail , later in this post and in the coming post.

**Cryptography Techniques**.

“**Crypt**” means “**encrypt**” and “**graphy**” means “**writing**“. Cryptography is the study of writing the code in an encrypted form.

Cryptography allows a sender to be-fool an Intruder by encrypting the message in some other format using certain technique or algorithm. The Intruder can be disguised, that he cannot get any information from the data if he is able to intercept it. Yes, the authenticated receiver will be able to receive the original data from the disguised data.

Let us suppose, James and Steve want to communicate. James wants to send a message to Steve. For example: James wants to ask “How are you, Steve”. Thus **James message in original form is known as Plain text**. James will use an encryption algorithm to encrypt his original message, to save it from the intruder attack. The **encrypted message is known as Cipher Text**. Cipher Text is not understandable by the intruder.

But as you know, in today’s global world, almost all the encryption and decryption algorithms are open to every person on the Internet. Even intruder knows these algorithms.So if the intruder knows the encryption method, he could have easily decrypt the message. But still something is there, that is preventing the intruder to decrypt and extract the transmitted message, that is known as a **key**.

A key can be anything like a string of characters or numbers etc. Say in this case, the encryption algorithm takes key **A**, message **m** as input and produces the cipher text as output. The cipher text here will be denoted as A(m).

Cipher Text ( C ) = A(m)

**m**is encrypted using a key

**A**. On the receiving side, Steve will provide a key B and the cipher text to the decryption algorithm, that will generate the plain text.

Plain Text (m) = B (C)

or

we can also write it as , Plain Text (m) = B(A(m))

There are two types of encryption Algorithms.

### i) Symmetric Key Algorithm

### ii) Public Key Algorithm

Let us start with our discussion over different encryption Algorithm. First, we will be going through Symmetric Key Algorithms and then moving on to various public key encryption techniques.

__Symmetric Key Algorithm in Cryptography__

Till now what have you understand about Cryptography? It must be that Cryptography is just putting one thing in place of the other using certain techniques so that it should not be understood by any wrong person. So we shall now look at various symmetric algorithms that are almost 500 years old or more. For Symmetric Key cryptography algorithms, we will use key as **K**.

**1. Caeser Cipher**

For example: James wants to send a message to Steve,

** “James, Meet me at University, Steve”. **

**K**letter later in the alphabet. Say if key K=5. Then every alphabet in the plain message will be replaced by its 5 letter later alphabet. Therefore, in this case, the cipher text will be as follows.

m= James, Meet me at University, Steve

K=5

‘J’ will be replaced by ‘O’, ‘a’ will be replaced by ‘f’ and similarly all the letters will be replaced. So our generated cipher text would be,

**Ofrjx, Rjjy rj fy Zsnajwxnyd, Xyjaj**

__Disadvantage of Caesar Cipher__

# 2. Mono Alphabetic Cipher

Let’s take an Example of a mono-alphabetic Cipher.

The plain message, “James, Meet me at University, Steve” will be encrypted as ,

**pqdtl, Dttz dt qz Xfoctklozn, Lztct**

Mono-alphabetic cipher can be extended to 26! ways to encrypt your text. It is almost equal 10^26. Thus, even if the intruder knows , that you have used Mono-alphabetic cipher algorithm, then using a Brute Force Approach ( Hit and Trial Approach ) also, it will be very brainstorming task for him to crack the code.

** 3. Poly Alphabetic Cipher**

**C**1

**, C**1

**, C**2

**, C**2

**, C**1. The 1st letter will be encrypted using 1st encryption key, the second will also be using 1st encryption key, 3rd will be using 2nd encryption key, 4th will use 2nd key also and the 5th letter will be encrypted using 1st encryption key. And this pattern will be followed for the coming letters also. Lets take our Example :

Plain Text (m) = “James, Meet me at the University, Steve”.

Cipher Text (C) = **Mdrjv, Phjy ph dy ykh Xsnyhuxnwb, Vyjyh**

**C**1

**, C**1

**, C**2

**, C**2

**, C**1.

- Nowadays , technology and the communication over Internet has grown to that extent, that these Symmetric Key Cryptographic Techniques also don’t work very effectively. As the 2 commodities communicating needs to share the key. Which is not feasible. Thus these techniques are hardly in use in today’s world. The techniques which are currently in use are PGP or Public Key Encryption.

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