Cryptography in Network Security
In network security,
 Cryptography is a method of storing and transmitting data in a particular form.
 It ensures that only the person for whom the message is intended can read the message.
The message exchange using cryptography involves the following steps
Step01:
At sender side,
 Using an encryption algorithm, the message is converted into an unreadable form.
 The message in unreadable form is called as cipher text.
Step02:
 The cipher text is sent to the receiver over the communication channel.
 Since the message is encrypted, the attackers can not read the message.
Step03:
At receiver side,
 Using a decryption algorithm, the message is again converted into the readable form.
 Then, receiver can read the message.
Cryptography Techniques
Cryptography techniques may be classified as
 Symmetric Key Cryptography
 Asymmetric Key Cryptography
In this article, we will discuss about symmetric key cryptography.
Symmetric Key Cryptography
In this technique,
 Both sender and receiver uses a common key to encrypt and decrypt the message.
 This secret key is known only to the sender and to the receiver.
 It is also called as secret key cryptography.
Working
The message exchange using symmetric key cryptography involves the following steps
 Before starting the communication, sender and receiver shares the secret key.
 This secret key is shared through some external means.
 At sender side, sender encrypts the message using his copy of the key.
 The cipher text is then sent to the receiver over the communication channel.
 At receiver side, receiver decrypts the cipher text using his copy of the key.
 After decryption, the message converts back into readable format.
Symmetric Encryption Algorithms
Some of the encryption algorithms that use symmetric key are
 Advanced Encryption Standard (AES)
 Data Encryption Standard (DES)
Advantages
The advantages of symmetric key algorithms are
 They are efficient.
 They take less time to encrypt and decrypt the message.
Disadvantages
Point01:
The number of keys required is very large. 
In symmetric key cryptography,
 Each pair of users require a unique secret key.
 If N people in the world wants to use this technique, then there needs to be N(N1) / 2 secret keys.
 For 1 million people to communicate, a half billion secret keys would be needed.
How N(N1)/2 Keys Will Be Required?

Point02:
 Sharing the secret key between the sender and receiver is an important issue.
 While sharing the key, attackers might intrude.
To overcome this disadvantage, Diffie Hellman Key Exchange Algorithm is used for exchanging the secret key. 
Important Points
Point01:
In symmetric key cryptography,
 Both sender and receiver uses the same key.
 Sender encrypts the message using his copy of the key.
 Receiver decrypts the message using his copy of the key.
 The key must not be known to anyone else other than sender and receiver.
 If the secret key is known to any intruder, he could decrypt the message.
Point02:
 This cryptography technique is called as symmetric key cryptography.
 It is because both sender and receiver use the same key on their sides.
Point03:
 This cryptography technique is called as secret key cryptography.
 It is because the key has to be kept secret between the sender and receiver.
To gain better understanding about Symmetric Key Cryptography,
Next Article Asymmetric Key Cryptography  RSA Algorithm
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