# Cryptography: Data Communication and Network Security

## Cryptography: Network Security

Definition of Cryptography:
Cryptography is the art and science of achieving security by encoding messages to make those non-readable. Cryptography is used for Information and Network Security. It is the cornerstone of modern electronic based security technologies which are used now-a-days to protect the valuable and confidential information resources on the internet, intranets and extranets.
Objectives of Cryptography:
• Confidentiality- Cryptography ensures confidentiality of the information must not be understood by the third party.
• Integrity- The information cannot be altered in case of transit or storage between intended receiver and sender without detecting any alternation.
• Authentication- Though this, the sender and receiver can confirm the identity of each other and the destination and origin of the information.
• Non-Repudiation- The sender of information can never deny the transmission or creation in later stages.

Definition of Cryptanalysis:

• The technique to decode back the data or messages from non-readable format to the readable format without knowing the starting procedure of the conversion from readable format to non-readable format is known as Cryptanalysis.
Cryptology:
The combination of Cryptography and Cryptanalysis is known as the Cryptology.
Cryptanalyst:
It refers to the person who attempts to break the cipher text message to obtain the original plain text message.
Types of Text:

• Plain Text or Clear Text- It signifies a message which can be understood by the sender, the recipient and the third party to whom the message is accessible.
• Cipher Text- When the plain text message is converted into the coded message using any suitable scheme, the resulting message is known as Cipher Text. The plain Text can be converted to the cipher text by Substitution and Transposition technique.

Types of Cipher:
• Homographic Substitution Cipher- It is the technique of encryption in which one plain text character is replaced with one cipher text character at a time. The cipher text character is not fixed.
• Polygram Substitution Cipher- It is the encryption technique where one block of plain text is replaced by another at a time.
• Mono Alphabetic Cipher- It is the technique of encryption in which one plain text character is replaced by one cipher text character.
• Block Cipher- It encrypts/decrypts a group of characters at a time.
• Book Cipher- It is the technique in which the randomly selected key from a page of a book is used.
• Caesar Cipher- It is the cryptography technique in which each of the plain text characters is replaced with an alphabet three places down the line.
• Hill Cipher- The technique works on multiple letters at same time. It belongs to the "Polygram Substitution Cipher".
• Playfair Cipher- The technique is used to encrypt the data manually.
• Running Key Cipher- In this encryption technique, some part of the text from one book is used as the key.
• Stream Cipher- In this technique, one bit is encrypted at a time.
• Transposition Cipher- In this cryptography technique, the re-arrangement of plain text characters in some other form is done.
• Vernam Cipher- It is also known as One-time pad which is discarded after single use.
Encryption:
The process of encoding plain text messages into the cipher text messages is known as Encryption.
Decryption:
It is the reverse process of Encryption. The process of converting back the cipher text messages to the plain text messages is known as decryption. Both of the encryption and decryption processes use one key and one algorithm to perform the function properly.