Cryptography is the concept of information security Cryptography is the practice and study of techniques for secure communication. The sender of an encrypted message shared the decoding technique also called as key needed to recover the encrypted information only with intended recipients, thereby avoiding unwanted persons to read information. Now days due to global acceptance of the internet every computer is connected to other so this has created many problems so cryptography plays very important role in information security. Cryptography is not new technique historical references were available of this technique date back to 2000BC. Cryptography played very important role in world war first and world war second. Decryption is reverse of encryption where coded message is decoded back in original message.
Principles of Security
Confidentiality means only sender of message and the desired receiver can access information no other person can access the information.
Authentication is the mechanism to proof the identity of user for example A is authorise user and B is unauthorised user and C is bank B may represent himself as A can do transaction to avoid this problem Authentication is done.
Integrity of information means information send from source to destination must not altered by hacker or any other person. For example sender A sends message “I LOVE YOU” and someone alters message as “I HATE YOU” this will be serious problem!
There are situations where user sends a information and later refuses that he had sent that message.
The principle of availability states that resources should available to authorised user all time.
Types of Attack
Criminal attacks are intentionally done to gain some information, destroy information, financial gain, revenge, sometime just for amusement. This is illegal and punishable.
Here attacker is tries to find weak point or defects in security system so that he can exploit the weakness in technology.
In passive attack attacker neither alters information nor access information he only measures parameters like size of information, frequency, length. Attacker just eavesdrops monitors the transmission so that he can get useful information. Further this attack can be classified in two attacks first is release of message content and traffic analysis. This attack is difficult to detect because information is not altered.
Unlike passive attack active attacks alters the information hence easy to detect.
Plain text as its name indicate it is not encrypted it can be understood by anyone suppose we send message to you friend “ good morning john” this message is plain text can understood by any one.
Cipher text is text in coded form only person having secrete key can decode the message.
We can transfer plain text into cipher text by two techniques
In substitution characters in original message are replaced by some other characters. Suppose my message is “WELCOME” and coded message would something like “ASFRHKM”.
In transposition we not only replace characters but also rearrange characters to change the word. This involves techniques like rail fence, simple column transposition.