Message Authentication Codes

Message Authentication Codes are the codes which plays their role in two important functions : Authentication Detection and Falsification Detection.

Where do we need these codes ?
Suppose a User A send message to user B with message – ‘abc’. A encrypts the message using Shared – Key Cryptosystem for encrypting the message. A sends the key to B using a source key. Key exchange is based on different protocols such as Public – Key Cryptosytem. B uses the key to decrypt the Ciphertext and obatins the message.
a

All this does not happen so easily. There lies a problem.
If a malicious user X has falsified the ciphertext during the transmission. Then in that case, B has no way to realize that it has been falsified.When B decrypts the message, it will get the wrong message. Unknown to the fact B will think wrong information to be the right. Although you can decrypt or encrypt the data later on but these operations, you are applying on the wrong data.
b

Here we need to detect the falsification in the message B has got.
Here A will create a key (used to create Message Authentication Code) and sends the key to B. A will create a value using Ciphertext and key and the value is obtained. This value Created by Cipertext + Key = Message Authentication Code. B has to check whether the ciphertext is falsified or not using Message Authentication Code. Now B can clearly know that whether the ciphertext is falsified or not.
c



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