Message Digest :

• A message digest is the summary of message

• Message Digest are used to verify the integrity of the data i.e. to check whether the message was modified or not after the sender sends it and before the receiver receives it.

• Consider an example of message of size 32 bits we arrange these 32 bits into 4 parts of each of 8 bits and place them row wise.

• Now we count the number of 1’s in each column if the numbers of 1’s are even then we say that the column has even parity and even parity is denoted by zero.

• If the number of 1’s are odd then we say that the column has odd parity and we denote it with ‘1’.

• After calculating the parity with the result is known as LRC i.e. Longitudinal Redundancy Check , this LRC is the summary of the original message.

• The sender sends the original message and the LRC to the receiver.

• The receiver separates the message and the LRC from the block.

• The receiver calculates its own LRC and check whether LRC matches with sender's LRC or not.

• If the values of senders LRC matches with the values of receiver LRC then the receiver has confidence that message was not modified during transmission.

• If the values of senders LRC does not match with the values of receiver LRC then the receiver understands that the message was modified during transmission.

Requirement of the Message Digest :

1. For a given message it must be very easy to calculate its message digest

2. For a given message digest it should very difficult to calculate original message

3. For given two messages if we calculate their message digest then their message digest should be difficult