Meassage Authentication In network Communication

Encryption protects against passive attack (eavesdropping). A different requirement is to protect against active attack (falsification of data and transactions). Protection against such attacks is known as message authentication.

A message, file, document, or other collection of data is said to be authentic when it is genuine and came from its alleged source. Message authentication is a procedure that allows communicating parties to verify that received message is authentic. The two important aspects are to verify that the contents of the message have not been altered and that the source is authentic. We may also wish to verify a message’s timeliness (it has been artificially delayed and replayed) and sequence relative to other messages flowing between two parties.

1 Authentication Using Conventional Encryption

It is possible to perform authentication simply by the use of conventional encryption. If we assume that only the sender and receiver share a key (which is as it should be), then only the genuine sender would be able to encrypt a message successfully for the other participant.

Furthermore, if the message includes an error-detection code and a sequence number, the receiver is assured that no alterations have been made and that sequencing is proper. If the message also includes a timestamp, the receiver is assured that the message has not been delayed beyond that normally expected for network transit.

1        Message Authentication without Message Encryption

We examine several approaches to message authentication that do not rely on encryption. In all of these approaches, an authentication tag is generated and appended to each message for transmission. The message itself is not encrypted and can be read at the destination independent of the authentication function at the destination.

Message Authentication Code

One authentication technique involves the use of a secret key to generate a small block of

data, known as a message authentication code that is appended to the message. This technique

assumes that two communicating parties, say A and B, share a common secret key K_AB.

When A has a message to send to B, it calculates the message authentication code as a function of the message and the key: MAC_M= F (K_AB, M). The message plus code are transmitted to the

intended recipient. The recipient performs the same calculation on the received message, using the same secret key, to generate a new message authentication code. The received code is compared to the calculated code. If we assume that only the receiver and the sender know the identity of the key, and if the received code matches the calculate code, then

1. The receiver is assured that the message has not been altered.

2. The receiver is assured that the message is from the alleged sender. Because no one else knows the secret key, no one else could prepare a message with a proper code.

3. If the message includes a sequence number, then the receiver can be assured of the proper sequence, because an attacker cannot successfully alter the sequence number. A number of algorithms could be used to generate the code. The national Bureau of Standards, in its publication DES Modes of Operation, recommends the use of Data Encryption Algorithm (DEA). 

 

                                             Message Authentication code