Question

In broadcast authentication, a senderaims to send oneauthentication code for multiple receivers to verify. Explain why symmetric cryptography (e.g., message authentication for 2-party communication) cannot be directly used for broadcast authentication.

Answer 1

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ANSWER :-

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=>The problem we face while securing a broadcast communication is source authentication, or to make sure the receivers of broadcast data can verify that the received data really has the same source of origin that it claims. Although broadcast network can efficiently distribute data to multiple receivers, their is always a chance that a malicious user will impersonate the sender and inject broadcast packets.

=>Simply using the standard point-to-point authentication mechanism does not provide secure broadcast authentication, because any user with secret key can forge data and impersonate the sender. An approach to provide broadcast authentication uses symmetric cryptography,i.e. is based on delayed disclosure of key by the sender.

=>But in symmetric cryptography, the sender and receiver use the same key for encryption and decryption so the key have to be kept secret and properly protected so if we want to communicate with multiple people then it would require separate key for every person.Now if we want to broadcast to hundreds of peoples then we have to handle hundreds of keys which can be a difficult task because we also have to keep those keys secret and well protected.

As the level of security of an symmetric cryptography is completely dependent on the privacy and
protection of the keys, if the key is compromised to any intruder the whole system could fail.

=>So, confidentiality may be promised by symmetric cryptography but it cannot be used for authentication while broadcasting directly as there is no way to ensure the origin of the data if more than one people are using the same key.