Question: We assume user A intends to send a message to multiple recipients, B through Bn. While maintaining security is not a priority, ensuring integrity

We assume user A intends to send a message to multiple recipients,

We assume user A intends to send a message to multiple recipients, B through Bn. While maintaining security is not a priority, ensuring integrity is crucial. This means that recipients B to B must have confidence that the message they receive originates from user A. To achieve this, user A opts to employ a Message Authentication Code (MAC). Assume all parties (A and B to B) share a secret key k. User A is using the secret key k to compute MAC value. Then, each receiver B, verifies the MAC using k as well. Please explain why this scheme is NOT secure. In other words, show that user B1 is not assured that messages he is receiving are from A.

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