Final Answer:
The given statement is: In the scenario where the sender and receiver do not agree on the message length in advance, what is the likely consequence when using the basic CBC-MAC construction with messages of different lengths? The correct answer is: The MAC verification will fail. Thus the correct option is A.
Step-by-step explanation:
In the scenario where the sender and receiver do not agree on the message length in advance, using the basic CBC-MAC construction with messages of different lengths is likely to result in the MAC verification failing. The CBC-MAC (Cipher Block Chaining Message Authentication Code) construction relies on consistent message lengths for proper operation. When messages of varying lengths are processed, the integrity of the MAC may be compromised, leading to verification failure.
The CBC-MAC construction involves dividing the message into blocks and processing them sequentially, where each block's output is used as the input for the next block. If messages of different lengths are used without proper alignment or padding, the final MAC value may not be accurately calculated or verified. The inconsistency in message lengths can disrupt the chaining mechanism integral to CBC-MAC, causing the generated MAC to be incompatible with the verification process.
To ensure the effectiveness of CBC-MAC, it is essential for the sender and receiver to agree on a standardized approach to handle messages of varying lengths, such as padding shorter messages to match the length of the longest one. Failure to establish this agreement may result in compromised message authentication, leading to the MAC verification failure stated in option (a).