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Why is Masking used in CMAC?

cn flag
harpreet

I am working on masking in CMAC.

If masking is not used, then what will be the behavior of CMAC?

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cn flag
Patriot
Please explain what you have done to answer the question.
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poncho avatar Crypto
my flag
poncho

if masking is not used then what will be the behavior of CMAC.

Well, if you take out the masking from CMAC, you effectively have CBC-MAC, and that has known issues.

The most immediate one is that the mask is used to distinguish a message that's exactly a multiple of 16 bytes in length (with 0's at the end), and one which is slightly shorter. This can be handled by including a padding scheme, so I'll assume that.

However, even with that addition, there are issues; for example, if you know the $CBCMac_k(A)$, you know that's the same MAC as $MACMac_k(Pad(A) || A')$, where $Pad$ is the padding function used within CBCMac, and $A'$ is exactly the same as $A$, except the first block is xor'ed with $CBCMac_k(A) || IV$ (where $IV$ is the fixed IV that CBCMac uses); what this xor does is reset the internal state of the MAC back to the standard starting IV for CBCMac, and then you'll process the rest of the message exactly the same as for the original message.

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