What will be the mac size of AES CBC-MAC if there are suppose 10 blocks in the AES CBC-MAC?
Will it always be 128 bits MAC size for no matter how many numbers of blocks?
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Yes, the CBC-MAC algorithm simply issues the last block of CBC mode as authentication tag. For AES this will always be 128 bit as that is the block size of AES (which is independent of the key size used).
The authentication tag is sometimes shortened to fewer bits (taken from the left hand side) for reasons of efficiency. This is however independent of the input size. $Tlen$, the size of the tag, is a security / configuration parameter of most MAC algorithms and authenticated modes of encryption such as CCM. $Tlen$ is commonly larger than 64 bits (smaller sizes will really influence security), a multiple of 8 bits (i.e. a certain number of bytes) and of course smaller than or equal to the output size of the algorithm that produces it.
Generally a protocol specifies the authentication tag size(s) that should be used. Generally the security in bits changes linearly with the authentication tag size for cipher based algorithms such as CBC-MAC. Other (more efficient) algorithms such as GMAC used in GCM however will degrade security faster (!).
Note that CBC-MAC is not AES CMAC, which should be used for dynamically sized messages.
answered Oct 7, 2020 at 7:40
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$\begingroup$ so mac value size will be same right for cmac and ccm also which will be 128 bits (fixed) $\endgroup$ Commented Oct 7, 2020 at 8:48
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$\begingroup$ 128 bits is what the MAC algorithm produces. It's commonly considered the default but can be configured explicitly by the protocol. $Tlen$, the size of the tag, is a security (configuration) parameter of most MAC algorithms and authenticated modes of encryption such as CCM. $Tlen$ is commonly larger than 64 bits (smaller sizes will really influence security), a multiple of 8 bits (i.e. a certain number of bytes) and of course smaller than or equal to the output size of the algorithm that produces it. $\endgroup$– Maarten Bodewes ♦Commented Oct 7, 2020 at 9:00