Problem - Kevin Tafuro

spc_mdc2_oneblock(c, c->bf);
memcpy(output, c->h1, SPC_BLOCK_SZ);
memcpy(output+SPC_BLOCK_SZ, c->h2, SPC_BLOCK_SZ);
}
6.17 Using Smaller MAC Tags
Problem
You want to trade off security for smaller authentication tags.
Solution
Truncate the least significant bytes of the MAC, but make sure to retain adequate
security.
Discussion
Normal software environments should not have a need for smaller MACs because
space is not at a premium. However, if you’re working in a space-constrained
embedded environment, it’s acceptable to truncate MAC tags if space is a requirement.
Note that doing so will not reduce computation costs. In addition, keep in
mind that security goes down as the tag size decreases, particularly if you are not
using a nonce (or are using a small nonce).
6.18 Making Encryption and Message Integrity
Work Together
Problem
You need to encrypt data and ensure the integrity of your data at the same time.
Solution
Use either an encryption mode that performs both encryption and message integrity
checking, such as CWC mode, or encrypt data with one secret key and use a second
key to MAC the encrypted data.
Discussion
Unbelievably, many subtle things can go wrong when you try to perform encryption
and message integrity checking in tandem. This is part of the reason encryption
298 | Chapter 6: Hashes and Message Authentication
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