Problem - Kevin Tafuro

easily last years. The 128-bit version generally does a bit better than half the
speed of the 64-bit version. Nevertheless, although there are a few things out
there using UMAC, we don’t recommend it. The algorithm is complex enough
that, as of this writing, the reference implementation of UMAC apparently has
never been validated. In addition, interoperability with UMAC is exceptionally
difficult because there are many different parameters that can be tweaked.
XMACC
XMACC can be built from a large variety of cryptographic primitives. It provides
good performance characteristics, and it is fully parallelizable. Unfortunately,
it is patented, and for this reason we won’t discuss it further in this book.
All in all, we personally prefer MAC127 or CMAC. When you want to avoid using a
nonce, OMAC1 is an excellent choice.
See Also
Recipes 4.11, 5.4, 5.10, 6.9 through 6.14
6.5 Incrementally Hashing Data
Problem
You want to use a hash function to process data incrementally, returning a result
when the last of the data is finally available.
Solution
Most hash functions use a standard interface for operation, following these steps:
1. The user creates a “context” object to hold intermediate state.
2. The context object gets initialized.
3. The context is “updated” by passing in the data to be hashed.
4. When the data is updated, “finalization” returns the output of the cryptographic
hash function.
Discussion
Hash functions are not secure by themselves—not for a password system,
not for message authentication, not for anything! If you do need a
hash function by itself, be sure to at least protect against length extension
attacks, as described in Recipes 6.7 and 6.8.
262 | Chapter 6: Hashes and Message Authentication
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