Problem - Kevin Tafuro

6.9 Checking Message Integrity
Problem
You want to provide integrity for messages in such a way that people with a secret
key can verify that the message has not changed since the integrity value (often called
a tag) was first calculated.
Solution
Use a message integrity check. As with hash functions, there are somewhat standard
interfaces, particularly an incremental interface.
Discussion
Libraries that support MACs tend to support incremental operation using a standard
structure, very similar to that used by hash functions:
1. Allocate and key a context object. The context object holds the internal state of
the MAC until data processing is complete. The type of the context object can be
specific to the MAC, or there can be a single type that works for all hash functions
in a library. OpenSSLsupports only one MAC and has only the associated
context type. The key can be reused numerous times without reallocating.
Often, you will need to specify the underlying algorithm you are using for your
MAC.
2. Reset the context object, setting the internal parameters of the MAC to their initial
state so that another message’s authentication tag can be calculated. Many
MACs accept a nonce, and this is where you would pass that in. This is often
combined with the “init” call when the algorithm does not take a nonce, such as
with OMAC and HMAC.
3. “Update” the context object by passing in data to be authenticated and the associated
length of that input. The results of the MAC’ing process will be dependent
on the order of the data that you pass, but you can pass in all the partial
data you wish. That is, calling the update routine with the strings “he” then “llo”
would produce the same results as calling it once with the string “hello”. The
update function generally takes as arguments the context object, the data to process,
and the associated length of that data.
4. “Finalize” the context object and produce the authentication tag. Most APIs will
generally take as arguments the context object and a buffer into which the message
digest is placed.
Often, you may have a block cipher or a hash function that you’d like to turn into a
MAC, but no associated code comes with the cryptographic primitive. Alternately,
274 | Chapter 6: Hashes and Message Authentication
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