Problem - Kevin Tafuro

Figure 6-2. The Mayas-Meyer-Oseas construct
MDC-2 is based on Matyas-Meyer-Oseas. There are two internal states instead of
one, and each is initialized with a different value. Each block of input is copied, and
the two copies go through one round of Matyas-Meyer-Oseas separately. Then,
before the next block of input is processed, the two internal states are shuffled a bit;
the lower halves of the two states are swapped. This is all illustrated for one block of
the message in Figure 6-3.
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Figure 6-3. The MDC-2 construct
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Clearly, input needs to be padded to the block size of the cipher. We do this internally
to our implementation by adding a 1 bit to the end of the input, then as many
zeros as are necessary to make the resulting string block-aligned.
One important thing to note about MDC-2 (as well as Matyas-Meyer-Oseas) is that
there are ways to extend a message to get the same hash as a result, unless you do
something to improve the function. The typical solution is to use MD-strengthening,
which involves adding to the end of the input a block that encodes the length of the
input. We do that in the code presented later in this section.
Using a Block Cipher to Build a Full-Strength Hash Function | 295
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