Introduction to Cryptography with Open-Source Software

272 Cryptography with Open-Source Software

and use an auxiliary function called a compression function f which takes

as input two blocks, one of q bits and another of n bits, and produces as

output an n-bit block. This construction also requires an n-bit initialization

vector IV, and the message m will have to be split into k blocks of q bits

each. This may require padding the last block (for example, with zeros and a

binary representation of the size of the message) to bring it up to size. The

hash value is the value of the last iteration. This construction is illustrated in

Figure 1 1.3.

IV

m

1

m

2

m

k−1

m

k

HASH

f f f f

Padding

FIGURE 11.3: The Merkle–Damg˚