Block Ciphers - Modern Cryptanalysis: Techniques for Advanced Code Breaking - page 114

The (A, B, C, D, E) values will represent the hash in the end. For now, we will calculate the (a, b, c, d, e)
values and eventually add them when finished processing this block.
There are 80 rounds, for i = 1, ... , 80:
The f i functions above change depending on the round:
The k i values change depending on the round:
Afterwehaveprocessedall80roundsforthecurrentblock,weaddtheroundvalues(using32-bitarithmetic)
to the ongoing hash values (A = A + a, B = B + b, C = C + c, D = D + d, and E = E + e) and start processing
the next block.
Both SHA-1 and MD5 have other “relatives,” such as MD4 (similar to MD5, but with a less thorough, and
therefore faster, digesting function). However, I shall not discuss them further.
4.13 Random Number Generators
Although we are primarily focused on block ciphers throughout this topic, it can be useful to understand a few
principles about random number generators in cryptanalysis.
A random number generator is a function that generates a random number: one that cannot be predicted.
A true random number generator, one whose output defies prediction, is difficult to obtain. These are normally
based on the measurement of some physical phenomenon, which, based on strong scientific support, has no pat-
terns or other characteristics. For example, measuring components of noise and radioactive decay is popular for
these numbers.
For these true random number generators, though, there is little to do, cryptanlytically speaking. If the num-
bers are truly random, no system could produce meaningful results in most cases. One exception to this rule is
found in the following section.
Any method for producing random numbers that aren't true is called a pseudorandom number generator .
4.13.1 Bias
Even a true random number generator can have a certain flaw: Certain numbers are generated more often than
others. For example, although flipping a coin is a fairly random process, there is a very slight, measurable dif-
ference in the output numbers generated by the coin toss. This can be due to any number of factors, such as