Problem - Kevin Tafuro

4.14 Timing Cryptographic Primitives
Problem
You want to compare the efficiency of two similar cryptographic primitives and
would like to ensure that you do so in a fair manner.
Solution
Time operations by calculating how many cycles it takes to process each byte, so that
you can compare numbers across processors of different speeds more fairly.
Focus on the expected average case, best case, and worst case.
Discussion
When you’re looking at timing information, you usually have one of two motivations:
either you’re interested in comparing the performance of two algorithms, or
you’d like to get a sense of how much data you’ll actually be able to pump through a
particular machine.
Measuring bytes per second is a useful thing when you’re comparing the performance
of multiple algorithms on a single box, but it gives no real indication of performance
on other machines. Therefore, cryptographers prefer to measure how many
processor clock cycles it takes to process each byte, because doing so allows for comparisons
that are more widely applicable. For example, such comparisons will generally
hold fast on the same line of processors running at different speeds.
If you’re directly comparing the speed of an algorithm on a 2GHz Pentium 4 against
the published speed of the same algorithm run on a 800 MHz Pentium 3, the first
one will always be faster when measured in bytes per second. However, if you convert
the numbers from bytes per second to cycles per byte, you’ll see that, if you run
the same implementation of an algorithm on a P3 and a P4, the P3 will generally be
faster by 25% or so, just because instructions on a P4 take longer to execute on average
than they do on a P3.
If you know the speed of an algorithm in bytes per second, you can calculate the
number of cycles per byte simply by dividing by the clock speed in hertz (giving you
bytes per cycle) and taking the reciprocal (getting cycles per byte). If you know the
speed measured in gigabytes per second, you can divide by the clock speed in gigahertz,
then take the reciprocal. For example, you can process data at 0.2 gigabytes
per second on a 3 GHz CPU as follows:
.2/3 = 0.066666666666666666 (bytes processed per cycle)
1/0.066666666666666666 = 15.0 cycles per byte
150 | Chapter 4: Symmetric Cryptography Fundamentals
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