The Official (ISC)2 SSCP CBK Reference by Wills Mike;

Author:Wills, Mike; [Wills, Michael S.]
Language: eng
Format: epub
Publisher: John Wiley & Sons, Incorporated
Published: 2022-04-12T00:00:00+00:00

Key Strength

Cryptographic key strength was introduced in the first section of this chapter as being strongly related to the number of bits in the encryption key used by a given encryption algorithm. This bit depth actually defines the size of the search space that a brute-force attack would have to go through, checking each possible value, until it found one that matched the hash value or ciphertext being attacked. Such brute-force attacks can, of course, get lucky and strike a match on one of the first few attempts…or have to run almost forever until a winner turns up. Computer scientists refer to this as a halting problem, in that while you know the maximum number of iterations the search loop must perform, you have no way of knowing whether it will take one iteration, a handful, or a full 2n times around the loop for an n-bit key. This search space size was the first practical way to estimate the work factor associated with cracking a particular cipher. Cryptanalysts could then estimate the number of CPU (or GPU) instruction cycles needed for each iteration, translate that into seconds (or microseconds), and then multiply to estimate the length of how long one's secrets might remain safe from an attacker in terms of hours, days, or weeks.

The search space argument regarding cryptographic strength started to fall apart as the cost of high-performance graphic processing units (GPUs) fell while their processing power soared. (Think about how many complex vector calculations have to be done each second to take a streaming HD video, uncompress it, render it into clusters of pixels of the right color and intensity, provide those pixels to the video driver circuits, and then send the information out to the display. One frame of video is about 6.221 MB of data; at 60 frames per second, one second of video involves about 374 MB of data total, all the result of parallel processing streams inside the GPU.) Software systems for designing massively parallel task streams, dispatching workloads to processors, and coordinating their efforts to produce a finished result have become far more commonplace and more powerful; the hypervisor contained in most modern desktop or laptop computers has many of these capabilities built right in. These factors all compound together to give cryptanalysts—hostile and friendly alike—entirely new and affordable ways to bring thousands or tens of thousands of CPUs (and GPUs) to a code-breaking task. Attempts have been made by cryptanalysts to express cryptographic strength in “MIP-years,” that is, the number of millions of instructions per second a CPU executes, across a whole year, but this has proven challenging to translate across dissimilar architectures of GPUs, CPUs, and such.

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