In the opening pages of the Simon Singh's The Code Book, he asserts that cryptanalysis - the science of de-encrypting encrypted messages and text - was only possible once the upper echelons of society had reached a sufficient level of mastery in mathematics, statistics, and linguistics. This argument is predicated on the idea that the very practice of encryption in and of itself was fairly new; an example of the rudimentary nature of the practice is the frequent use of simple substitution and shift ciphers, both of which intrinsically limit the number of possible permutations of the encrypted message i.e. the 25 possibilities for a shift cipher. In that sense, the encryption techniques of yesteryear were on the bleeding edge of espionage, and thus demanded the most qualified minds of the time to ponder and decipher them.
However, this ultimately begs the question of how modernity has rendered some of the most complex ciphers of the past obsolete, mere puzzles to occupy one's time on a long flight or car ride; even a grade-school child could decode a simple shift cipher in a reasonable amount of time.
The reason for this paradigm shift is two-fold. Firstly, modern media is inundated and saturated with puzzles for people to solve. The advent of technology has turned codebreaking into a game, a passtime, one that millions upon millions of people enjoy on a day-to-day basis. Some of the highest grossing apps on both iTunes and Google Play in recent years have involved unscrambling words or connecting certain images to keywords. Altogether, everyone from children to adults are nigh constantly training themselves in rudimentary codebreaking, unconsciously creating heuristics and algorithms to solve any other such puzzles that may come their way. In that sense, amateur codebreakers have, unknowingly or not, likely already been through an intensive training program in cryptanalysis.
Secondly, for codes that escape the powers of the human mind, there exists the accessibility of the modern computer. With billions upon billions transistors available to anyone with the monetary capital, the bulk of the mathematical and statistical expertise has been outsourced to the raw computing power of the computer. Able to test millions of cases in the fraction of a second, amateur codebreakers with a powerful enough processor and a bit of creativity are able to decode messages that would have taken the scholars of the past days or weeks in a matter of minutes.
Technology and the ubiquity of cryptography has thus made cryptanalysis into a hobby of sorts, turning a tool of high stakes espionage into a low stakes passtime.