Why Classical cryptography is weak, while some of the classical cyphers have never been broken?

Based on my my experiences doing my PhD in crypto I'd like to take a step back here and give a short overview about what I believe is the current state of .

There are comparably few who really are working with "old-school" cryptanalysis. Most people in research today do not try to attack, lets say AES directly, but working with the protocols, implementations etc. IMHO that is due to several facts:

For almost every application we have very well-researched (and therefore considered secure) algorithms (if correctly applied). Many people look at the algorithms as a "given", something that has been researched and that we are done with. Most people do not expect a breakthrough in e.g. solving multivariate polynomial equation systems efficiently or any serious flaw in those algorithms.

Due to this it is very hard to publish significant papers in cryptanalysis. Well, actually it is hard to find anything really new on this front and funding goes into other channels, i.e., research on the security of protocols, applications, theory or generalization surrounding the algorithms. Just take a look at the CRYPTO 2014 program.

Now, to get to your question. There you have a tiny group of people (in comparison to other scientific fields) of very specialized cryptographers, some of them really the absolute experts in the whole field (and some which I had the pleasure to meet). Most of them (except the tenured probably) must create output, i.e., papers. (The driving force, if not to say currency, of today's success in research.) It is very risky to delve into a topic like, e.g., the Voynich manuscript, or other historical ciphers, since the perspective of finding anything significant you could publish is small to begin with. So it is very unlikely that for example a PhD student will be working on such a thing full time, maybe even because he wouldn't find a supervisor or funding for such an endeavor. Furthermore, all these cryptanalysts are just that: cryptanalysts. For historical ciphers you often need a background in history, and even more often languages. Of the cryptanalysts I met most worked exclusively with modern stuff. Some looked at historical ciphers as pet projects, but that was it, at most.

The Voynich manuscript is a prime example. To my knowledge, there is today no (professional) cross disciplinary group or project involving cryptanalysts, historians and others researching this topic full time. It is hard, yes, but not only because of the ciphers used themselves, but also because of the funding and energy invested in it.

/r/crypto Thread