TL;DR: If you have to decrypt a cipher for example, and you don't know which encryption method(s) have been used.... Where do you start? Or "do you just start"? Or are there certain questions you have to ask yourself first to determine a certain strategy (to be as efficient and productive as possible)?
Let's assume I receive some kind of cipher in the form of a file or just a string text. I would like to determine the actually content, so I'll start trying to determine the encryption method used. The first thing I can think of, in case I might already know the cipher contains readable text, is looking for the most common letters and assume those are
a, and trying to figure out the pattern (so brute force)
But I guess that method is quite useless with modern encryption methods. So how do you choose the right strategy to use? Where to start? Those courses and books about cryptography are great, but with all those information it's still hard for me to determine and choose a logic strategy, before just start trying to decrypt the cipher with the first thing that crosses my mind.
Do you guys know some valuable content (book or website) where I can learn more about actually applying my decryption knowledge in real-life cases? So I'll learn to choose the right approach to start with.
My thinking, first determime what kind of software is used and try to reverse engineer them. But if you really don't know, are there good ways to fallback on?