# Decide strategy to decrypt cipher [duplicate]

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 e and 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?

• Linked or possible duplicate: Deciphering “easy” ciphers without hints or Possible ways to crack simple hand ciphers – tylo Jan 30 '17 at 12:12
• Welcome to crypto-SE. Above questions should adress the general idea, when you're dealing with classical ciphers. All of them have some kind of tell, but you might end up trying all of them one after the other. For modern cryptography, as you have guessed, this is bound to be practically impossible. Knowing the cryptosystem is a given in any kind of cryptanalysis. In mordern protocols, this is basically always public information anyway. – tylo Jan 30 '17 at 12:18
• Thanks @tylo and MaartenBodewes, I'll have a look at your links. Tylo, so you mean that in most cryptanalysis it's normal to know the cryptosystem? Guess it's almost kinda hopeless if you don't? – Erik van de Ven Jan 30 '17 at 15:02
• @ErikvandeVen Modern cryptography is designed to be secure even if you know everything but the key. No patterns, nothing but the length of the ciphertext leaking information about the maximum length of the plaintext. For weak classical ciphers you might manage to find an attack without knowing the cipher beforehand, but that's not really what most modern cryptographers are interested in. – CodesInChaos Jan 30 '17 at 15:48
• @ErikvandeVen Hint: Hypothetical unknown cipher - security in obscurity? – e-sushi Jan 30 '17 at 18:52