Does Cracking Encryption Involve Some Precognition?

jcapell asks: "Let's say I sent an encrypted message using any method that could normally be 'cracked' relatively quickly using brute-force or complex algorithms. Wouldn't any decoding require some minimal knowledge of the format of the contents? What if I (for example) printed out my message, took a photo of the text with a digital camera, and then ran the resulting .jpg file through rot-128? How would an unintended recipient know where to start decoding the message?"