How does a decrypter know it has been successful? How does it know the difference between gibberish and clear text? It’s a computer program and doesn’t understand anything. Does it look for words like “the” and “bomb”?
Don’t real terrorists use words like “the” and “Package”? Wouldn’t it write in code, like “Aunt Susie is going to deliver the package to New York”
And anyway, why would a true terrorist write in English?
In other words, if the decrypter doesn’t know what it’s looking for, what does it look for?
I would like to know. Not being flippant, for a change.
It’s mathematics, it’s not hard to tell if a bunch of bits is random or contains a pattern. True randomness is very hard to do. Once data is encrypted it still can contain some non-randomness that can be discerned. The job is to decrypt to the most non-random state you can. The most non-random state might still be something like a simple book cypher so it won’t be readable yet...or it could be plain-text.
Subtle steganography is a real headache for those looking for secret meaning in masses of data.
You are more or less on to it. Bobalu’s answer is correct, but let me elaborate some. They might look for common English words. See here for how it was done a Bletchley Park:
Read the entire article it’s terribly interesting.
Ever heard of Google®?