To be clear, it's if you know more or less the right passphrase. This is why your passphrase should be randomly generated. Word based passwords are still fine (

To be clear, it's if you know more or less the right passphrase. This is why your passphrase should be randomly generated. Word based passwords are still fine (for memorability, a la xkcd), but you need real entropy that can't be guessed. For example, if you choose your daughter's middle name your favorite animal, and the street you grew up on, then you have a small amount of entropy, and you'll be screwed in an offline attack scenario like this. The attacker can narrow it down to combinations of words and numbers related to you, and the brute force becomes feasible. If you use four randomly chosen words from a dictionary of 2000 words, then cracking it is roughly equivalent to cracking 13 random digits. Side note: cycling through multiple random pass-phrases of this type until you find something memorable has difficult-to-analyze negative effects on the final entropy. Ideally, you should be using the first output it gives you.