3 ms·

This isn’t how Sha works

• mlyle 5y ago
  If they were dictionary words, or a similarly constrained search space, fed through SHA, this is exactly how it works. The information given after one guess excludes a whole lot of guesses as being possible solutions. Here, there's 91.7 bits of entropy in what goes into the hash function. Each guess shaves off more than 10 bits of entropy. After 9 guesses, only one password conforming to the generation format will be possible... yes, it will be very (impractically) hard to find this password, but the rest could be done offline to find the 10th value and solve it in 10 guesses. e.g. Make 9 random guesses. Then, for each of the 2^92 possible input strings: 1. Hash it. 2. See if the hash matches the things we know about the hash from the previous guesses.
  • acchow 5y ago
    Right, it's a random password tho not a dictionary word
    • mlyle 5y ago
      Even so, I just edited my comment and elaborated. You can do this in 9 online guesses with feedback + a very large number of offline guesses, and have the solution for the 10th. The information is there-- just the best search strategies known are very expensive.
      • acchow 5y ago
        > a very large number of offline guesses Right, the entire search space of random passwords. The matching hash characters are tongue-in-cheek. They don't help you. They could've just given you the entire hash up front and you would still have to search the entire random password space. Sure, you could do it "offline", but it would still take forever to compute
        • gitgud 5y ago
          This is the best description of why it's completely infeasible to make a system to guess it. It would be only be possible if the password length was below a certain threshold (maybe 30 characters) beyond that limit, there wouldn't be enough atoms in the known universe in order to store every hash/password combination.... making it physically impossible....
          • mlyle 5y ago
            In passwordle, the input is a 14 character password made up of letters, numbers, and punctuation, chosen with some bias. There's less than 92 bits of entropy (the bias shaves off a few bits of effective entropy but I'm too lazy to calculate it). That is-- out of the range of current brute force, but if it were just a few characters shorter, it could be attacked with this oracle technique no problem.
            • selestify 5y ago
              How would the oracle technique help at all? Like the other commenter said, they could just give you the hash upfront, and you'd still be stuck with bruteforcing the entire space of characters.
              • mlyle 5y ago
                > How would the oracle technique help at all? If they give you the hash upfront (or this oracle), you can test passwords offline without using up a limited number of guesses. It may be very computationally expensive to brute force the space, but the information is there. If they don't, you get 10 guesses, and you have effectively no chance of guessing the password.
                • selestify 5y ago
                  Ah, I see what you mean. Yes, if you don't even have the entire hash, you're kind of out of luck. > It may be very computationally expensive to brute force the space, but the information is there. If the password is long enough, it will take longer than the heat death of the universe to brute force the space. So in practice, brute forcing secure passwords might as well be impossible.
• ehsankia 5y ago
  Sure it is, dictionary attacks are extremely efficient, if the password is made out of dictionary words.