4 ms·

What is k-anonymity?

• ec109685 8y ago
  Rather than providing the service a password hash for it to look up in its database, you download k password hashes that have the same prefix and do the matching locally. That way, the user’s real password hash is never sent over.
  • bipvanwinkle 8y ago
    Ah, cool. Thanks for the explanation.
  • solipsism 8y ago
    You download however many passwords match the prefix (N). If the process has k-anonymity then it's guaranteed that N >= k for any possible prefix.
    • Ajedi32 8y ago
      And in this case, k = 381. (Though more by coincidence than design.) https://www.troyhunt.com/ive-just-launched-pwned-passwords-version-2/ https://www.troyhunt.com/ive-just-launched-pwned-passwords-v...
      • andrewaylett 8y ago
        It's much larger than that, really -- if you know that the user is definitely looking for a hash that's present, there are 381 they could have been looking for. But here we're just as interested in elements that are outside the set, and there are a lot more of them.
        • ocdtrekkie 8y ago
          I was initially extremely skeptical of sites using Pwned Passwords' API until I learned how this worked/realized this. I was super skeptical of "send Troy your whole SHA1 hash", because many many SHA1 hashes have been reversed. Then when I was looking at V2, I was considering the fact that in a given set of 381 passwords, there were probably only a couple you might be meaning to check (if you look at a range set, each one generally has a couple of repeatedly used password hashes). But if my passwords fall outside of that set, someone might think I am maybe checking against one of the more commonly leaked passwords, when mine isn't even in that range set at all. Partially for my own comfort, and for fun, I ended up writing my own short checker in a .NET console app: https://github.com/ocdtrekkie/PwnPsswords https://github.com/ocdtrekkie/PwnPsswords