Entropy or randomness is a quantitative measure of security strength for various types of passcodes, but in current times with digital technology, breaches, and cracking tools it seems that lifetime should also be a factor in determining “strength”. For example, a low entropy password with a short lifetime may be as strong as a high entropy password that is seldom replaced.

Is there a quantitative approach to determine passcode strength using both entropy and time?

  • Lifetime as a measure of strength is dependant on external factors which are dependant on entropy. Entropy is an independent quality. – schroeder Oct 11 at 11:55
  • @schroeder Could you explain further? The answer sounds circular. What external factors are you inferring and how would they be dependent on entropy? – gethopr Oct 11 at 21:02
  • It's a tree of dependencies. Lifetime is only a measure of strength in terms of how long it might take to crack. Lifespan should be shorter than time to crack. Time to crack is largely a factor of entropy (on a few vectors). So, you want to calculate strength based on entropy and a factor that is dependent on entropy. Taken to extremes, a password with extremely high entropy need not have a short lifespan, right? Lower entropy password need shorter lifespans. So, why not just stick to entropy? – schroeder Oct 11 at 21:20

As others have noted in comments, there will not be a general answer to this. There are far too many other things to take into consideration when trying to asses the lifetime of a password of some given strength.[^1]

The major things that can vary and need to be considered include

  1. The details of the Key Derivation Function (or hashing scheme) that is used. With some schemes, tens of millions of guesses can be made per second on ordinary machines. Other schemes can grind to just a few thousand guesses per second on the same hardware.

  2. What hardware an attacker is willing to through at the problem? Is it a high end gaming computer? Is it a rig built with 4 GPUs specifically for cracking purposes? Is it one of the $30,000 rigs that you can buy from specialist vendors. Is it a coordinated suite of such machines?

  3. How will prices and speeds change in the future

I helped organize an (expensive) experiment to get some idea of the cost of cracking 42 bit passwords hashed with 100,000 rounds PBKDF2-H256. Our goal was to get a sense of how expensive it is to crack these, so we offered substantial prizes.

Anyway, I still haven't done a final write up of the whole thing, but you can see information in our (1Password's) discussion forum topic. The actual contest details.

Money, not time

What we found was that at the scale of this contest, it cost about 6USD to make 232 guesses for something hashed that way in 2018. Any attempts to judge about the future will involve making estimates about how rapidly the cost of the relevant computing changes over time.

Again, that exercise only tells us about the host of cracking PBKDF2-H256 100K rounds. But I do believe that trying to put things into money terms instead of in to time estimates is more useful. We deliberately make the contest hard enough so that there would be a good mix of trade-offs between fixed costs and running costs.

So while I don't think that anyone can give you an answer to your precise question, as it depends on far too many other variables that will differ in each case, I do believe that it is useful to think in terms of costs instead of in terms of time. And, with enough incentives, it is possible to get decent estimates of those costs for certain kinds of hashing/KDF mechanisms.

[^1]: I will speak informally of the "entropy (or strength) of a password", but, of course, the strength of a password is a function of the system by which it was generated.

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