How to hash a time interval?

Question

I need to compute the hash string that will match any future hash "within the past hour" starting from now.

I mistakenly did:

now = datetime.now()
hash = now.strftime("%D %H")

but that simply truncates the minutes so if I am at say 4:55, within 5 minutes the "hour" is up.

The past hour is necessary to be part of the hashing since the consumer of the hash string needs to know if the hash was computed within the past hour.

Answer 1

The simple, approximate solution, similar to mgibsonbr's suggestion, would be to generate a new token with the current hour and then if that fails, check again with the past hour.

Answer 2

I don't think that's possible at all. Take for example the values 4:15, 4:55, 5:30, with hashes X, Y and Z. 4:55 is within the past hour of both 4:55 and 5:30, so Y must be equals to Z. However, 4:15 is withing the past hour of 4:55, but not of 5:30, so X must be equals to Y and different from Z.

Better avoid hashes and do as katrielalex suggested, storing the datetime (or timedelta) and using that in your checks.

Update: Seems I misundestood you, you want hashes for crypto, not for storing things in a hash table for quick access... Maybe if you provide more details of your needs we can help you better, like who will construct the hash, who will check the hash, against what, etc.

There aren't many minutes in an hour, even many seconds, so in principle you could simply hash the initial time (truncated to the nearest minute/second) and, to see if the hash is still valid, take the current time and check the hashes of every minute/second before that. A naïve solution, but can be a starting point for something better.