***The following is my updated algorithm for determining the lottery winners***
Ok, so this uses your already provably fair method for determining rolls.
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At the start of the round, have the server create a seed, and display that seed hashed to the public on the lottery page. At the end of the round, automatically post an excel spreadsheet to Dropbox (I believe they have an API) showing the acquired tickets for the round and who they belong to (like this:
https://docs.google.com/spreadsheets/d/16FCMiXgVXI9unoFMm2qgCoK7gGA5BaZqth2BCoddT7k/edit?usp=sharing). Posting the file to Dropbox leaves an upload timestamp created by a server INDEPENDENT of freebitco.in, meaning the file cannot be tampered with or it would be evident in the timestamp.
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using such a spreadsheet in Dropbox does have some big disadvantages (in my opinion):
a) the size of file can get gigantic:
imagine tons of entries with just 1 or 2 tickets (from free rolls). lets calculate. 8 byte for timestamp, 4 byte for userID, 8 byte for ticket window (2 integers) makes a total of 20 bytes for possibly each ticket. say that we have only entries with 2 tickets, that would mean we could have 50 million entries in the spreadsheet. That is around 1 Gigabyte of Data. This will also grow with more active users. Also it gets very messy and confusing to quickly find the winning ticket number.
b) you are relying on 3rd parties
Using dropbox (or any other "trustworthy" partner) is always a risk for freebitco. If dropbox is not available, we cannot draw lottery winners. Are we willing to risk that? What if dropbox changes their services, APIs, or financial plans?
c) you are publishing sensitive data
publishing the exact date and amount of aquired tickets combined with the userID they belong to will reveal many other things as well. Now everybody can track specific users, check when they received tickets (and with that, they can backtrace the action you did (playing free rolls? 2 tickets. 100k tickets? probably bought directly. 12489 tickets? oh, you probably bet 12489*1500 (current rate) = 18,773,000 satoshis in high-lo game. Now you can also see a users Referer, since he will also receive 1 ticket at the same time you receive 2 from a free roll. With the timestamp you can now find out at which times of the day a user is active which will be an indicator of his timezone. This is a leak of personal data many people should be concerned of.