Is a brute force attack a viable option in this event ticketing scheme

Question

I plan on creating an ticket "pass" platform. Basically, imagine you come to a specific city, you buy a "pass" for several days (for which you get things like free entrance to museums and other attractions).

Now, the main question that bothered me for several days is: How will museum staff VALIDATE if the pass is valid? I see platforms like EventBrite etc. using barcodes/QR codes, but that is not quite a viable solution because we'll need to get a good camera phone for every museum to scan the code and that's over-budget. So I was thinking of something like a simple 6-letter code, for eg: GHY-AGF. There are 26^6 = 308 million combinations, which is a tough nut to crack.

I've asked a question on the StackExchange security site about this, and the main concern was the brute forcing. However, I imagine someone doing this kind of attack if: they had access of doing pass lookup. The only people that will be able to do this are:

1) The museum staff (for which there will be a secure user/pass app, and rate limits of no more than 1000 look-ups per day)

b) Actual customers to check the validity of their pass, and this will be protected with Google ReCaptcha v3, which doesn't sacrifice user experience like with v1. Also rate limits and IP bans will be applied

Is a brute force STILL a viable attack if I implement these 2 measures in place? Also, is there something else I'm missing in terms of security, when using this approach?

By the way, Using a max. 6-character-long string as a unique "pass" has many advantages portable-wise, for eg. you could print "blank" passes, where the user will be give instructions on how to obtain it. After they pay, they'll be given a code like: GAS-GFS, which they can easily write with a pen on the pass. This is not possible with a QR/barcode. Also, the staff can check the validity in less than 10 seconds, by typing it in a web-app, or sending an SMS to check if it's valid. If you're aware of any other portable system like this, that may be more secure, let me know.

Answer 1

Brute forcing is a function of sparseness. How many codes at any given time are valid out of how large a space? For example, if out of your 308M possibilities, 10M are valid (for a given museum), then I only need ~30 guesses to hit a collision. If only 1000 are valid, then I need more like 300k guesses. If those values are valid indefinitely, I should expect to hit one in less than a year at 1000/day. It depends on how much they're worth to figure out if that's something anyone would do.

This whole question is around orders of magnitude. You want as many as you can get away with. 7 characters would be better than 6 (exactly 26x better). 8 would be better than that. It depends on how devoted your attackers are and how big the window is.

But that's how you think about the problem to choose your space.

What's much more important is making sure that codes can't be reused, and are limited to a single venue. In all problems like this, reconciliation (i.e. keeping track of what's been issued and what's been used) is more important than brute-force protection. Posting a number online and having everyone use it is dramatically simpler than making millions of guesses.