I am developing a cross platform game for which I needs to generate unique identifier (User ID) for each user. I known some platform (Android or iOS) specific approaches to get device related identifiers but I am looking for a solution independent of the device identifiers.

User ID Requirements:

 1. Independent of the device's platform
 2. Offline implementation (no communication with any servers)
 3. Without sign-up process

I have implemented one approach to create User IDs where I store the system time when the game was launched for the first time on the device.

I have following questions:

 1. Are there any other approaches to generate User IDs (which will meet the above requirements)?
 2. What are the common approaches to create unique identifiers with taking any information from the user?
 3. Are there any third party plug-ins to implement User IDs?

I would appreciate any suggestions and thoughts on this topic.

EDIT:

There are lot of responses to use UUID/GUID. Generally, this approach looks fine but I am looking for a solution which can generate same User ID even if the user reinstall the game.

Generate a random 128-bit integer in some way. As iedoc said, you can do that by computing the md5sum of some machine-specific details, but you can also throw the timestamp of when the program was started in there, or some hardware-generated random numbers. You can also ask the user to take a picture (do you need a profile picture?) and take the md5sum of the image.

If you do a decent job of making the numbers random, 128 bits is enough to not expect any collisions ever. For instance, if you have 26 billion users, the probability of a collision is 10^-18. If for some reason you want to use 64 bits only, it might be OK, depending on how many users you expect: With 6 million users, the probability of a collision would be about 10^-6. I wouldn't go lower than 64 bits. See https://en.wikipedia.org/wiki/Birthday_problem

UUID is what you are looking for. There are plenty open source implementations for all kinds of languages.

15 hours ago, iedoc said:

Guids are one thing you could look into.

So they have to be able to get an ID while they are offline and not signed up. If they are offline then it won't really matter what their ID is until they come online. Once they are online you can assign them a new ID that is unique throughout everyone else that is online.

I'd use an integer as their ID. when they sign up, give them the next available integer. that can be their user ID, so even when they are offline they will use that ID. If they need to get an ID without signing up, maybe do something like a session ID, so once someone connects online, they will get the next available session ID. again if they are offline the ID won't matter what it is, only when they come online.

One last thing you can do is get device specific information, like the hard drive serial number plus mac address plus some other hardware serial. This would of course go into your platform dependent section of code, since each platform is going to have different ways of getting this information. once you have that string of serials, just md5 hash it or something to get a normalized string so everyone uses the same string size for their ID.

I wouldn't use the time they started the app with, because it would be possible for multiple people to have started the app at the same time in their local machines time

Thank you for the reply. I agree with your suggestion of using GUID (UUID) as User ID. I don't want to generate any identifier based device's hardware so that I don't violate Apple's or Google's guidelines.

Given I am fetching local machine's time in milliseconds, what are the chances of collision?

8 hours ago, Mike2343 said:

Some things to also consider:

1) Will the game keep scores online for others to see?

2) Are there achievements?

3) Are there levels/progress points/character(s) stats?

If yes, you will have to support them using multiple devices with the same ID or to transfer their profiles to a new device without losing all their progress too.  So having them type 64-128 bit integer is likely not a great idea.  You might have to make a database that assigns IDs.

Answers matching your questions:

1. No
2. No
3. No

I am fine with if a new User ID is generated when a  player play the game on other device.

19 hours ago, Octane_Test said:

There are lot of responses to use UUID/GUID. Generally, this approach looks fine but I am looking for a solution which can generate same User ID even if the user reinstall the game

If you want some kind of staticness you should take a look at Bitcoin. Some wallets give the feature to ressemble an Id by getting some kind of personal information into the generator process, a password for example or PIN. So even if a user would loose his wallet for some reason, chances are there to restore it from those informations.

But you know when going for Android there is a little chance that those data will be keeped for you even if a user uninstalles your app. We have had (even on newer Android versions) the problem that old game data was still there when we did not hard uninstall the app from ADB, an uninstall on the device gave those zombie data

Use the device's mac address?  If the user changes hardware, yeah, it'll change, but that's not super often.