[foresto]

I once pondered how I might generate IDs that were as compact as a machine word, without a value (or small set of values) revealing the size of the data set. One application might be user-visible customer numbers that don't easily reveal how many customers there are.

I eventually came across the idea of using maximal period linear-feedback shift registers to transform an integer variable through every possible value (minus one), but in a non-incremental sequence that depends on the LFSR arrangement.

I never ended up putting the idea to use, but I've always been curious about people who have and how it worked out for them. [Edit to clarify: It was meant for obfuscation, not security against a determined attacker.]

[dpifke]

I've used a small block cipher like Skip32 or Speck to obfuscate database sequences, either on INSERT or as part of the encoding scheme.

This works well against the German Tank Problem when there's no oracle allowing an attacker to guess lots of IDs quickly (such as when there are reasonable rate limits). It does not provide enough entropy when such an oracle exists (especially an offline one).

For something like a password reset token, it still needs to be paired with suitably random bytes.

[BatteryMountain]

Please see my previous comment, feel free to give feedback.

So far I haven't encountered any problems in the short term by using the approach described.

[slver]

The problem is that if your encoding algorithm leaks, it’s game over.