Question

I want to generate a unique id which will be used in URLs to identify a specific resource. In principle it is the same as pastebin.com etc. does.

The id and resource is not very secret but I want it to be so you just can’t decrement a id and then get another users resource. I´m thinking of a CHAR(8) which will look nice in a URL and still be large enough to reduce the chance of guesses. But how do I generate this? For an INT, you can use auto_increment and primary key to insure the uniqueness.

But if I do the following in order

1. Generate a CHAR(8) in my application
2. Insure that this ID doesn’t exists.
3. If it not exists, store, else goto 1.

I have to wrap 2. and 3. in a atomic transaction.

But is there a better way? or shouldn’t I care about the the check (2.) because a clash doesn’t occur regularly. I use MySql and .Net (C#) if that helps. Is it possible to somehow ‘encrypt’ a auto-incremented int as the text-id and decrypt it again in precisely 8 (or 10) characters.

I have read Need a smaller alternative to GUID for DB ID but still unique and random for URL which was useful, but the use of GUID is not supported in MySql (as far as I know). But a comment on the quality of the LongToShortCode method in the thread would also be appreciated.

Note: the resources can’t be changed, only viewed.

Best regards,
Lasse

Answer 1

I think I’ll do it like this:
A 8 character text id can store a number up to 64^8 = 2^48.

I will then use two columns:

• ID, INT 2^32 auto-increment
• Rand, INT 2^16

Then, when I add a row, I will generate a random 2^16 integer and put it in a new row. The text id is then simply generated from from the two numbers combined. And retrieval is easy two – just splitting it up and a simple lookup in the database. Ridiculous simple solution which should eliminate row clashes and be random enough (2^16) to reduce guesses.

Feedback on this approach will be appreciated.