For every file that is saved to disk a database record is created that contains the filename (and other fields). I need to generate a unique filename for each file. The URL containing the filename is posted on the internet. I want to generate unique (and preferably short) URL's that are not sequential so that the next URL cannot be guessed based on any existing URL. I could get the id from the database row and hash it in base62 (lowecase and uppercase letters and numbers) but this would generate sequential URL's allowing filenames for other files to be easily guessed. What do you suggest?