An anon-id is composed of two parts: an a-id (public, username, 16 characters) and an h-id (private, password, 32 characters).
Once an anon-id is created, the a-id can be displayed and, only together with the h-id, can be validated.
As long as the h-id is not compromised, it is virtually impossible to validate an anon-id solely from the public a-id.
This enables each user to build a reputation within a website/app using their provided a-id, with the assurance that these anonymous identifiers are extremely difficult to forge.
This demo can be used to gauge the difficulty of finding a valid anon-id by guessing its h-id.
Example of a website using this concept of anonymous identifiers on its leaderboards: Cross-Pollinator.
Note that the actual a-id representation (image or avatar) can vary to fit the website/app graphic design.
a-id Python implementation on GitHub. Enjoy!