In theory you can use cryptography to give a signature confirming you belong to a group, without saying who you are in the group (ring signature, or more specif

In theory you can use cryptography to give a signature confirming you belong to a group, without saying who you are in the group (ring signature, or more specifically for our case, traceable ring signature or linkable ring signatures). But we don't have the infrastructure to do such things atm, so which other option do you have to prove the validity of a petition?