Here in Sweden the murder of Salwan Momika makes me feel that states wanting to limit the anonymity of protesters must take rather great care to protect their rights.

This, I think must involve at least two things: strong laws protecting against creating any kind of registry of people who have participated, or who are likely to sympathize with any particular kind of protest, exceptions in cases where it there is a possibility of retaliation against protesters and that the police take care to actually protect protesters who are targeted because of their participation of protests, so that cases like Momika's murder do not happen again.

Good thing this isn't done in the US - we'd be cracking down on the police themselves