Function rekey_pseudonym 

pub fn rekey_pseudonym(
    server_root: &ServerRootKey,
    channel_id: &ChannelId,
    epoch: Epoch,
) -> Pseudonym

The relay-filterable address of a channel REKEY event for (channel, epoch). Derived from the server-root key (NOT the channel key) + the channel id + the epoch the rekey introduces. Because the IKM is the server root — which every member always holds and which is stable across a channel rotation — any member can compute this for ANY epoch directly, WITHOUT holding that epoch’s (or the prior epoch’s) channel key. That is what makes epochs independently recoverable: a member fetches the rekey for whichever epoch(s) they choose (latest only, or all, in parallel), rather than chaining forward one key at a time. Distinct from the channel message pseudonym (channel key IKM) and the control pseudonym (community-id binding) by IKM/id, and domain-separated by label.