Function derive_dm_key 

pub fn derive_dm_key(
    our_ed25519_seed: &[u8; 32],
    partner_ed25519_pubkey: &[u8; 32],
    canonical_room_id: &str,
) -> Result<[u8; 32]>

Derive the symmetric DM room key from one side’s Ed25519 secret seed and the other side’s Ed25519 public key, plus the canonical DM room_id (which binds the key to this specific 1-1 channel).

Both peers, swapping seed ↔ pubkey, derive identical output.