Function rho_input 

pub fn rho_input(
    msg: &[u8],
    commitments: &[(u32, [u8; 32], [u8; 32])],
) -> Vec<u8> 

Build the “rho input” for a signing instance — the canonical bytes that all participating parties hash to derive each rho_i.

Format: msg_len:u32 BE | msg | party_count:u32 BE | for each party (sorted by index): idx:u32 BE | D_i | E_i.