Function gg18_mta_proved 

pub fn gg18_mta_proved(
    i_keypair: &PaillierKeypair,
    ck_i: &CommitmentKey,
    ck_j: &CommitmentKey,
    q: &BigUint,
    k_i: &BigUint,
    x_j: &BigUint,
) -> Result<(BigUint, BigUint), MtaProofError>

GG18-specific MtA with the Appendix A proofs on every ciphertext (identical sub-protocol to CMP20 — both per Gennaro-Goldfeder).