Function decrypt_pairwise_payload 

pub fn decrypt_pairwise_payload(
    recipient_transport_secret: &[u8; 32],
    sender_transport_public: &[u8; 32],
    encrypted: &PairwiseEncryptedPayload,
    associated_data: &[u8],
) -> Result<Vec<u8>>

Decrypt a DKG round-two payload from one sender.