Function decrypt 

pub async fn decrypt(
    encrypted: &Value,
    pair: &KeyPair,
    their_epub: Option<&str>,
) -> Result<Value, SeaError>

Decrypt data using AES-256-GCM

Parses {ct: ciphertext, iv: nonce, s: salt} format. Shared decrypt: their_epub = Some Self decrypt: their_epub = None