code: public byte [] open(byte [] box) {
              if (box==null) return null;
  
              // prepare shared key
              if (this.sharedKey == null) before();
  
              return open_after(box, 0, box.length);
          }
docs: /*
  @description
  Authenticates and decrypts the given box with peer's public key,
  our secret key, and the given nonce.
  
  Returns the original message, or null if authentication fails.