Careful Onboarding

Don't: Do not call onBoard on a value that has not previously had offBoard called on it

contract BadContract {
  ctUint64 public sum;
  //...
  function addWithoutCarefulOnboard() public {
    gtUint64 a = MpcCore.setPublic64(1);
    gtUint64 sum_ = MpcCore.onBoard(sum); // THIS WILL REVERT
   
    sum_ = MpcCore.add(sum_, a);
    
    sum = MpcCore.offBoard(sum_);
  }
  //...
}

Do: Initialize the value by calling offBoard inside the constructor

contract GoodContract {
  ctUint64 public sum;
  
  constructor() {
    // By initializing sum with an encrypted value, we ensure
    // calling onBoard on sum does not revert
    gtUint64 sum_ = MpcCore.setPublic64(0);
    sum = MpcCore.offBoard(sum_);
  }
  //...
  function add() public {
    gtUint64 a = MpcCore.setPublic64(1);
    gtUint64 sum_ = MpcCore.onBoard(sum);
   
    sum_ = MpcCore.add(sum_, a);
    
    sum = MpcCore.offBoard(sum_);
  }
  //...
}

Or: Check if the value is zero before calling onBoard

contract GoodContract {
  ctUint64 public sum;
  //...
  function addWithCarefulOnboard() public {
    gtUint64 a = MpcCore.setPublic64(1);
    gtUint64 sum_;
    
    // By checking if sum is equal to zero, we can verify whether
    // or not it is safe to call onBoard on sum
    if (ctUint64.unwrap(sum) == 0) {
        sum_ = MpcCore.setPublic64(0);
    } else {
        sum_ = MpcCore.onBoard(sum);
    }
   
    sum_ = MpcCore.add(sum_, a);
    
    sum = MpcCore.offBoard(sum_);
  }
  //...
}