Access modifier on optional parameter

gistfile1.txt

public class SecureTransactor
{
  public bool DoManySecureTransactions(string[] data)
  {
    if (!SecurityCheck()) return false;

    bool doCheck = false;   // We already did the check, no need to repeat ourselves!
    foreach(string datum in data)
    {
      DoSecureTranaction(data, doCheck);
    }
  }
  
  public bool DoSecureTransaction(string data, protected bool doSecurityCheck=true)
  {
    // Notice the protected default argument. Externally, this function can only be
    //  called with 1 argument. Internally, we can supply whatever value we choose
  
    if (doSecurityChecks && !SecurityCheck())
    {
      return false;
    }
    
    // ...
    
    return true;
  }

  protected bool SecurityCheck()
  {
    // ...
  }
}