NSAttributedString

    private func makeCriteriaMessage() -> NSAttributedString {
        var plainTextAttributes = [NSAttributedString.Key: AnyObject]()
        plainTextAttributes[.font] = UIFont.preferredFont(forTextStyle: .subheadline)
        plainTextAttributes[.foregroundColor] = UIColor.secondaryLabel
        
        var boldTextAttributes = [NSAttributedString.Key: AnyObject]()
        boldTextAttributes[.foregroundColor] = UIColor.label
        boldTextAttributes[.font] = UIFont.preferredFont(forTextStyle: .subheadline)

        let attrText = NSMutableAttributedString(string: "Use at least ", attributes: plainTextAttributes)
        attrText.append(NSAttributedString(string: "3 of these 4 ", attributes: boldTextAttributes))
        attrText.append(NSAttributedString(string: "criteria when setting your password:", attributes: plainTextAttributes))

        return attrText
    }

attrText는 append 해야하기 때문에 NSMutableAttributeString으로 선언