## 1 Introduction  
Quantum key distribution (QKD) is a secure communication protocol with strong security guarantees. In 1984, the BB84 protocol [1] emerged, marking the beginning of quantum communication. Since then, various protocols have emerged, including the BBM92 protocol [2], SARG04 protocol [3], differential phase protocol [4], and six-state protocol [5]. On the other hand, rapid theoretical advancements in QKD have been accompanied by significant experimental achievements. In 1992, [PERSON] and [PERSON] successfully built the first QKD experimental system [6] and demonstrated the key distribution process of the BB84 protocol. In 1999, [PERSON] and [PERSON] proposed the use of high-dimensional quantum qubits [7] for QKD to enhance its information-carrying capacity, replacing traditional two-dimensional qubits. In 2002, [PERSON] and [PERSON] analyzed the security of d-level QKD systems [8]. The feasibility of high-dimensional QKD has been gradually demonstrated by researchers with studies focusing on transmission distance [9, 10] and security [11, 12, 13]. Although the communication distance and key length of this system are limited, it proved that QKD is feasible. This indicates that QKD research is continuously evolving and becoming more sophisticated. However, as research progresses, scholars have identified several security vulnerabilities in QKD applications. These include the photon number-splitting (PNS) attack [14], the fake state (FS) attack [15], the time-shift (TS) attack [16], and the detector-blinding (DB) attack [17]. Scholars have proposed various methods and protocols to counter the above attacks. In 2004, [PERSON], [PERSON], [PERSON], and [PERSON] proposed an analytical method for assessing the security of the BB84 protocol with non-ideal devices, which was known as the GLLP [18] theory. This theory effectively extends the security of the BB84 protocol to practical applications, analyzing the real-world security of QKD. Subsequent theoretical advancements, such as the decoy state method [19] and optimized parameter estimation techniques, have been developed to improve the performance of practical systems [20; 21; 22], enhancing both security and efficiency. To address side-channel attack threats faced by detectors, scholars have proposed the measurement-device-independent (MDI) protocol [23]. To improve the performance of practical QKD systems, [PERSON] et al. proposed the twin-field (TF) protocol [24] in 2018. Due to its significant advantages in transmission, it has sparked widespread research. Many subsequent enhanced protocols, referred to as TF-type protocols, further improved both the security and performance of the system. Among the TF-type protocols, sending-or-not-sending (SNS) protocol [25] is more widely used. The SNS protocol achieves long-distance secure transmission by configuring a series of sending or not sending signals. Recently, researchers have analyzed the security of the SNS protocol under different situations [26; 27; 28; 29; 30]. Researchers also have proposed various methods to enhance the performance of the SNS protocol. For example, by using the independent lasers [31], introducing the actively odd parity pairing (AOPP) method [32; 33; 34] and applying the phase postselection [35], the transmission range of the SNS protocol can be significantly extended. Nonetheless, practical implementation challenges persist. Researchers have addressed the practical security issues of the SNS protocol, particularly those related to the light source [36; 37; 38; 39; 40; 41; 42; 43; 44]. In our preliminary work, we investigated light source monitoring. Although this work is theoretically significant, it is not practical for real-world system implementation. In this paper, we propose a practical solution, untagged bits (UBs) [45], to effectively address the light source security challenges in SNS protocol. This paper is organized as follows. In Section 2, we describe the steps of the SNS protocol, analyze its security, and estimate key parameters.