**3. Challenges of anti-counterfeiting technology based on halide perovskites**

We hereby briefly discuss the current challenges encountered by perovskite fluorescent tags prior to their real-world applications, including the potential overuse of toxic lead, the poor durability, and many clonable functions that can be easily reproduced by counterfeiters. Possible solutions are also provided with respect to each challenging case.

### **3.1 Lead contamination**

Lead's toxicity has been widely recognized due to its damage to the nervous system of biological individuals. Therefore, lead-based wastes are now under strict control in many developed countries. Despite perovskite security tags made by lead compounds feature many intriguing fluorescent properties, they can be highly risky when adhere to daily goods and cause potential lead leakage. Alternatively, more environmentalfriendly perovskites (e.g. tin-, antimony-, bismuth-, and copper-based) can be developed to replace lead-based ones while maintaining the bright luminescence and high processability of tags [55, 57, 58, 79, 82, 83, 96].

#### **3.2 Poor durability**

The phase stability of halide perovskites, especially 3D ones, can be susceptible to environmental perturbations and hence fail to work during long term or repeated authentication. Lowering down the dimensionality of perovskites as well as composite strategies enable more robust perovskite phase, yet the stability of fluorescent tags can hardly rival the simple-patterned tags (e.g. QR code). Advanced sealing techniques alleviate this problem by isolating perovskites from environment; however, they are limit for those tags that need direct exposure to atmosphere, chemicals, or solvents. Inert matrix has been demonstrated to enhance the durability of both common and special perovskite fluorescent tags. Beside glass, silica, and polymers [66, 82, 101], other durable matrix materials remain to be exploited.

#### **3.3 Clonable functions**

Single-mode perovskite fluorescent tags work as response to certain stimulus, making their functions clonable by commercial phosphors or other functionalized luminescent materials. A safer communication between users and server database requires physically unclonable functions (PUFs) that generated by irregular encryption and decryption methods. In this view, multimodal anti-counterfeiting that combines two or more encoding and decoding pathways (see Section 2.3) is prompt to be developed for highly confidential security tags. In addition, authentication based on the digital readout of sophisticated machines can also fulfill the demands of PUFs [97, 100].
