# Offline Payment of Central Bank Digital Currency Based on a Trusted Platform Module
## 4 Results and Discussion  
### Experimental Results of Offline CBDC Token (JWT) and Comparisons  
The proposed model represents a sophisticated framework designed to offer a comprehensive and holistic solution for secure offline CBDC payments. This paper presents the results of a preliminary experimental study that investigates the feasibility of the model. A small-scale proof-of-concept prototype was developed to test the cryptographic functions of the TPM, specifically focusing on digital signature verification. The testing environment is detailed as follows [29]. Additionally, a combination of shell commands from TPM tools was employed for the testing process.  
OS-Ubuntu 20.04 LTS,  
TPM simulator-IBMtpm1661,  
Figure 6: Saved tokens in OffCW_A, B (Tokens Chain).  
TPM software stack-tpm2-tss-3.1.0, Resource management daemon-tpm2-abrmd-2.3.1, TPM tool-tpm2-tools-4.3.2  
Initially, the counter increments were examined within the TPM simulator test environment. Subsequently, the signature value and the verification procedure, as well as the token header and payload, were tested, as illustrated in Figure 7.  
And the JWT with RSA-SHA256 is as follows:  
Token = eylJhGciOijU5 uzI1 NilsInR5 cC16 lkpXVCJ9.eyjYlN0 YSI6 Ik8 yIiiviY2J5 dC16 ljY3 OSI sImNiT3 duljoiV09 mZjEwMzikLCljYk90Y24 ioIxNiTslmNiQW10 jojDYwli viY2 JFZHMiO iJXT2 ZmOTeyMCISImNiRXRjkbil6 ljk4 MSlj.ABQACwEAJM2 EVJ7B8 eOs4a3 KKKKTFWabZ8W yNZZsltDMghd06 ahFTM8 qwH11 EtgN-h8 PSo4 tJYQhmg Lp6 aLsJZ7 jOR7 dyMtikhE0 SO_ZZAY3 gEqojyHTEm12c4 yw8 DnPrKzSzSf3 IvX_VlchXiYoSRYQ9 MQQsgSeb20 NbO2m2 srA cLmIII-ywK4 uZp0 fMjhIJeU-eUWhr-q3g6c80 bd5400M_e9 iZosXH8 bUzuGpJjIciU9 qTCC0a LSSodaOHb1 aaf-2 CX1 NAjSHIZ wxOaB5 jepuVLtBa7 vqBGsvqLGjSQqCmcnb9 JJJCT8 pYhQ5D gQk4q248 HJ4 NbZe7 QxXAlRhJ5u3 eD-blg==  
Our model distinguishes itself from the hybrid CBDC transaction models discussed in Section 2, including those of Stripe, UPI Lite, Matera, and Crunchfish's digital cash, by being a CBDC transaction protocol designed for implementation in a fully offline environment, as opposed to a hybrid model. Conversely, it shares several similarities with the OPERA model, particularly in its operation within a completely offline environment, its token-based structure, and its utilization of verifiable signatures. Nonetheless, there are significant differences between the two models, which are outlined in Table 3.  
Figure 7: Offline CBDC token and TPM simulation result. (**a**) TPM monotonic counter, (**b**) TPM signing and verification, (**c**) JWT decoding (offline CBDC token).  
The OPERA model is predicated on the utilization of secure hardware components, such as one-time readable memory, to facilitate transactions in a complete offline environment. However, as highlighted in the report from Section 2 [15], the requirement for a specific type of memory and a complex token infrastructure may impede its widespread adoption. In contrast, the proposed model employs a secure protocol that incorporates a TPM, counters, and a JWT with a signature, eschewing reliance on hardware techniques. And our model offers the flexibility to segregate transaction amounts unlike the OPERA model that circulates the fixed amount tokens. Furthermore, the proposed model demonstrates high scalability by leveraging a combination of general-purpose technologies that are currently prevalent.  
The detailed security considerations for the proposed model, in relation to the requirements outlined in Section 2, are as follows.