This paper introduces a privacy-preserving framework for blockchain systems using the Smart Contract for Data Expiry (SCDE). SCDE governs data registration, retention, and erasure through on-chain policies and off- chain encrypted storage. It combines AES-256 encryption, a Key Management System (KMS) for cryptographic erasure, and Zero-Knowledge Proofs (ZKPs) for verifiable deletion without revealing data. Decentralized Identifiers (DIDs) enable pseudonymization and user accountability.
Comparative results show that traditional and off-chain approaches lack automated, verifiable erasure. SCDE achieves full GDPR compliance with moderate overhead, demonstrating that privacy, transparency, and immutability can coexist in decentralized environments
[1] Tripathi, G., Ahad, M. A., & Casalino, G. (2023). A comprehensive review of blockchain technology: Underlying principles and historical background with future challenges. Digital Applications and Technology, 3, 100344. https://doi.org/10.1016/j.dajour.2023.100344
[2] European Data Protection Board (2025). Guidelines 02/2025 on processing of personal data through blockchain technologies. Official publication.
[3] Zafar, A. (2025). Reconciling blockchain technology and data protection laws. Journal of Cybersecurity, 11(1), tyaf002. https://doi.org/10.1093/cybsec/tyaf002
[4] Bodó, B., Brekke, J. K., & Hoepman, J. H. (2021). Decentralisation: A multidisciplinary perspective. Internet Policy Review, 10(2). https://doi.org/10.14763/2021.2.1563
[5] Godyn, M., Kedziora, M., Ren, Y., Liu, Y., & Song, H. H. (2022). Analysis of solutions for a blockchain compliance with GDPR. Scientific Reports, 12, 15021. https://doi.org/10.1038/s41598-022-19341-y
[6] Monteiro, S., Oliveira, D., António, J., Martins, P., & Abbasi, M. (2024). Data anonymization: Techniques and models. In ICMarkTech 2022. Springer. https://doi.org/10.1007/978- 981-99-0333-7_6
[7] Shao, Y., Liu, J., Shi, S., & Zhang, Y. (2019). Fast de- anonymization of social networks with structural information. Data Science and Engineering, 4, 76–92. https://doi.org/10.1007/s41019-019-0086-8
[8] Quang, Q. N., Pham, A., & Nguyen, T. (2025). Privacy- Preserving Cyberattack Detection in Blockchain-Based IoT Systems Using AI and Homomorphic Encryption. IEEE Internet of Things Journal, 12(11), 16478–16492. https://doi.org/10.1109/JIOT.2025.3535792
[9] Bao, H., Yuan, M., Deng, H., Xu, J., & Zhao, Y. (2024). Secure multiparty computation protocol based on homomorphic encryption and its application in blockchain. Future Generation Computer Systems, 153, 22–35. https://doi.org/10.1016/j.heliyon.2024.e34458
[10] Capraz, S., & Ozsoy, A. (2021). Personal data protection in blockchain with zero-knowledge proof. In Blockchain Technology and Innovations in Business Processes. Springer. https://doi.org/10.1007/978-981-33-6470-7_7
[11] Shashidhara, R., Nair, R. C., & Panakalapati, P. (2024). Promise of Zero-Knowledge Proofs (ZKPs) for blockchain privacy and security. Security and Privacy, e461. https://doi.org/10.1002/spy2.461
[12] Guo, F., Gao, Y., Jiang, J., Chen, X., Chen, X., & Jiang, Z. (2025). Linkable Ring Signature for Privacy Protection in Blockchain-Enabled IIoT. Sensors, 25(12), 3684. https://doi.org/10.3390/s25123684
[13] Boumaouche, O., Ghenai, A., & Zeghib, N. (2020). Data- Oriented Blockchain: Off-chain storage with data-dedicated and prunable transactions. In ACOSIS 2019. Springer. https://doi.org/10.1007/978-3-030-61143-9_16
[14] Sun, S., Zhang, Y., Wu, Q., & Huang, Z. (2022). Blockchain- based verifiable data deletion using smart contracts and cryptographic erasure. Computers & Security, 120, 102806. https://doi.org/10.1016/j.cose.2022.102806
[15] Kumar, R., Ruj, S., & Nayak, A. (2021). Decentralized key management in blockchain-based systems. IEEE Communications Surveys & Tutorials, 23(4), 2154–2180. https://doi.org/10.1109/COMST.2021.3106764
[16] Zhang, X., Wang, T., Li, J., & Chen, L. (2023). A survey on blockchain-based privacy-preserving mechanisms: Cryptographic foundations and applications. Information Sciences, 621, 72–99. https://doi.org/10.1016/j.ins.2022.12.044