The Snap4City blockchain can be used in multiple domains. For example, in the context of mobility and transport it could be used to certify the way we interact with the world, connecting everyday devices/entities, in the cities and industries to certify: (i) distribution of goods and services in last mile collaborative framework, such as city hubs; (ii) vehicle identity, mileages, maintenance operations, reparation from incidents, on board unit data for insurances, on board unit data for professional drivers (to certify their readiness and attention level); (iii) tickets sold on the Mobility as a Service platforms, MaaS, also using it for revenue sharing towards the operators of multimodal traveling; (iv) computation of KPI indicators of the city with the aim of comparing them with other cities and for taxation, etc. In environmental applications, it can be used to certify measured values of pollution metrics, which are used for city taxations and to assess the achievement of the targeted values according to the European Commission. In the context of health to certify the source of organs and blood and their delivery chain of cold, the process for cleaning surgery instruments and hospital sheets, etc. In the context of energy and gas, it can be useful to certify the energy produced and exchanged in the communities of energies. It can also be used to certify, unicity, ownership and/or provenance of digital content, art, collectibles, and more through NFTs (Non-Fungible Tokens) that are unique digital assets authenticated on an immutable ledger. Blockchain can enhance smart home applications in regard to the fundamental security goals of confidentiality, integrity, and availability. 
In the literature many specific frameworks have been developed, while the IoT/WoT platforms may be used to implement all those cases. IoT/WoT platforms are capable to manage device messages exchanged with any kinds of data/device structure. In this context, a strong push on defining data models has been realized, for example the FIWARE Smart Data Models, SDM. A data model provides a formal template format for IoT/WoT entity/messages with formalized variables/attributes including data types, units, etc., to produce identical entities, and this process should be certified as well. As to the platforms under analysis, messages from IoT devices are freely shaped, to assure data flexibility. For example, IBM Watson uses formats such as JSON or XML, without supporting FIWARE SDMs, which also are published in several versions over time. The IoT/WoT models are the formal template from which the devices/entities can be generated/registered, and in turn the devices/entities with their registration formalize the structure of the messages which can be received/sent on them by the brokers and storage. Whenever a message arrives from a device (which can partially provide pieces of information into its body, typically not the metadata, since most devices minimize data transmission), the platform is not capable to register the device, nor to correct the message link to former devices. 

Snap4City added blockchain on IoT/WoT infrastructure adding a set of general-purpose features for certification/verification, and at the same time leaving flexibility to the developers of IoT/WoT solutions, allowing them to use the blockchain technology to set up their scenarios with a mix of certified and non-certified entities in a federated distributed architecture. The main contributions are on: (i) certification of any kind of IoT/WoT data models, devices and messages (with a particular focus on the certification of data models and its implications); (ii) automation of certifications according to the relationship among models and devices/entities, and among devices/entities and messages to guarantee consistency; (iii) certification of the hash for data messages by defining specific rules to cope for the flexibility of JSON objects of the simple schema validation; (iv) certification of time series, within a certain interval, for example to certify a trip, a mission, a delivery travel, etc.; (v) an architectural multi-organization solution to guarantee satisfactory performance in the certification of messages which is the most critical performance aspects.

• "Certifying Entity Models, Entities and Data Messages on IoT/WoT Platforms via Blockchain", Computer Networks, Elsevier, 2025 https://authors.elsevier.com/sd/article/S1389-1286(25)00249-X

The blockchain support is accessible via Entity/IoT Directory in which the developer can:

·         Specify which Entity Models have to be certified, implying the production of Entities / Devices which are certified.

·         Specify which Entities Instances / IoT Devices have to be certified, implying that messages on them will be certified

·         Verify the certifications performed over time, on the time series of the devices / entities.

The developers can use these facilities to implement a large range of applications: voting, access rights, rewards, revenue distribution, healthiness, driver assessment, emission of coupons, digital collection, NFT, complimentary currencies, loyalty tools, crowdfunding, etc.

Snap4City Block chain support is focused on certifications/unicity, immutability of models, corresponding produced entities and corresponding messages (and related formal implications) by enforcing blockchain in a general IoT/WoT framework with a number of rules according to the elements to be certified. The main contributions and novelty with respect to state of the art are on:

• certification of integrity for data models and formal implications for the usage/certification of corresponding produced instances of entities and their messages in a platform that may present both non- certified and certified models, entities and messages;
• automation of certification and verification processes, in a platform that also manages non-certified objects. This implies mechanisms and formal rules to:
  • A. guarantee consistency of connected certifications according to the relationships among models and entities, and among en tities and their messages;
  • B. certify partial data messages (from JSON objects, despite their format, and considering that messages may contain partial data) of certified devices and models, enforcing efficiency, energy saving and sustainability in the data management;
  • C. certify segments of time series, of partial data messages, to certify delivery trips, missions, shared trips, etc.;
• enforcing the approach into a multi-organization distributed solution to actually provide support for:
  • A. satisfactory performance in the phases of (i) certification, and (ii) verification;
  • B. both certified via blockchain and non-certified elements in the same framework according to the developers and designers.
• Performance analysis provides evidence of the performance costs in adopting the blockchain, and a strategy to exploit the best per formance on the basis of the number of data streams that the achitecture has to sustain. The estimation of the overhead of the solution is also provided. A comparison of the proposed solution with respect to other blockchains for IoT/WoT is also provided in terms of throughput, latency, efficiency, cost and energy.
• Validation on the Snap4City federated network, demonstrating practical applicability including mobility, environmental monitoring, healthcare, and energy management. satisfactory performance in the phases of (i) certification, and (ii) verification; both certified via blockchain and non-certified elements in the same framework according to developers and designers.
• Performance analysis providing evidence of performance costs in adopting the blockchain, and a strategy to exploit the best perfor-mance on the basis of the number of data streams that the archi-tecture has to sustain. The estimation of the overhead of the solution is also provided.

Overview and Motivation

Context and Challenges

The Internet of Things (IoT) and Web of Things (WoT) are increasingly integral to modern applications across domains like mobility, healthcare, energy, and environmental monitoring. As the complexity and scale of these platforms grow, so do the challenges associated with managing data securely, transparently, and efficiently. Common issues include the need for data integrity, privacy, consistency, and interoperability across a distributed network of devices and systems.

Blockchain as a Solution

Blockchain technology, with its inherent characteristics of decentralization, immutability, and verifiability, is proposed as a foundational solution. The paper explores how blockchain can be applied to certify not only data messages but also the entities (devices) that generate them and the models they are based on. This layered certification enables enhanced trust, especially in multi-stakeholder environments.

Figure 1: Certification Chain (Described)

The certification chain illustrates how models define entities, which in turn produce data messages over time. Each level can be independently or jointly certified to ensure consistency and traceability. The blockchain stores hashes of these elements to verify their authenticity without storing actual data, thus remaining GDPR compliant.

Contributions and Architecture

Contributions

1. Formal certification of models, entities, and data messages using blockchain.
2. Automation of the certification and verification processes through rule-based mechanisms.
3. Certification of partial messages and time-bound segments (e.g., for trip delivery validation).
4. Capability to support multi-tenant and multi-organizational deployments.
5. Performance evaluation using Snap4City with Hyperledger Fabric integration.

Architecture Highlights

The architecture is built on Snap4City, an open-source FIWARE platform supporting smart city applications. Key components include:
- Entity Directory: Defines models and entities.
- Broker Filter: Enforces data structure validation and triggers certification.
- Blockchain Abstraction Layer: Interfaces with Hyperledger Fabric for storing and verifying certifications.
- Storage: Utilizes OpenSearch and Virtuoso for storing data messages and metadata.

Blockchain Design and Implementation

Technology Stack

Hyperledger Fabric was selected for its enterprise-friendly features and permissioned nature, ideal for secure multi-party applications. The blockchain interface is abstracted through a Node.js-based service layer that handles API requests and communicates with the blockchain backend.

Certification Workflow

The workflow involves:
- Certifying entity models and propagating certification requirements to derived entities.
- Verifying entity structure at data ingestion via the Broker Filter.
- Normalizing and hashing JSON messages to store on the blockchain.
- Enabling time-series certification by grouping message hashes over intervals.

Comparative Evaluation

Platform Comparison

Compared to other blockchain-enabled IoT platforms (Helium, IOTA, IoTeX, etc.), Snap4City is the only solution that fully meets all identified requirements (R1.1 to R8). Other platforms often miss support for time series, model-level certification, or hybrid certified/non-certified deployments.

Performance Results

Snap4City, with its blockchain layer, handles millions of daily messages efficiently. Certification introduces a 10–20% overhead in processing time, which is acceptable given the benefits. The platform outperforms public blockchains like Ethereum in terms of latency, cost, and energy consumption.

Applications and Future Directions

Use Cases

- Mobility: Transparent transaction validation for MaaS services.
- Environment: Certification of air quality and waste metrics for policy enforcement.
- Healthcare: Cold-chain and procedure traceability.
- Energy: Certified peer-to-peer exchanges in energy communities.
- Digital Assets: NFT provenance and AI model certification.

Last: Ask to Snap4City@disit.org to get information and support to use this advanced feature on Snap4City.org infrastructure and on MicroX installations.

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