Oracles and blockchain
Blockchain is a growing list of records (blocks) that are securely linked together: each block contains a cryptographic hash of the previous block, a timestamp, and transaction data. Consequently, the data in any given block can only be tampered with by altering all subsequent blocks.
Blockchain as a technology lays a foundation for Web 3.0, mainly due to its distributed nature of decision-making and verifying, i.e., making any intermediaries or authorities obsolete. To establish trust in blockchain technology, it is necessary to ensure the tripod: security, authenticity, and integrity. Once the data are on the internet, we can, with reasonable certainty, be sure that the tripod is strong and secure in the case of the blockchain.

- Monitoring the blockchain network to check for incoming user or smart contract requests for measured data.
- Performing some type of computation, such as calculating a median or more complex parameters from multiple oracle submissions (e.g., extended Kalman filter) and calculating a critical value for threshold defined in the calibration procedure.
- Verifying (signing) and sending measured (or calculated) data to the blockchain for processing by the smart contract.
- Software oracles — oracles that provide online information to the blockchain, e.g., additional data about the calibration document and laboratory.
- Hardware oracles — oracles that provide information from physical devices, in our case from IoT devices, to the blockchain.
- Inbound oracles — oracles that provide smart contracts with data from the external world, e.g., from accreditation institutions.
- Outbound oracles — oracles that send information to the outside world, e.g., to users interested in measurement traceability.
- Consensus-based oracles—data passed to the blockchain are treated as a result of a consensus of multiple oracles, e.g., if it is required to decide about data based on multiple hardware oracles (sensors).
Concept of complete trust hierarchy in IoT system as shown in [4]

[1] Melo, W.S. Blockchains and Legal Metrology: Applications and Possibilities. In OIML Bulletin; 2021; Volume LXII, Number 3 (https://www.oiml.org/en/publications/bulletin/pdf/oiml_bulletin_july_2021.pdf)
[2] Chainlink. What Is the Blockchain Oracle Problem? Available online: (https://blog.chain.link/what-is-the-blockchain-oracle-problem/)
[3] Mammadzada, K.; Iqbal, M.; Milani, F.; García-Bañuelos, L.; Matulevičius, R. “Blockchain Oracles: A Framework for Blockchain-Based Applications” in Business Process Management: Blockchain and Robotic Process Automation Forum; Springer: Berlin/Heidelberg, Germany, 2020; Volume 393. (https://link.springer.com/chapter/10.1007/978-3-030-58779-6_2)
[4] Miličević, K.; Omrčen, L.; Kohler, M.; Lukić, I. Trust Model Concept for IoT Blockchain Applications as Part of the Digital Transformation of Metrology. Sensors 2022, 22, 4708. https://doi.org/10.3390/s22134708For details see: Miličević, K.; Omrčen, L.; Kohler, M.; Lukić, I. Trust Model Concept for IoT Blockchain Applications as Part of the Digital Transformation of Metrology. Sensors 2022, 22, 4708. https://doi.org/10.3390/s22134708