# Lightweight Blockchain Processing. Case Study: Scanned Document Tracking on Tezos Blockchain
## 1 Introduction  
In the context of the 2007-2008 worldwide financial crisis, Blockchains emerged as an attempt to ensure trust and to enhance the security side of financial transactions in peer-to-peer networks. Blockchains are specialized computing machines, optimized to perform a reduced set of repetitive operations related to user/message/transaction authentication, and achieved by public-key protocols, hash computation, and consensus protocols. Their potential benefits, including decentralized trust and resilience, are generally put into balance against their highly intensive computing requirements. Illustrating this point, it was reported that in 2008 the BitCoin [1] energy consumption was 16% larger than the whole energy consumption of Ireland [2]. Since then, Blockchain is no longer intrinsically connected to financial applications and has gradually become an appealing solution for a large variety of business verticals, suchas industry 4.0 [3], logistics [4], agriculture [5], e-health [6], and fake news detection [7], to mention but a few. Yet, the applicative needs of such verticals are generally catered for by conventional, general-purpose computing machines, increasingly accommodated by elastic cloud resources. Faced with these challenges, conventional Blockchain architectures reach their inner limitations and often become prohibitive in terms of memory, computation and energy-resources consumption required to achieve a predefined task into a limited time lapse [8]. Moreover, a second type of limitation relates to the complexity of code development: each Blockchain comes across with specific programming languages and with different syntactic constraints. To jointly solve these limitations and having in view a lightweight generation of Blockchains, the present paper advances a distributed on-chain/off-chain architecture, allowing the global applicative logical workload to be securely distributed between on-chain and off-chain resources, while keeping the global orchestration of the underlying execution processes. We understand off-chain resources to mean: a general-purpose computing machine with conventional computing and storage resources (elastic or not) and with a generic programming language (e.g., C or Python). The applicative illustrations correspond to the Tezos Blockchain and to a scanned document tracking system based on visual fingerprinting. Note that scanned documents are characterized by their inner semantic/digital representation duality: actually, a unique document, from the point of view of human understanding, can have multiple digital representations, as illustrated in Figure 1. This is the result of the fact that the scanning conditions are never identical and, even in would-be identical conditions, the physical characteristics of each sensor result in a unique scanned document. Consequently, digital documents cannot be directly tracked by Blockchains (for which a single bit change in the digital representation corresponds to a new document) and solutions related to visual fingerprinting should also be investigated. Visual fingerprints are compact and salient visual content features computed from the content itself, which can uniquely identify duplicated and/or replicated versions of it in a reference database. It is also referred to as content-based copy detection (CBCD) or near duplicate detection [9]. Figure 1: Digital documents: heterogeneous content (text, image, etc.) with various digital representations for the same semantic content.