=Paper=
{{Paper
|id=Vol-2542/MOD-DLT5
|storemode=property
|title=Towards Modeling Privity and Enforceability Requirements for BPM based Smart Contracts (invited paper)
|pdfUrl=https://ceur-ws.org/Vol-2542/MOD-DLT5.pdf
|volume=Vol-2542
|authors=Julius Köpke
|dblpUrl=https://dblp.org/rec/conf/modellierung/Kopke20
}}
==Towards Modeling Privity and Enforceability Requirements for BPM based Smart Contracts (invited paper)==
https://ceur-ws.org/Vol-2542/MOD-DLT5.pdf
Joint Proceedings of Modellierung 2020 Short, Workshop and Tools & Demo Papers
Int. Workshop on Conceptual Modeling for Distributed Ledger Technologies 53
Towards Modeling Privity and Enforceability Requirements
for BPM based Smart Contracts
Julius Köpke1
Abstract: Blockchains are a good foundation for the realization of inter-organizational business
processes and smart contracts. Existing approaches for BPM on blockchains focus on supporting
observability and enforceability. However, they fall short in providing privity. Since there are tradeoffs
between privity, enforceability and costs, we propose to explicitly model privity and enforceability
requirements for BPM based blockchain approaches. Such extended models are the foundation for
detecting conflicts, to balance conflicting requirements, and to derive compliant implementations.
Keywords: Blockchain; Distributed ledger; Privity; Privacy; Enforceability; Conceptual Modeling;
Business Process Modeling
1 Introduction
Blockchain technology has gained attention in the Business Process Management community
in the recent years [Di19, Me18]. On the one hand, blockchains are seen as a good basis
for inter-organizational business processes. On the other hand methods from the business
process management community are considered as a good foundation for the model-driven
development of smart contracts on blockchains [LW19, Hu16].
Smart contracts, originally introduced in [Sz97] have the objectives observability, privity and
enforceability. Blockchains naturally provide a good basis for addressing observability and
enforceability requirements. This is also witnessed by numerous approaches for executing
business process models on blockchains (see [Di19, Hä18b, St19] for some examples)
focusing on these aspects.
Privity describes the property that knowledge about the contract should only be spread to
the participants with a contractual need to know. However, this property is not addressed by
existing approaches or rather simple assumptions like encrypting all data or using off-chain
data are proposed.
In our earlier work [KFE19] we defined privity spheres of data items in order to express
privity requirements of business processes. Such spheres limit the read-access of data-values
to certain sets of participants. In particular, the most general public sphere allows the entire
1 Department of Informatics Systems, Alpen-Adria-Universität Klagenfurt, Austria, Julius.Koepke@aau.at
Copyright © 2020 for this paper by its authors.
Use permitted under Creative Commons License Attribution 4.0 International (CC BY 4.0).
54 Julius Köpke
blockchain network to read the data. The most restricted sphere only allows participants to
read some data value if they will certainly need the data for some future step.
However, following the existing approaches for BPM on blockchain, limiting access to data
items has negative impacts on enforceability. I.e. when data referenced by some decision
node is encrypted or stored off-chain, the blockchain network fails short in validating the
correctness of the decision [Ha18a]. This problem can be tackled in a reactive manner
as proposed in [Ha19], where encrypted data is only revealed in case of a dispute. For
supporting proactive enforcement, we have proposed solutions based on voting schemes in
[KFE19]. Following these solutions, the larger the set of participants with data access for
some decision, the larger the potential set of voters. This requires to balance the requirements
on enforceability and privity.
2 Implementing Privity and Enforceability Requirements
In the talk, we will report on our current work on annotating process models with privity-
and enforceability requirements. Having such extended models allows to derive optimized
implementations using encryption and voting schemes on public blockchains as sketched
in [KFE19]. However, alternative implementations using private/permissioned chains or
Zero Knowledge Proofs such as [Gr16, Be19] are also viable options. In the talk, we will
comment on these alternatives and discuss their strength and weaknesses. I.e. implementing
the most restrictive privity sphere efficiently on a permissioned blockchain with channels
can be challenging as the channel to write the data to may depend on runtime decisions
that are taken after the data should be written. The applicability of Zero Knowledge Proofs
has recently made significant advances [ET18]. However, these methods still come with
substantial amounts of costs in terms of CPU time, RAM, storage space and gas costs.
Consequently, there is no simple one-size fits all solution and implementation decisions
should be grounded on requirements for privity and on enforceability.
We currently aim in restricting read access to data elements. Potential future work is to
additionally encrypt or obfuscate the control-flow in the spirit of [MGC19, HGF12].
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