Certificates are integral to the security of today’s Internet. Protocols like BlockVoke allow secure, timely and efficient revocation of certificates that need to be invalidated. ACME, a scheme used by the non-profit Let’s Encrypt Certificate Authority to handle most parts of the certificate lifecycle, allows automatic and seamless certificate issuance. In this work, we bring together both protocols by describing and formalizing an extension of the ACME protocol to support BlockVoke, combining the benefits of ACME’s certificate lifecycle management and BlockVoke’s timely and secure revocations. We then formally verify this extension through formal methods such as Colored Petri Nets (CPNs) and conduct a risk and threat analysis of the ACME/BlockVoke extension using the ISSRM domain model. Identified risks and threats are mitigated to secure our novel extension. Furthermore, a proof-of-concept implementation of the ACME/BlockVoke extension is provided, bridging the gap towards deployment in the real world.

Protocol flaws such as the well-known Heartbleed bug, security and privacy issues or incomplete specifications, in general, pose risks to the direct users of a protocol and further stakeholders. Formal methods, such as Colored Petri Nets (CPNs), facilitate the design, development, analysis and verification of new protocols; the detection of flaws; and the mitigation of identified security risks. BlockVoke is a blockchain-based scheme that decentralizes certificate revocations, allows certificate owners and certificate authorities to revoke certificates and rapidly distributes revocation information. CPNs in particular are well-suited to formalize blockchain-based protocols—thus, in this work, we formalize the BlockVoke protocol using CPNs, resulting in a verifiable CPN model and a formal specification of the protocol. We utilize an agent-oriented modeling (AOM) methodology to create goal models and corresponding behavior interface models of BlockVoke. Subsequently, protocols semantics are defined, and the CPN models are derived and implemented using CPN Tools. Moreover, a full state-space analysis of the resulting CPN model is performed to derive relevant model properties of the protocol. The result is a complete and correct formal BlockVoke specification used to guide future implementations and security assessments.