Building trust in e-commerce is especially challenging in complex transactions. Complex transactions, involving a combination of aggregate, optional, and partial transactions, are increasingly relevant in real-world e-commerce scenarios. Existing multi-party contract signing protocols are not suitable for such transactional complexity or rely on trusted third parties (TTPs), resulting in inefficiency and limited applicability. In this paper, we propose a blockchain-based multi-party contract signing protocol specifically designed to ensure trust in complex e-commerce scenarios. Our approach introduces a Pre-agreement sub-protocol which reduces contract signing in complex transactions to contract signing in aggregate transactions, allowing the actual signing process to proceed efficiently and involve only relevant parties. The protocol uses Merkle tree structures to compute a unique identifier for the multi-party contract, reducing computation overhead compared to existing solutions. By invoking smart contracts only in the event of disputes, our solution ensures fairness while minimizing blockchain costs. Formal verification with the Tamarin Prover confirms that the protocol satisfies key security properties, including fairness, confidentiality, and non-repudiation. We also provide an Ethereum-based implementation that further demonstrates its practical viability and cost-effectiveness, providing a trusted coordination mechanism for multi-party contract signing in decentralized e-commerce environments.

Fair exchange and customer's and merchant's anonymity are two important properties of e-commerce transactions. There is to date a variety of proposed e-commerce protocols to achieve fair exchange and customer's anonymity for transactions involving digital products. For physical products delivery there is no e-commerce protocol to provide fair exchange and customer's and merchant's anonymity. In this paper, we propose the first e-commerce protocol for physical products delivery that will provide fair exchange in all circumstances, anonymity of customer and merchant for any collusion that can be formed, non-repudiation, integrity and confidentiality of data exchanged between the parties.

The secrecy problem for security protocols is the problem to decide whether or not a given security protocol has leaky runs. In this paper, the (initial) secrecy problem for bounded protocols with freshness check is shown to be NEXPTIME-complete. Relating the formalism in this paper to the multiset rewriting (MSR) formalism we obtain that the initial secrecy problem for protocols in restricted form, with bounded length messages, bounded existentials, with or without disequality tests, and an intruder with no existentials, is NEXPTIME-complete. If existentials for the intruder are allowed but disequality tests are not allowed, the initial secrecy problem still is NEXPTIME-complete. However, if both existentials for the intruder and disequality tests are allowed and the protocols are not well-founded (and, therefore, not in restricted form), then the problem is undecidable. These results also correct some wrong statements in Durgin et al., JCS 12 (2004), 247–311.