Appending Data to Blockchain is not Sufficient for Non-repudiation of Receipt

Exchanging data while ensuring non-repudiation is a challenge, especially if no trusted third party exists. Blockchain promises to provide many of the required guarantees, which is why it has been used in many non-repudiable data exchange protocols. Specifically, some authors propose to append data to blockchain to achieve non-repudiation of receipt. In this position paper, we show that this approach is insufficient. While appending data to blockchain can guarantee non-repudiation of origin in some cases, it is not sufficient for non-repudiation of receipt. For confidential data, we find a catch-22 that makes it impossible. For non-confidential data, meanwhile, plausible deniability remains. We discuss potential solutions and suggest smart contracts as a promising approach.