  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.
