  In a blockchain Data Availability Attack (DAA), a malicious node publishes a
block header but withholds part of the block, which contains invalid
transactions. Honest full nodes, which can download and store the full
blockchain, are aware that some data are not available but they have no formal
way to prove it to light nodes, i.e., nodes that have limited resources and are
not able to access the whole blockchain data. A common solution to counter
these attacks exploits linear error correcting codes to encode the block
content. A recent protocol, called SPAR, employs coded Merkle trees and
low-density parity-check codes to counter DAAs. In this paper, we show that the
protocol is less secure than claimed, owing to a redefinition of the
adversarial success probability. As a consequence we show that, for some
realistic choices of the parameters, the total amount of data downloaded by
light nodes is larger than that obtainable with competitor solutions.
