  The adversary's goal in mounting Long Range Attacks (LRAs) is to fool
potential victims into using and relying on a side chain, i.e., a false,
alternate history of transactions, and into proposing transactions that end up
harming themselves or others. Previous research work on LRAs on blockchain
systems have used, at a high level, one of two approaches. They either try to
(1) prevent the creation of a bogus side chain or (2) make it possible to
distinguish such a side chain from the main consensus chain.
  In this paper, we take a different approach. We start with the
indistinguishability of side chains from the consensus chain -- for the
eclipsed victim -- as a given and assume the potential victim will be fooled.
Instead, we protect the victim via harm reduction applying "short leashes" to
transactions. The leashes prevent transactions from being used in the wrong
context.
  The primary contribution of this paper is the design and analysis of leashes.
A secondary contribution is the careful explication of the LRA threat model in
the context of BAR fault tolerance, and using it to analyze related work to
identify their limitations.
