  A principal vulnerability of a proof-of-work ("PoW") blockchain is that an
attacker can re-write the history of transactions by forking a previously
published block and build a new chain segment containing a different sequence
of transactions. If the attacker's chain has the most cumulative mining puzzle
difficulty, nodes will recognize it as canonical. We propose a modification to
PoW protocols, called ADESS, that contains two novel features. The first
modification enables a node to identify the attacker chain by comparing the
temporal sequence of blocks on competing chains. The second modification
penalizes the attacker by requiring it to apply exponentially increasing
hashrate in order to make its chain canonical. We demonstrate two things; (i)
the expected cost of carrying out a double-spend attack is weakly higher under
ADESS compared to the current PoW protocols and (ii) for any value of
transaction, there is a penalty setting in ADESS that renders the expected
profit of a double-spend attack negative.
