Fabien Coelho An (Almost) Constant-Effort Solution-Verification Proof-of-Work Protocol Based on Merkle Trees. 80-93 2008 AFRICACRYPT http://dx.doi.org/10.1007/978-3-540-68164-9_6 conf/africacrypt/2008 db/conf/africacrypt/africacrypt2008.html#Coelho08