This paper studies mechanisms for allocating rights to forge new blocks
on a blockchain. The proof of work contest (PoW) where all block
generators, or miners, compete for finding the next block is
characterized. The central feature of the PoW contest is that no miner
has an incentive to participate with multiple identities, a property
called sybil proofness. There are several alternatives for the PoW that
involve payments and messaging between the miners and the protocol,
features that are not used in the PoW contest. A class of mechanisms
that generalizes the proof of stake procedure is introduced for
allocating multiple rights to carry out the mining of blocks. These
mechanisms are characterized by a set of elementary properties.
Auctioning the rights to add blocks into a blockchain is also
considered.