Sybil Control (Proof of…)

Con­sen­sus pro­to­cols can only deal with a lim­it­ed amount of mali­cious nodes. Sybil con­trol pre­vents the cheap intro­duc­tion of a large num­ber of mali­cious nodes, the basis for a sybil attack. Effec­tive sybil con­trol forces net­work par­tic­i­pants to put real-world resources at stake. Like this, par­tic­i­pants have “skin in the game” and are less like­ly to com­pro­mise the network’s integri­ty. The most com­mon sybil con­trol mech­a­nisms for today’s blockchains are proof of work (PoW) and proof of stake (PoS).

List of Sybil Control Mechanisms

• Proof of Work (PoW): to val­i­date blocks for the net­work, nodes have to solve hard math­e­mat­i­cal prob­lems in a com­pet­i­tive set­ting. The hard­ware and ener­gy spend­ings nec­es­sary to “win” the block cre­ation com­pe­ti­tion bind actors to the blockchain’s fate. This is the orig­i­nal sybil con­trol mech­a­nism for Bit­coin, estab­lished by Satoshi Nakamo­to. Apart from the work costs, PoW adds secu­ri­ty by its nat­u­ral­ly ran­dom selec­tion process of the win­ning block producer.
• Proof of Stake (PoS): block pro­duc­er rights are assigned in pro­por­tion to a node’s hold­ings of the network’s native token. This mech­a­nism binds block pro­duc­ers to the network’s val­ue and ide­al­ly keeps them from act­ing against col­lec­tive interest.
• Proof of Elapsed Time (POET): in a POET sys­tem, nodes go to sleep for a ran­dom amount of time. The first node to wake up obtains the right to pro­duce the next block. POET sys­tems have to ensure that nodes do indeed select a ran­dom sleep time, and that they do indeed wait for the said amount of time before wak­ing up again.
• Proof of Space and Time (PoST): this sybil con­trol mech­a­nism asks block pro­duc­ers to pro­vide a cer­tain amount of stor­age space to the sys­tem as a proof of their engage­ment. Block pro­duc­tion time is fac­tored into the chain’s weight to pre­vent cheap stor­age attacks on the his­tor­i­cal chain.
• Proof of Author­i­ty (PoA): here, the right to pro­duce a block is assigned to a num­ber of known nodes. This high­ly cen­tral­ized approach is used for pri­vate blockchains, or dur­ing the start­up phase of new pub­lic con­sen­sus mechanisms. 
• Proof of Diver­si­ty (PoD): PoD uses IPV4 as a scarce resource lim­it­ing net­work participation.

Sources

• https://files.avalabs.org/papers/platform.pdf
• https://www.investopedia.com/terms/p/proof-elapsed-time-cryptocurrency.asp