# Proof-of-work system

**Príncipe**

One of the methods extraction cryptocoins is PoW.This method consists in the continued execution of the calculation or solution a task.Most of the crypto currency uses this method.The main feature of it is the amount of time expended.PoW is oriented on finding a solution according to the algorithm known in advance for some finite time, but to check the obtained solution, a relatively small number of operations.

**Birth**

For the first time the presentation of the requirement for proving the performance of the work was given in 1993. The authors proposed - to access the common resource, the user must compute a function that is very complex and resource-intensive, but at the same time being solved in an acceptable time.The term "Proof of Work" or POW was first coined and formalized in a 1999 paper by Markus Jakobsson and Ari Juels.

**Background**

One popular system, used in Hashcash, uses partial hash inversions to prove that work was done.In past we talked about this.

PoW have two variants

Challenge response

And

Solution verification

Moreover, the underlying functions used by these schemes may be:

1)cpu-bound=where the computation runs at the speed of the processor

2)memory-bound=where the computation speed is bound by main memory accesses

3)network-bound=collect tokens from remote servers before querying the final server provider