Editing Cryptographic hash function (section)

=== Proof-of-work ===
{{ main | Proof of work }}

A proof-of-work system (or protocol, or function) is an economic measure to deter [[denial-of-service attack]]s and other service abuses such as spam on a network by requiring some work from the service requester, usually meaning processing time by a computer. A key feature of these schemes is their asymmetry: the work must be moderately hard (but feasible) on the requester side but easy to check for the service provider. One popular system – used in [[Bitcoin mining]] and [[Hashcash]] – uses partial hash inversions to prove that work was done, to unlock a mining reward in Bitcoin, and as a good-will token to send an e-mail in Hashcash.  The sender is required to find a message whose hash value begins with a number of zero bits. The average work that the sender needs to perform in order to find a valid message is exponential in the number of zero bits required in the hash value, while the recipient can verify the validity of the message by executing a single hash function. For instance, in Hashcash, a sender is asked to generate a header whose 160-bit SHA-1 hash value has the first 20 bits as zeros. The sender will, on average, have to try {{math|2<sup>19</sup>}} times to find a valid header.