Hashcash and Linear Cost
TL;DR
Hashcash introduced Proof of Work to create “digital scarcity” of CPU time, making it expensive to send bulk spam emails.
What Is It?
Invented by Adam Back in 1997, Hashcash was a system designed to limit email spam and denial-of-service attacks. It required the sender to attach a “stamp” to every email. This stamp was a Proof of Work—a hash that met certain criteria (e.g., starting with 20 zeros).
Why Does It Matter?
- Linear Cost: In digital communication, sending 1 message or 1 million messages normally costs nearly the same (sublinear cost). Hashcash makes the cost linear: to send 1 million messages, you must expend 1 million times the CPU energy.
- Digital Scarcity: It proved that you could make “cheap” digital actions “expensive” by anchoring them to physical CPU time and electricity.
- Foundation of Mining: Bitcoin uses a modified version of Hashcash as its mining algorithm.
Significance
Hashcash didn’t solve the “double-spend” problem (you could reuse a stamp if the recipient didn’t check), but it provided the mechanism for Proof of Work that Bitcoin eventually used to secure the Timechain.