POW (Proof-of-Work)

Proof of Work (PoW) algorithm or protocol or function is not necessarily tied to the world of Cryptocurrency alone. In fact, its roots are deeper than the inception of any of the contemporary cryptocurrencies or blockchain techniques. Originally, the proof of work (PoW) idea was thrown to deter the brute force attacks such as Denial of Service (Dos) and Spamming. The idea is very simple. It is sometimes considered analogous to the postal mails where the users have to affix stamps on the mails that they want to send out to others. When a user affixes a stamp on the envelope, he has proven his authenticity by demonstrating that he has spent some money to buy a stamp. Hence, the mailing system, such as USPS, accepts his mails. A similar concept was applied in the digital world, which we now call proof-of-work. But, instead of requiring to spend money (buying stamps), the users or their computers are required to spend some CPU cycles as the proof of their work. Consider the Captcha Turing test that distinguishes a robot from humans and hence prevents DoS attacks.