Proof of Work To add a block B,a node needs to find a nonce,n,such that the value of the hash function h applied to the concatenation of n and B(n B)is less that some specified value. The function h must have the puzzle-friendliness property:Given k and an n-bit value y,it is infeasible to find x such that h(x k)=y in time significantly less than 2h. ■Forks If more than one node solves the puzzle around the same time,two blocks could be added after the most recent block ·The result is a fork Since nodes attempt to add to the most recent block of the longest chain,eventually blocks on shorter forks are orphaned(slide 7) Database System Concepts-7th Edition 26.12 ©Silberscha乜，Korth and SudarshanDatabase System Concepts - 7 26.12 ©Silberschatz, Korth and Sudarshan th Edition Proof of Work ▪ To add a block B, a node needs to find a nonce, n, such that the value of the hash function h applied to the concatenation of n and B (n || B) is less that some specified value. ▪ The function h must have the puzzle-friendliness property: Given k and an n-bit value y, it is infeasible to find x such that h(x || k) = y in time significantly less than 2n . ▪ Forks • If more than one node solves the puzzle around the same time, two blocks could be added after the most recent block • The result is a fork • Since nodes attempt to add to the most recent block of the longest chain, eventually blocks on shorter forks are orphaned (slide 7)