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Introduction to Crypto and Cryptocurrencies

Cryptographic Hash Functions

Hash Function:

• takes any strings as input
• fixd-size output (we'll use 256bits)
• efficiently computable
  

Security properties:

• collision-free
  • Nobody can find x ang y such that x != y and H(x)=H(y), but collision do exists.
• hiding
  • Give H(x), it is infeasiable to find x.
• puzzle-friendly
  • For every possible output value y, if k is chosen from a distribution with high min-entropy, then it is infeasible to find x such that H(k | x)=y.
  • 就是说看x不知道H（x）是什么？这个无法从输入数据，判断到底输出是什么样子。就是说，知道输入的信息，无法一眼看出来输出的hash值是什么，谜题友好性值得就是这一点：你无法通过控制输入值x来获得想要的输出值H（x）。

SHA-256 hash function
https://sha256algorithm.com/

Hash Pointers and Data Structures