Cryptocurrency uses blockchain to record transactions between entities.When users send and receive a given number of cryptocurrencies,these transactions will be V+(cc13422088639) broadcast on P2P networks.Other entities monitor these transactions,combine them into a"block"at regular intervals,and then broadcast the new block to the network after calculating the workload proof.
这些应用的流行哈希函数算法有:
md5:2^128个值（找一个冲突键，需要哈希大约2^64个值）
sha-1：2^160个值（找一个冲突键，需要大约2^80个值）
Java字符串的hashCode()就是一个哈希算法，它的输入是任意字符串，输出是固定的4字节int整数：
"hello".hashCode(); // 0x5e918d2
"hello, java".hashCode(); // 0x7a9d88e8
"hello, bob".hashCode(); // 0xa0dbae2f
两个相同的字符串永远会计算出相同的hashCode，否则基于hashCode定位的HashMap就无法正常工作。这也是为什么当我们自定义一个class时，覆写equals()方法时我们必须正确覆写hashCode()方法。
哈希碰撞
哈希碰撞是指，两个不同的输入得到了相同的输出：
"AaAaAa".hashCode(); // 0x7460e8c0
"BBAaBB".hashCode(); // 0x7460e8c0
The most important feature of hash algorithm is:
The same input must be get the same output;
Different input large probability to get different output.
Hash algorithm is the purpose of to test whether the raw data been tampered with, hash game development V hkkf5566 docking, the rules of the game can be customized.
不能猜测输出是指，输入的任意一个bit的变化会造成输出完全不同，这样就很难从输出反推输入（只能依靠暴力穷举）。假设一种哈希算法有如下规律：
hashA("java001") = "123456"
hashA("java002") = "123457"
hashA("java003") = "123458"
那么很容易从输出123459反推输入，这种哈希算法就不安全。安全的哈希算法从输出是看不出任何规律的：
hashB("java001") = "123456"
hashB("java002") = "580271"
hashB("java003") = ???
There are children's shoes will ask: collision can avoid? The answer is not. Collision will occur, because of the output byte length is fixed, the String of hashCode () output is 4 bytes integer, only up to 4294967296 kinds of output, but the input data length is not fixed, there are numerous kinds of input. So hash algorithm is an infinite input set mapped to a limited output set, will inevitably produce collision.
Collision is not terrible, we worry about is not the collision, but the probability of collision, because of the discretion of the collision probability is related to the security of hashing algorithm. A secure hash algorithm must be satisfied:
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