哈希函数

In computer programming hash functions map text (or other data) to integer numbers. Usually different inputs maps to different outputs, but sometimes a collision may happen (different input with the same output).

在计算机编程中，哈希函数将文本（或其他数据）映射为整数。通常情况下不同的输入将映射得到不同的输出，但有时也可能会发生碰撞（不同的输入具有相同的输出）。

Cryptographic hash functions transform text or binary data to fixed-length hash value and are known to be collision-resistant and irreversible. Example of cryptographic hash function is SHA3-256:

加密哈希函数用来将文本或二进制数据转换为固定长度的哈希值，并且它具有抗碰撞性和不可逆性。加密哈希函数的一个例子是 SHA3-256：

SHA3-256("hello") = "3338be694f50c5f338814986cdf0686453a888b84f424d792af4b9202398f392"

The above SHA3-256 hash calculation can be coded in Python like this:

上述 SHA3-256 哈希计算可用 Python 编码如下：

import hashlib, binascii
sha3_256hash = hashlib.sha3_256(b'hello').digest()
print("SHA3-256('hello') =", binascii.hexlify(sha3_256hash))

Run the above code example: https://repl.it/@nakov/SHA3-256-hello-in-Python.

可在此链接中运行该示例代码：https://repl.it/@nakov/SHA3-256-hello-in-Python。

The same SHA3-256 hash calculation can be coded in JavaScript like this (after npm install js-sha3):

类似的，SHA3-256 哈希计算可以用 JavaScript 编码如下（需先执行 npm install js-sha3）：

sha3 = require('js-sha3');
let sha3_256hash = sha3.sha3_256('hello').toString();
console.log("SHA3-256('hello') =", sha3_256hash);

Run the above code example: https://repl.it/@nakov/SHA3-256-hello-in-JS.

可在此链接中运行该示例代码：https://repl.it/@nakov/SHA3-256-hello-in-JS。

软件工程中的哈希

The process of calculating the value of certain hash function is called "hashing".

计算某个哈希函数值的过程称为“哈希”。

In the above example the text John Smith is hashed to the hash value 02 and Lisa Smith is hashed to 01. The input texts John Smith and Sandra Dee both are hashed to 02 and this is called "collision".

在上面的例子中，文本 John Smith 被哈希为哈希值 02，Lisa Smith 被哈希为 01。文本 John Smith 和 Sandra Dee 都被哈希为 02，这就是所谓的“碰撞”。

Hash functions are irreversible by design, which means that there is no fast algorithm to restore the input message from its hash value.

哈希函数在设计上是不可逆的，这意味着没有快速的算法来从其哈希值恢复输入信息。

In programming hash functions are used in the implementation of the data structure "hash-table" (associative array) which maps values of certain input type to values of another type, e.g. map product name (text) to product price (decimal number).

在编程中，哈希函数用于“哈希表”（关联数组）数据结构的实现，该结构将某种输入类型的值映射到另一种类型的值，例如，将产品名称（文本）映射到产品价格（小数）。

A naive hash function is just to sum the bytes of the input data / text. It causes a lot of collisions, e.g. hello and ehllo will have the same hash code. Better hash functions may use the Merkle–Damgård construction scheme, which takes the first byte as state, then transforms the state (e.g. multiplies it by a prime number like 31), then adds the next byte to the state, then again transforms the state and adds the next byte, etc. This significantly reduces the rate of collisions and produces better distribution.

简单的哈希函数就是对输入数据或文本的字节求和。这会引起很多碰撞，例如 hello 和 ehllo 会有相同的哈希值。更好的哈希函数可以使用 Merkle–Damgård 构造方案，它将第一个字节作为状态，然后转换状态(例如将其乘以一个质数，如 31)，然后将下一个字节添加到状态中，然后再次转换状态并添加下一个字节，依次重复。这将大大降低碰撞率并产生更好的分布。

加密哈希函数

In cryptography, hash functions transform input data of arbitrary size (e.g. a text message) to a result of fixed size (e.g. 256 bits), which is called hash value (or hash code, message digest, or simply hash). Hash functions (hashing algorithms) used in computer cryptography are known as "cryptographic hash functions". Examples of such functions are SHA-256 and SHA3-256, which transform arbitrary input to 256-bit output.

在密码学中，哈希函数将任意大小的输入数据（如文本信息）转换为固定长度的哈希值（也可称为哈希码、信息摘要或简单哈希）结果（例如 256 位）。计算机密码学中使用的哈希函数（哈希算法）称为”加密哈希函数“。这类函数的例子有 SHA-256 和 SHA-256，它们将任意输入转换为 256 位的输出。

加密哈希函数——示例

As an example, we can take the cryptographic hash function SHA-256 and calculate the hash value of certain text message hello:

以加密哈希函数 SHA-256 为例，计算文本信息 hello 的哈希值：

SHA-256("hello") = "2cf24dba5fb0a30e26e83b2ac5b9e29e1b161e5c1fa7425e73043362938b9824"

The above SHA-256 calculation can be coded in Python like this:

上述 SHA-256 哈希计算可用 Python 编码如下：

import hashlib, binascii

sha256hash = hashlib.sha256(b'hello').digest()
print("SHA-256('hello') = ", binascii.hexlify(sha256hash))

Run the above code example: https://repl.it/@nakov/SHA-256-hello-in-Python.

可在此链接中运行该示例代码：https://repl.it/@nakov/SHA-256-hello-in-Python。

There is no efficient algorithm to find the input message (in the above example hello) from its hash value (in the above example 2cf24dba5fb0a30e26e83b2ac5b9e29e1b161e5c1fa7425e73043362938b9824). It is well-known that cryptographic hash functions cannot be reversed back, so they are used widely to encode an input without revealing it (e.g. encode a private key to a blockchain address without revealing the key).

没有一种有效的算法可以从其哈希值（在上述示例中为 2cf24dba5fb0a30e26e83b2ac5b9e29e1b161e5c1fa7425e73043362938b9824）中找到输入信息（在上述示例中为 hello）。 众所周知，加密哈希函数无法逆推，因此它们被广泛地用于在不显示输入的情况下对输入进行编码（例如，在不显示密钥的情况下对一个区块链地址的私钥进行编码）。

As another example, we can take the cryptographic hash function SHA3-512 and calculate the hash value of the same text message hello:

再举一个例子，我们可以采用加密哈希函数 SHA3-512 计算同一文本信息 hello 的哈希值：

SHA3-512("hello") = "75d527c368f2efe848ecf6b073a36767800805e9eef2b1857d5f984f036eb6df891d75f72d9b154518c1cd58835286d1da9a38deba3de98b5a53e5ed78a84976"

加密哈希函数——在线演示

Play with most popular cryptographic hash functions online: https://www.fileformat.info/tool/hash.htm.

可在线使用最流行的加密哈希函数：https://www.fileformat.info/tool/hash.htm

Cryptographic hash functions are widely used in cryptography, in computer programming and in blockchain systems.

加密哈希函数广泛用于加密、计算机编程和区块链系统中。