What is a hash value? How to use a hash value

Many friends don't know what the hash value is. I want to know how to use the hash value.

What is the hash value?

The hash value is the ID of the file, but it is stricter than the ID card. He is calculated based on file size, time, type, creation, machine, etc. It is easy to change, no one can predict how much the next number is, nor change his software. The hash algorithm maps binary values ​​of arbitrary length to smaller fixed-length binary values. This small binary value is called a hash value. A hash value is a unique and extremely compact numerical representation of a piece of data. If you hash a plaintext and even change only one letter of the paragraph, subsequent hashes will produce different values. It is computationally impossible to find two different inputs that are hashed to the same value. Message Authentication Code (MAC) Hash functions are often used with digital signatures to sign data, while Message Detection Code (MDC) hash functions are used for data integrity. For example, Xiaohong and Xiaoming can use hash functions in the following way to ensure data integrity. If Xiaohong writes a message to Xiao Ming and creates a hash of the message, Xiao Ming can hash the message later and compare his hash to the original hash. If the two hash values ​​are the same, the message has not been changed; however, if the values ​​are not the same, the message has been changed since Xiaohong wrote it. In order for this system to work, Xiaohong must keep the original hash value secret to everyone except Xiao Ming.

Hash value application

Since the fixed hash value can be used in file search, for example, you can use the file hash value to search for files on the download tool. Take the emule as an example to illustrate the method of use.

I. Know how the resource HASH value searches for a file Expression: ed2k::file hash value For example, I know that the hash value of a file is: E8C636D0C0486378BF61E6A3000D0FB7 I can select the search method "server” or “Global Server”, then type: ed2k::E8C636D0C0486378BF61E6A3000D0FB7 This will find a file with a hash value equal to this value. If there are many file names, you can click the small plus sign in front of the file name “+”, View all file names.
2. Look for related file expressions: related:: file hash value is the same as above, select the search method "server" or "global server", then enter: related::E8C636D0C0486378BF61E6A3000D0FB7 to find the file related to this file Documents, but how specific is related, Xiaobian does not know for the time being.