What is a hash value

Many friends do not know what the hash value is, want to know how to use the hash, small series here to do some popular science.

What is the hash value

The hash value is the ID card of the file, but it is stricter than the ID card. He is based on file size, time, type, creation, machine, etc. calculated, it is easy to change, who can not predict the next number is how much, and did not change his software. The hash algorithm maps the binary value of any length to a smaller binary value of a fixed length, which is called a hash value. A hash value is a unique and extremely compact numeric representation of a piece of data. If you hash a clear text 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 hash the same value.

Message authentication Code (MAC) hash functions are typically used with digital signatures to sign data, while message detection code (MDC) hash functions are used for data integrity.

For example, Xiao Hong and xiaoming can use hash functions in the following ways to ensure data integrity. If Xiao Hong writes a message to xiaoming and creates a hash of the message, Xiaoming can hash the message later and compare his hash with the original hash. If the two hashes are the same, the message is not changed, but if the value is not the same, the message is changed after it has been written by Little Red. In order for this system to run, the red must keep the original hash value for everyone except Xiaoming.

Application of hash value

Because of its own fixed hash value can be used in file search, such as the use of file hashes on the download tool to search for files, the following small set to the electric Donkey (emule) as an example, explain the use of the method.

I. Know how a resource hash value searches for a file

An expression:

ED2K:: File hash value

For example, I know the hash value of a file is: E8c636d0c0486378bf61e6a3000d0fb7

I can select "Server" or "Global server" in the search mode, and then enter:

Ed2k::e8c636d0c0486378bf61e6a3000d0fb7

This will be able to find the hash value is equal to this value of the file, if there are many kinds of file name, you can point to the file name before the small plus "+" to see all the file names.

As shown in the figure:

Known file hash value search for a file on the donkey server

Two. Find relevant documents

An expression:

Related:: File hash value

Ditto select the search method "server" or "Global Server", and then enter:

Related::e8c636d0c0486378bf61e6a3000d0fb7

You can find the file related to this file, but specifically how relevant, the small part of the temporary don't know.

Finally, attach a handy hash value computing tool (Hashchecker):