What exactly is MD5? ? _PHP Tutorial

Question
What exactly is MD5? ?
Solution
MD5 is the abbreviation of message-digest algorithm 5. It is widely used in encryption and decryption technology. It can be said to be the "digital fingerprint" of a file. Any file, whether it is an executable program, an image file, a temporary file or any other type of file, regardless of its size, has and has only one unique MD5 information value, and if the file has been modified, its MD5 The value will also change accordingly. Therefore, we can verify whether the file has been "tampered with" by comparing the MD5 value of the same file.

What is the use of MD5

After we download a file like the one shown in Figure 1, if we want to know whether the downloaded file is exactly the same as the original file on the website, we can do an MD5 check for the downloaded file. If the MD5 value obtained is the same as that published on the website, it can be confirmed that the downloaded file is complete. If there is a difference, it means that the file you downloaded is incomplete: either an error occurred during the network download process, or the file has been modified by someone else. To prevent others from inserting viruses when they change the file, it is best not to use it.

When we send a file to a friend by e-mail, we can tell the other party the MD5 value of the file to be sent, so that the friend can verify it after receiving the file to determine whether the file is safe.

Another example: After the system has just been installed, you can do an MD5 check on the system files. After a while, if you suspect that some files have been replaced, you can do an MD5 check on the suspected files. If it is different from the MD5 check code obtained before, then there is definitely something wrong.

How to read and verify MD5 information

After understanding the MD5 information, let's take a look at how to read and verify the MD5 information of the file. This requires a special small software to detect the MD5 value. This is a green software. Just decompress it and run the MD5.EXE file. The software is very simple to use. Click the "Open" button, select and open the file you want to verify. After a moment, the MD5 value of the file will be displayed in the MD5 column. Compare this value with the value published on the website. Compare to determine whether the file is complete. Click the "Save" button to save the read MD5 as an .MD5 file. Open the file with Notepad and copy the MD5 value.


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