MD5 is often used in forums and software releases. It is a verification system designed to ensure the correctness of files and prevent some people from stealing programs, adding some Trojans or tampering with copyright. Each file can use MD5 validator to calculate a fixed MD5 code. Software authors often calculate the MD5 code of his program in advance and publish it on the Internet. So when you see the MD5 code next to a program download on the Internet, you can write it down. After downloading this program, use the MD5 validator to calculate the MD5 code of the file you downloaded. Compare the MD5 code you wrote before, and you will know if you downloaded the original. If they are the same, then you downloaded the original. If the calculated value does not match what is displayed on the Internet, then the file you downloaded is incomplete, or someone else has tampered with it.

I wrote it myself. I don't know if I made myself clear.

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I found some relevant information:

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The full name of MD5 is Message-Digest Algorithm 5, which was invented by MIT Computer Science Laboratory and RSA Data Security Company in the early 1990s and developed from MD2, MD3 and MD4.

Message-Digest generally refers to the hash transformation of messages, that is, the byte string of any length is transformed into a large integer with a certain length. Please note that I use the word "byte string" instead of "string" because this conversion is only related to the value of bytes, not to the character set or encoding method.

MD5 converts a "byte string" of any length into a large integer of 128bit, which is an irreversible string conversion algorithm. In other words, even if you see the description of the source program and algorithm, you can't convert an MD5 value back to the original string. Mathematically, it is because there are infinite original strings, which is a bit like a mathematical function without inverse function.

The typical application of MD5 is to generate fingerprints for messages (byte strings) to prevent them from being tampered with. For example, if you write a paragraph in a file named readme.txt and generate an MD5 value for this readme.txt and record it, then you can spread this file to others. If someone else modifies anything in the file, when you recalculate the MD5 of this file (the two MD5 values are different), you will find that. If there is a third-party certification authority, MD5 can also prevent the "denial" of the document author, which is the so-called digital signature application.

MD5 is also widely used in encryption and decryption technology. In many operating systems, users' passwords are saved in the form of MD5 values (or similar other algorithms). When the user logs in, the system calculates the password entered by the user as the MD5 value, and then compares it with the MD5 value saved in the system, but the system does not "know" what the user's password is.