MD5 is a one-way operation to convert a data string of arbitrary length into a short fixed-length value. No two strings should have the same hash value (that is, there is a "high probability" that they are different, and it should be difficult to artificially create two strings with the same hash value).
MD5? Checksum checks the correctness of received transmission data by performing hash operation. The calculated hash value is compared with the hash value transmitted with the data.
If the two values are the same, it means that the transmitted data is complete and has not been tampered with (provided that the hash value has not been tampered with), so it can be used safely.
Extended data:
MD5 authentication can be used in many fields, such as the authentication of confidential information, the authentication of downloaded files, and the encryption of plaintext passwords.
MD5 application
Consistency verification
The typical application of MD5 is to generate a message digest for a message to prevent it from being tampered with. For example, under Unix, many softwares have a file with the same file name and a file extension of .md5 when downloading. In this file, there is usually only one line of text, and the general structure is as follows:
MD5(tanajiya.tar.gz)= 38 b8 C2 c 1093 dd0 FEC 383 a9 d9 AC 9405 15
This is the digital signature of tanajiya.tar.gz document. MD5 regards the whole file as a big text message, and generates this unique MD5 message digest through its irreversible string transformation algorithm.
The MD5 value of a file is like the "digital fingerprint" of this file. The MD5 value of each file is different. If someone makes any changes to the file, its MD5 value, that is, the corresponding "digital fingerprint", will change.
For example, the download server provides an MD5 value for the file in advance. After the user downloads the file, use my algorithm to recalculate the MD5 value of the downloaded file. By comparing whether these two values are the same, you can judge whether the downloaded file is wrong or whether the downloaded file has been tampered with.
In fact, MD5 is a lossy compression technique. Before compression, files must have the same MD5 value, and the same MD5 value cannot guarantee the same data before compression. The probability of this happening in cryptography is very small, so MD5 has a place in the field of cryptography.
Professional hackers and even ordinary hackers can use the principle that MD5 value is actually a lossy compression technology, and use the inverse value of MD5 as a hash table commonly known as rainbow table to crack passwords.
The scheme of checking files with MD5 algorithm is widely used in software download station, forum database, system file security and so on.
digital signature
The typical application of MD5 is to generate fingerprints for messages (byte strings) to prevent them from being tampered with.
For example, write a paragraph in a file named readme.txt, 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.
Secure access authentication
MD5 is also widely used in login authentication of operating systems, such as Unix, login passwords of various BSD systems, digital signatures and many other aspects. For example, in a Unix system, the user's password is hashed with MD5 (or other similar algorithm) and stored in the file system.
Baidu Encyclopedia -MD5 Verification