What is MD5 checksum and what is MD5 checksum tool used for?

A typical application of MD5 is to generate a message-digest of a message to prevent tampering. For example, there are many software under UNIX that have the same file name and file extension when **. MD5 files, in which there is usually only one line of text, are roughly structured as follows:

md5 ( = 0ca175b9c0f726a831d895e269332461

This is the digital signature of the file. MD5 treats the entire file as a large text message, and through its irreversible string transformation algorithm, produces this unique md5 information digest. If there is any change in the content of the unspecified file in the process of disseminating the file in the future (including human modification or transmission errors caused by unstable lines in the process, etc.), as long as you recalculate the md5 of the file, you will find that the information summary is not the same, so you can be sure that you are only getting an incorrect file.

If there is a third-party certification body, using MD5 can also prevent the authorship of the document"Repudiation", which is known as a digital signature application.

MD5 is also widely used in encryption and decryption techniques. For example, in UNIX, the user's password is encrypted in MD5 (or other similar algorithms) and stored in the file system. When a 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 file system to determine whether the password entered is correct.

Through such steps, the system can determine the legitimacy of the user's login to the system without knowing the user's password. This not only prevents the user's password from being known by a user with system administrator privileges, but also increases the difficulty of password cracking to a certain extent.

It is for this reason that one of the most used methods by hackers to decipher passwords right now is one known as"Run the dictionary"method. There are two ways to obtain a dictionary, one is to collect a list of strings used as passwords on a daily basis, and the other is to use the permutation and combination method to generate the md5 values of these dictionary items with the md5 program, and then use the md5 value of the target to retrieve in this dictionary. Let's assume that the maximum length of the password is 8 bytes, and the password can only be letters and numbers, a total of 26 + 26 + 10 = 62 characters, and the number of entries in the permutated dictionary is p(62,1)+p(62,2)....

p (62,8), which is already a very astronomical number, and storing this dictionary requires terabytes of disk arrays, and this method also has a prerequisite, that is, if you can obtain the password md5 value of the target account. This encryption technology is widely used in UNIX systems, which is an important reason why UNIX systems are more robust than ordinary operating systems.

The MD5 verification tool is actually an MD5 encryption calculation software. The software can calculate the md5 value of the file, and then compare it with the official md5 value, which means that the software has not been modified.

MD5 can be said to be the "digital fingerprint" of files, and is often used for encryption and decryption of files. Any file has only one unique MD5 information value, and if the file is modified, its MD5 value will change accordingly.

Therefore, we can verify whether the file has been "tampered with" by comparing the MD5 value of the same file. This requires the use of the MD5 validation tool.

MD5 stands for Message Digest, which means message digest, but this message digest is not an abbreviation of information content, but a 128-bit feature code obtained by mathematically transforming the original information according to the publicly available MD5 algorithm.

This feature code has the following characteristics, first of all, it is irreversible, for example, I have a secret text such as:"my secret words"After the algorithm transformation, the md5 code (b9944e9367d2e40dd1f0c4040d4daaf7) is obtained, and the code is told to others, they have no systematic way to know what your original text is according to this md5 code.

The main purpose of the mathematical modeling graduate student upload contest is to prevent the tampering of the competition. The MD5 code is the characteristic code of the submitted ** and the supporting materials, and the code that uniquely identifies the work. If you change MD5 after you have committed it, it will be discovered, which is a means to eliminate cheating and package transfers.

MD5 stands for Message-Digest Algorithm 5 and was invented in the early 90's by MIT's Computer Science Lab and RSA Data Security Inc. The practical application of MD5 is to generate a fingerprint (fingerprint) of a message (byte string), which can prevent "tampering".