What is the value of MD5? What is the MD5 value

MD5 is often used on forums and software releases, in order to ensure the correctness of the file, to prevent some people from stealing the program, adding some Trojans or tampering with the copyright, a set of verification system designed. Each file can be calculated with a fixed MD5 code using the MD5 validator. Software authors often calculate the MD5 code of their programs in advance and post them on the Internet.

Therefore, when you see a program on the Internet that indicates the MD5 code, you can write it down, and use the MD5 verification program to calculate the MD5 code of the file you have, and compare it with the MD5 code you wrote down before, so you can know whether you are under the original version, if the two are the same, then what you are ** is the original. If the calculation does not match the one noted on the Internet, then the document of your ** is incomplete, or it has been tampered with by others.

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

I found some relevant information:

MD5 stands for Message-Digest Algorithm 5, which was invented in the early 90's by MIT's Computer Science Lab and RSA Data Security Inc, and developed from MD2, MD3 and MD4.

message-digest generally refers to the hash transformation of a byte string (message), that is, the transformation of a byte string of any length into a large integer of a certain length. Note that I used the word "byte string" instead of "string" because this transformation is only about the value of the byte, not about the character set or how it is encoded.

MD5 converts arbitrary length of "string of bytes" into a large integer of 128 bits, and it is an irreversible string conversion algorithm, in other words, even if you see the source program and the description of the algorithm, you can't convert a value of MD5 back to the original string, mathematically speaking, because there are infinitely many original strings, which is a bit like a mathematical function with no inverse function.

A typical application of MD5 is to generate a fingerprint on a message to prevent it from being "tampered with". For example, if you write a paragraph in a file called and generate an md5 value for this and record it, then you can spread the file to someone else, and if someone changes anything in the file, you will find out when you recalculate the md5 of the file (the two md5 values are not the same). If there is a third-party certification authority, MD5 can also prevent "repudiation" by the author of the document, which is called a digital signature application.

MD5 is also widely used in encryption and decryption technology, in many operating systems, the user's password is saved in the form of MD5 value (or similar other algorithms), when the user login, 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, and the system does not "know" what the user's password is.

The MD5 value is a widely used cryptographic hash function that produces a 128-bit (16-byte) hash value that is used to ensure that information is transmitted intact and consistent.

The MD5 value is an increase from MD4"Seat belts"MD5 is more complex than MD4, but it is more secure. This algorithm is obviously composed of four steps that are slightly different from the MD4 design. In the MD5 algorithm, the size of the information-digest is exactly the same as in MD4.

What is MD5??? The full name of MD5 is message-digest

algorithm

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:

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 form of change in the content of the unsuccessful file in the process of propagating the file in the future (including manual modification or transmission errors caused by unstable lines in the process), as long as you recalculate the md5 of this file, you will find that the information summary is not the same, so you can be sure that what you get is only 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 a list of strings used as passwords collected on a daily basis, and the other is generated by the permutation and combination method of such things, and the md5 of these dictionary items is calculated by the md5 program

value, and then retrieve it in this dictionary with the md5 value of the target. Let's assume that the maximum length of the password is 8 bytes (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 dictionary is p(62,1)+p(62,2)....p

62,8), that is already a very astronomical number, and the storage of this dictionary requires a terabyte-level disk array, and this method also has a premise, 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.