Why is MD5 irreversible, but it may also be decrypted by MD5 free decryption website

md5 It plays a very important role in network security , It is used to ensure the integrity of data , And the data is not tampered . meanwhile ,md5 It is also used for password encryption . Some friends don't know why md5 Irreversible , But it may also be claimed to be able to decrypt md5 Decryption of . therefore , Some people say md5 Not enough security , It should not be widely used .

 

md5 Why not reversible ？

md5 Irreversible because it is a hash function , Also called hash functions . It is a one-way cryptosystem , That is, the irreversible mapping from plaintext to ciphertext , Only the encryption process, no decryption process . The hash function can change the input of any length to obtain a fixed length output , This fixed length output is called the hash of the original message or the message mapping . Be careful ： In the process of calculation, some information of the original text is lost .

md5 Not enough security , Should not be widely used

MD5 Is a widely used cryptographic hash function , It is one of the methods to realize data integrity verification .md5 It is mainly used to ensure the integrity of data , Ensure that data is not lost in transmission , It will not be tampered with . Some people say md5 Not enough security , It should not be widely used , This view is right . however , actually md5 It is also widely used . Because the current data communication project , The old software and hardware only support md5, Level protection has not been abolished for the time being md5 Use . Although there are security loopholes , But many places have not updated the hash algorithm .

Why? md5 May be decrypted by some websites

md5 The reason why it can be decrypted , Because many people store a large number of strings calculated in advance into the database , For inquiry . such md5 Decrypt , In fact, it is to query from the database , This is not really decryption .md5 Decryption is an irreversible Algorithm , It can't be decrypted .

Those websites on the Internet that say they can be decrypted , Equivalent to md5 Dictionary library , It is the corresponding table of the original text and ciphertext . This kind of website has a huge amount of data , Trillions of levels , If the user's ciphertext just exists in the dictionary , Just check the corresponding table . But this kind of website is not all md5 Can be decrypted , If the user's password is complex , It depends on the database and data volume of this website , Generally speaking, too complex passwords cannot be found .

md5 Declassification website declassification md5 test

So , Let's do a test , Find one on the Internet md5 Online decryption website ：https://www.md5.cn/, Log in to your account first , Then enter the ciphertext ：a141c47927929bc2d1fb6d336a256df4. Then enter the verification code , Then click blue to decrypt security , Then get the decryption result ：abc1234. Before and after 5 Seconds to find out , This speed is ok .

 

But not all ciphertext can be decrypted , We enter ：ba59abbe56e058dejw5edh, Then enter the verification code , Click the decryption button , After a while, there will be such a line of words ： Decryption failed , Has joined the background decryption of this site , Please wait patiently for the decryption result ！ The system will automatically notify you of the latest results ！ This indicates that the corresponding original text is not included in the database , Can't find out .

 

in summary ,md5 Irreversible because it is a one-way cryptosystem , Some information of the original text is lost in the process of calculation . Many websites on the Internet say they can decrypt md5, In fact, it is not really decryption , It just queries the matching original text from the database .