Md5 hash encryption and decryption

Question

I have a confusion regarding md5. I know that we cannot decrypt MD5 without attempting something like brute force hacking which is extremely tough. Now, For one md5 hash i visit this website. MD5Online For curiosity i decrypt that encrypted password to and i got the decrypted password. Then i tried 4-5 password which are previously stored in my database and this site decrypted all of them.

Then i tried with below code.

<?php
        $password = 'cool@123@!';
        $secure_md5password = md5($password);
        echo $secure_md5password;
?>

i got this md5 hash : 6234c13c3e1b965dbdd32d604151bd1b

I tried this hash in decryption of this site and i got 'cool@123@!'. I tried with other toughest passwords also.

So now i'm confuse about md5 algorithm. Is that website doing brute force or any thing and can we use any code in php which that site is using. I visit these links for answer but i can't find the answer. 1).encrypt-and-decrypt-md5 2).is-md5-decryption-possible 3).how-to-decrypt-an-md5-string-in-php

Answer 1

MD5 has been proven to be a weak algorithm. As all your 'passwords' are basic, the website has already stored each password with a hash that was cracked long ago.

You cannot decrypt a hash, but you can brute-force and find out what it is.

Read more here: https://en.wikipedia.org/wiki/MD5

and

here: https://security.stackexchange.com/questions/19906/is-md5-considered-insecure

EDIT: Saw that you updated your question.

Assuming that you were to have a complex password of 'ajfn3inf' and you were to hash it. Running a md5 cracker will be easy to unhash it due to it's relatively short length and the power of GPU's to crack a hash. Read the links above to understand more about MD5.

Answer 2

That website is probably using rainbow tables.

Information about this topic: https://en.wikipedia.org/wiki/Rainbow_table

Simple said: if they ever brute forced a hash, they'll save the hash and the password in a table. When someone enters a hash, they'll search the table and retrieve the unhashed value.