It starts out with a secret password input.
Like other levels, I looked at the source code to see what was going on in the background.
Based on this code, you can see that it has a stored secret value, that is base64 encoded, then reversed, and then converted to a hex string. That value is then compared to the stored secret value, and if it’s a match, it will give you the next level’s password.
I decided to write a reversal program in php, since it was super easy. The only problem it gave me, was that there is no built-in hex2bin method until a newer version of PHP than I had on my machine. Luckily, on the bin2hex documentation page, someone kindly wrote a reversal function that I stole.
Once I executed that program in php, it gave me the decoded password. Once I entered it into the page, it gave me the password to the next level.