reverse string obfuscation that uses base64

Question

I found below javascript code to convert a string to an obfuscated base64 string.

But how to convert it back?

` 

function strobuscate(a) {
      return (a.split('').map(function(c, i) {
        return String.fromCharCode(c.charCodeAt(0) + i % 80);
      }).join('');
    }

`

use atob to convert back. check this https://www.geeksforgeeks.org/html-window-atob-method/#:~:text=The%20Window%20atob()%20method,which%20represents%20the%20decoded%20string. — Harmandeep Singh Kalsi, Jun 14 '20 at 12:38
@Harmandeep Singh Kalsi Yeah,i know it. but this base64 string is obfuscated, we need deobfuscate before base64decode. — k123c123, Jun 14 '20 at 12:41
What have you tried? Invert every operation in the inverse order. — luk2302, Jun 14 '20 at 12:41
@luk2302 hello,i tried to replace back "_" to "=", "." with "+" and "-" with "/". but i dont understand following code and how to reverse it. ` return y + obbtoa(a.split('').map(function(e, i) { return String.fromCharCode(e.charCodeAt(0) + i % t); }).join(''));` — k123c123, Jun 14 '20 at 12:52
Do you keep `t` somewhere? Otherwise might be quite hard, if I read correctly, since it seems to be a pseudo random seed. — Kaiido, Jun 14 '20 at 12:56
@Kaiido `t` is in `y`, which is prepended to the resulting string. — luk2302, Jun 14 '20 at 12:59

score 1 · Accepted Answer · answered Jun 14 '20 at 12:59

You have to invert each operation and invert the order of the operations. I will give you a breakdown of every operation and leave it to you to actually do the inversion:

Preparation:

t = parseInt((+new Date()).toString().substr(11)): it gets the the last two digits of the current UTC timestamp, aka the 100th and the 1000th of a second. The || 1 is just for error handling in case the Date does not give a useful result.
y = ('0' + t.toString(16)).substr(-2): convert the previous t into base 16. The 0 prefix and -2 substring are once again just corner case error handling

Obfuscation:

a.split('').map(function(e, i) { return String.fromCharCode(e.charCodeAt(0) + i % t); }).join(''): iterate over every char of the input string and apply the inner function:
- get the char code and add the index modulo t and get a String back from that char code
pass the char transformed string into obbtoa:
- btoa the string
- replace different special characters
the y is prepended to the result string so the decoding can know both the t and y.

To get an idea of the reversal:

take the first two characters to get y, inverse the operation we used to compute y from t to get "our" t
inverse the replacements, inverse the btoa
inverse the char transformation by basically doing a - instead of a +

"0 prefix and -2 substring" is actually a padStart(2, '0'), to get a fixed length in case it produced 0x0n — Kaiido, Jun 14 '20 at 13:02

reverse string obfuscation that uses base64

1 Answers1