Post

Obfuscation

Posted Updated
By Noah Balencourt 1 min read

Context

This challenge takes place in the source code of a web page :

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<html>
<head>
    <title>Obfuscation JS</title>
    <script type="text/javascript">
    function dechiffre(pass_enc){
        var pass = "70,65,85,88,32,80,65,83,83,87,79,82,68,32,72,65,72,65";
        var tab  = pass_enc.split(',');
                var tab2 = pass.split(',');var i,j,k,l=0,m,n,o,p = "";i = 0;j = tab.length;
                        k = j + (l) + (n=0);
                        n = tab2.length;
                        for(i = (o=0); i < (k = j = n); i++ ){o = tab[i-l];p += String.fromCharCode((o = tab2[i]));
                                if(i == 5)break;}
                        for(i = (o=0); i < (k = j = n); i++ ){
                        o = tab[i-l]; 
                                if(i > 5 && i < k-1)
                                        p += String.fromCharCode((o = tab2[i]));
                        }
        p += String.fromCharCode(tab2[17]);
        pass = p;return pass;
    }
    String["fromCharCode"](dechiffre("\x35\x35\x2c\x35\x36\x2c\x35\x34\x2c\x37\x39\x2c\x31\x31\x35\x2c\x36\x39\x2c\x31\x31\x34\x2c\x31\x31\x36\x2c\x31\x30\x37\x2c\x34\x39\x2c\x35\x30"));
    
    h = window.prompt('Entrez le mot de passe / Enter password');
    alert( dechiffre(h) );
    
</script>
</head>

</html>

We can see a call to a function named dechiffre :

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String["fromCharCode"](dechiffre("\x35\x35\x2c\x35\x36\x2c\x35\x34\x2c\x37\x39\x2c\x31\x31\x35\x2c\x36\x39\x2c\x31\x31\x34\x2c\x31\x31\x36\x2c\x31\x30\x37\x2c\x34\x39\x2c\x35\x30"));

Resolution

I decrypted the string in argument using the command :

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unescape("\x35\x35\x2c\x35\x36\x2c\x35\x34\x2c\x37\x39\x2c\x31\x31\x35\x2c\x36\x39\x2c\x31\x31\x34\x2c\x31\x31\x36\x2c\x31\x30\x37\x2c\x34\x39\x2c\x35\x30");

Then, I got a Unicode encoded string : 55,56,54,79,115,69,114,116,107,49,50

Unicode is a text encoding standard designed to support the use of text written in all of the world’s major writing systems. Version 15.1 of the standard defines 149 813 characters and 161 scripts used in various ordinary, literary, academic, and technical contexts. It has inherited ASCII standard in its 128 first code points. Unicode Table Unicode Table Sources : Wikipedia, Microsoft

Using the Javascript command :

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String.fromCharCode(55,56,54,79,115,69,114,116,107,49,50)

I finally got the password !

Root-me, Web Client
javascript
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