CODEWITHSUNDEEP
javascriptencryptionxor
James Harzs
James Harzs

Reputation: 1913

String XOR encryption fails with some keys

I am trying to "encrypt" a string (js code) with php, and then decode it using javascript.

This is the php function:

function xor_string( $text, $xorKey ) {
    $xored = '';
    $chars = str_split( $text );

    $i = 0;

    while ( $i < count( $chars ) ) {
        $xored .= chr( ord( $chars[$i] ) ^ $xorKey );
        $i++;
    }

    return $xored;
}

This is the js function:

function xor_string( str, key ) {
    var xored = "";
    for (i=0; i<str.length;i++) {
        var a = str.charCodeAt(i);
        var b = a ^ key;
        xored = xored+String.fromCharCode(b);
    }
    console.log(xored);
}

This work way both ways with certain keys but with others it fail, for example:

echo urlencode( xor_string( 'document.location.href.search', 67 ) );

Returns:

%27%2C+6.%26-7m%2F%2C+%227%2A%2C-m%2B1%26%25m0%26%221+%2B

And when I try to "decode" it with javascript using:

var str = decodeURIComponent("%27%2C+6.%26-7m%2F%2C+%227%2A%2C-m%2B1%26%25m0%26%221+%2B");
xor_string( str, 67 );

It returns:

dohument.lohation.href.searhh

Anyone knows why this is happening?

With some "keys" such as 120 and others it works fine, but with many others it fails.

Upvotes: 0

Views: 2193

Answers (2)

Tigger
Tigger

Reputation: 9130

A better solution is to use rawurlencode() instead of urleconde().

rawurlencode() will convert a space to '%20' but urlencode() will convert a space to '+'. '%20' is what decodeURIComponent() is expecting for a space.

See the following full example:

<?php
    function xor_string( $text, $xorKey ) {
    $xored = '';
    $chars = str_split( $text );
    $i = 0;
    while ( $i < count( $chars ) ) {
        $xored .= chr( ord( $chars[$i] ) ^ $xorKey );
        $i++;
    }
    return $xored;
    }
?><html>
<body>
Encoded (php):
<div id="phpUrlEncode">
<?=urlencode( xor_string( 'document.location.href.search', 67 ) )?>
</div>
<div id="phpRawUrlEncode">
<?=rawurlencode( xor_string( 'document.location.href.search', 67 ) )?>
</div>
<br />
Decoded (js):
<div id="jsDecodeUrl"></div>
<div id="jsDecodeRawUrl"></div>
<script type="text/javascript">
function decodeStr(rawId,displayId) {
    var raw = document.getElementById(rawId).innerHTML;
    document.getElementById(displayId).innerHTML = xor_string(decodeURIComponent(raw),67);
}

function xor_string( str, key ) {
    var xored = "";
    for (i=0; i<str.length;i++) {
        var a = str.charCodeAt(i);
        var b = a ^ key;
        xored = xored+String.fromCharCode(b);
    }
    //console.log(xored);
    return xored;
}

decodeStr('phpUrlEncode','jsDecodeUrl');
decodeStr('phpRawUrlEncode','jsDecodeRawUrl');
</script>
</body>
</html>

Upvotes: 1

Bergi
Bergi

Reputation: 665364

A classic :-)

PHP's urlencode does not exactly the same as JavaScript's encodeURIComonent: they handle blanks different; one uses +, the other %20.

You'll need to handle that, and e.g. phpjs offers a PHP-compliant decodeURI function.

> var phpstring = "%27%2C+6.%26-7m%2F%2C+%227%2A%2C-m%2B1%26%25m0%26%221+%2B";
> xor_string(decodeURIComponent(phpstring.replace(/\+/g, "%20")), 67 );
"document.location.href.search"

As you might notice, this error hits only the character that is encoded to a space with your xor function (and its parameter).

Upvotes: 4

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