Implement PHP string xor with JavaScript

Question

I would like to convert this PHP code$a ^ $b(where length of a is 16, b is 32) to javascript code. Here is my implementation.

    var xor = "";
    for (var i = 0; i < a.length; i++) {
        xor += String.fromCharCode(a.charCodeAt(i) ^ b.charCodeAt(i));
    }
    xor = b.substr(-16) + xor;

However, the result not the same. Please help me to figure it out. Thanks.

By the way, here's the part of code I'm working around:

    var secret = "secret";
    var password = "password";
    var chal = "2ba5565a539c57c1ce2356e218faa321";
    var hexchal = php.pack('H32', chal);
    var newchal, newpwd, pappassword;

    newchal = php.pack('H*', php.md5(hexchal + secret));
    newpwd = php.pack("a32", password);
    var xor = "";
    for (var i = 0; i < newchal.length; i++) {
        xor += String.fromCharCode(newchal.charCodeAt(i) ^ newpwd.charCodeAt(i));
    }
    xor = newpwd.substr(-16) + xor;

The corresponding PHP code:

    <?php
    $secret = "secret";
    $password = "password";
    $chal = "2ba5565a539c57c1ce2356e218faa321";

    $hexchal = pack ("H32", $chal);
    $newchal = pack ("H*", md5($hexchal . $secret));

    $newpwd = pack("a32", $password);
    $pappassword = implode ("", unpack("H32", ($newpwd ^ $newchal)));
    echo $pappassword;
    ?>

Where a and b are newchal and newpwd, respectively. And php.func()s come from http://phpjs.org/. The expected output is "821f984aa1062e56dbdc8f77454e5eb3".

Can you post sample input and output (both actual and expected)? — David Harkness
– David Harkness, Commented Jun 6, 2012 at 5:32
@trantor with PHP I get f27eeb39d6695c32dbdc8f77454e5eb3 xor 7461746573740000000000000000000000000000000000000000000000000000 == 861f9f5ca51d5c32dbdc8f77454e5eb3 - care to also post a working PHP version? :) — Ja͢ck
– Ja͢ck, Commented Jun 6, 2012 at 6:14
@Jack I added corresponding PHP code and modified the password. Hope no mistake this time. — Trantor Liu
– Trantor Liu, Commented Jun 6, 2012 at 6:25
@trantor thanks! I've modified my answer, it produces the correct output — Ja͢ck
– Ja͢ck, Commented Jun 6, 2012 at 6:34

Ja͢ck · Accepted Answer · 2012-06-09 14:06:13Z

5

I've used your code almost literally:

String.prototype.xor = function(other)
{
  var xor = "";
  for (var i = 0; i < this.length && i < other.length; ++i) {
      xor += String.fromCharCode(this.charCodeAt(i) ^ other.charCodeAt(i));
  }
  return xor;
}

The loop should stop when the end of either string is reached.

Tested with:

String.prototype.toHex = function() {
    var hex = '', tmp;
    for(var i=0; i<this.length; i++) {
        tmp = this.charCodeAt(i).toString(16)
        if (tmp.length == 1) {
            tmp = '0' + tmp;
        }
        hex += tmp
    }
    return hex;
}

var x = "password\u0000\u0000\u0000\u0000\u0000\u0000\u0000\u0000\u0000\u0000\u0000\u0000\u0000\u0000\u0000\u0000\u0000\u0000\u0000\u0000\u0000\u0000\u0000\u0000";
var y = "\u00f2~\u00eb9\u00d6i\\2\u00db\u00dc\u008fwEN^\u00b3";

x.xor(y).toHex();

Output:

821f984aa162e56dbdc8f77454e5eb3

edited Jun 9, 2012 at 14:06

answered Jun 6, 2012 at 5:37

Ja͢ck

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4 Comments

Trantor Liu Over a year ago

toString(16) doesn't guaranteed to return to char. For example, '\t'.charCodeAt(0).toString(16) return '9'. So my for-loop in toHex() is tmp = ''+this.charCodeAt(i).toString(16); if (tmp.length == 1) tmp = '0' + tmp; hex += tmp;

Ja͢ck Over a year ago

@trantor isn't that what I have? Maybe it's my phone but I don't see the diff :)

Trantor Liu Over a year ago

For example, chal = '15ff497252bb7c5ce5a9da7a85d07121'. Then with your toHex(), newpwd.xor(newchal).toHex() returns 'd93b42daab792cc8f601a99a2e28c6b', and with mine '0d93b42daab792cc8f601a99a2e28c6b'. The only difference is the leading '0'. Because the first char code of newpwd.xor(newchal) is 13, and Number(13).toString(16) is 'd' not '0d'.

Ja͢ck Over a year ago

@trantor right, that make absolute sense :) weird how I couldn't see that on my phone .. i'll edit my answer, thanks!!

Bergi · Accepted Answer · 2012-06-06 05:48:36Z

1

You forgot to add a check if the length of b exceeds:

if(i < b.length){
    xor += String.fromCharCode(a.charCodeAt(i) ^ b.charCodeAt(i));
}else{
    xor += a.charAt(i);
}

edited Jun 6, 2012 at 5:48

Bergi

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answered Jun 6, 2012 at 5:32

Corsair

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