[Webkit-unassigned] [Bug 40367] New: BigInteger Math Library Javascript Bug on Safari 5 (webkit 533.16) under "32bit" mode

bugzilla-daemon at webkit.org bugzilla-daemon at webkit.org
Wed Jun 9 10:00:45 PDT 2010


https://bugs.webkit.org/show_bug.cgi?id=40367

           Summary: BigInteger Math Library Javascript Bug on Safari 5
                    (webkit 533.16) under "32bit" mode
           Product: WebKit
           Version: 528+ (Nightly build)
          Platform: All
        OS/Version: All
            Status: UNCONFIRMED
          Severity: Blocker
          Priority: P1
         Component: JavaScriptCore
        AssignedTo: webkit-unassigned at lists.webkit.org
        ReportedBy: priyajeet.hora at gmail.com


Summary:

This issue comes with the use of BigInteger library from the link below.
The issue happens only in Safari 5 32bit on mac and windows
This library works fine on every other browser under 32bit.

This bug is reproducible with webkit nightly in 32bit

Library in use
-----------
http://www-cs-students.stanford.edu/~tjw/jsbn/

Files
---- 
jsbn.js - basic BigInteger implementation, just enough for RSA encryption and not much more.
jsbn2.js - the rest of the library, including most public BigInteger methods.


Steps to Reproduce:

1] Goto http://www-cs-students.stanford.edu/~tjw/jsbn/rsa2.html
This is a demo page of the above libraries. Once this page is loaded, the js files above should be in the browsers cache and usable.

2] Once the page is loaded, type this in the address bar
javascript:var x = new BigInteger("10000"); var y = new BigInteger("10000"); alert(x.multiply(y));


Expected Results:

All browsers on any platform: An alert popup with the value 100000000


Actual Results:

Safari 5 32bit - an alert with the value 184217728
All other browsers on any platform - an alert popup with the value 100000000


Notes:

A word from the author of that math library

"The "am" functions are various implementations of the core inner 
add-and-multiply loop.  There are several implementations to account for 
the fact that different browsers' JS engines have different bugs when 
doing bitwise math on large Numbers, and also vary in performance.

It sounds like Safari on 32-bit systems has a bug or unexpected 
behavior that prevents am1 and/or am3 from working properly.  The fix 
would be to identify what assumption am1/am3 depend on that isn't being 
met by that JS engine and codify it into a test that forces use of am2 
under those circumstances.

Thanks for the report - I will try to reproduce it on a 32-bit Windows 
Safari and if successful, will add the appropriate test to the next 
version of jsbn."


Methods in question -
Based on the logic below, the am3 algorithm is chosen for safari, firefox and chrome


// am: Compute w_j += (x*this_i), propagate carries,
// c is initial carry, returns final carry.
// c < 3*dvalue, x < 2*dvalue, this_i < dvalue
// We need to select the fastest one that works in this environment.

// am1: use a single mult and divide to get the high bits,
// max digit bits should be 26 because
// max internal value = 2*dvalue^2-2*dvalue (< 2^53)
function am1(i,x,w,j,c,n) {
  while(--n >= 0) {
    var v = x*this[i++]+w[j]+c;
    c = Math.floor(v/0x4000000);
    w[j++] = v&0x3ffffff;
  }
  return c;
}
// am2 avoids a big mult-and-extract completely.
// Max digit bits should be <= 30 because we do bitwise ops
// on values up to 2*hdvalue^2-hdvalue-1 (< 2^31)
function am2(i,x,w,j,c,n) {
  var xl = x&0x7fff, xh = x>>15;
  while(--n >= 0) {
    var l = this[i]&0x7fff;
    var h = this[i++]>>15;
    var m = xh*l+h*xl;
    l = xl*l+((m&0x7fff)<<15)+w[j]+(c&0x3fffffff);
    c = (l>>>30)+(m>>>15)+xh*h+(c>>>30);
    w[j++] = l&0x3fffffff;
  }
  return c;
}
// Alternately, set max digit bits to 28 since some
// browsers slow down when dealing with 32-bit numbers.
function am3(i,x,w,j,c,n) {
  var xl = x&0x3fff, xh = x>>14;
  while(--n >= 0) {
    var l = this[i]&0x3fff;
    var h = this[i++]>>14;
    var m = xh*l+h*xl;
    l = xl*l+((m&0x3fff)<<14)+w[j]+c;
    c = (l>>28)+(m>>14)+xh*h;
    w[j++] = l&0xfffffff;
  }
  return c;
}
if(j_lm && (navigator.appName == "Microsoft Internet Explorer")) {
  BigInteger.prototype.am = am2;
  dbits = 30;
}
else if(j_lm && (navigator.appName != "Netscape")) {
  BigInteger.prototype.am = am1;
  dbits = 26;
}
else { // Mozilla/Netscape seems to prefer am3
  BigInteger.prototype.am = am3;
  dbits = 28; <<<<<<<<<<<<<<<< changing to 30 bits fixes the issue
}

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