Re: [PHP-DEV] bcpowmod() and bigIntegers

[Michael Bretterklieber]

Hi,

Robert Kofler wrote:
is there a possibility to implement bcpowmod() in a 
version prior than PHP5?

bcpowmod() ist the only way to calculate a blind signature
as described by David CHAUM 1982 -> digicash
(openssl does not work here).
try this:

function bcPowMod($n, $e, $m) {
   //if ($m == 0 || $e < 0) return 0;
   if (bccomp($m, 0) == 0 || bccomp($e, 0) == -1) return 0;
   $res = 1;
   $pow = $n;
   $e1  = $e;
   //while ($e1 != 0) {
   while (bccomp($e1, 0) != 0) {
      $d = bcmod($e1, 2);
      //$e1  = floor(bcdiv($e1, 2));
      $e1  = bcdiv($e1, 2, 0);
      //if ($d == 1) {
      if (bccomp($d, 1) == 0) {
         $res = bcMod(bcMul($res, $pow), $m);
      }
      $pow = bcMod(bcMul($pow, $pow), $m);
   }
        // if ($res < 0)
        if (bccomp($res, 0) == -1) $res = bcAdd($res, $m);
        return $res;
}
bye,
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Michael Bretterklieber        - [EMAIL PROTECTED]
JAWA Management Software GmbH - http://www.jawa.at
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