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110 lines
2.9 KiB
PHP
110 lines
2.9 KiB
PHP
<?php
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/**
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* BCMath Dynamic Barrett Modular Exponentiation Engine
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*
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* PHP version 5 and 7
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*
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* @category Math
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* @package BigInteger
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* @author Jim Wigginton <terrafrost@php.net>
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* @copyright 2017 Jim Wigginton
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* @license http://www.opensource.org/licenses/mit-license.html MIT License
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* @link http://pear.php.net/package/Math_BigInteger
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*/
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namespace phpseclib\Math\BigInteger\Engines\BCMath\Reductions;
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use phpseclib\Math\BigInteger\Engines\BCMath\Base;
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use phpseclib\Math\BigInteger\Engines\BCMath;
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/**
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* PHP Barrett Modular Exponentiation Engine
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*
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* @package PHP
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* @author Jim Wigginton <terrafrost@php.net>
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* @access public
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*/
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abstract class EvalBarrett extends Base
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{
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/**
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* Custom Reduction Function
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*
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* @see self::generateCustomReduction
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*/
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private static $custom_reduction;
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/**
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* Barrett Modular Reduction
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*
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* This calls a dynamically generated loop unrolled function that's specific to a given modulo.
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* Array lookups are avoided as are if statements testing for how many bits the host OS supports, etc.
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*
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* @param string $n
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* @param string $m
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* @return string
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*/
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protected static function reduce($n, $m)
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{
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$inline = self::$custom_reduction;
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return $inline($n);
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}
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/**
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* Generate Custom Reduction
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*
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* @param BCMath $m
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* @param string $class
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* @return callable|void
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*/
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protected static function generateCustomReduction(BCMath $m, $class)
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{
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$m_length = strlen($m);
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if ($m_length < 5) {
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$code = 'return bcmod($x, $n);';
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eval('$func = function ($n) { ' . $code . '};');
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self::$custom_reduction = $func;
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return;
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}
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$lhs = '1' . str_repeat('0', $m_length + ($m_length >> 1));
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$u = bcdiv($lhs, $m, 0);
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$m1 = bcsub($lhs, bcmul($u, $m));
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$cutoff = $m_length + ($m_length >> 1);
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$m = "'$m'";
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$u = "'$u'";
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$m1= "'$m1'";
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$code.= '
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$lsd = substr($n, -' . $cutoff . ');
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$msd = substr($n, 0, -' . $cutoff . ');
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$temp = bcmul($msd, ' . $m1 . ');
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$n = bcadd($lsd, $temp);
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$temp = substr($n, 0, ' . (-$m_length + 1) . ');
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$temp = bcmul($temp, ' . $u . ');
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$temp = substr($temp, 0, ' . (-($m_length >> 1) - 1) . ');
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$temp = bcmul($temp, ' . $m . ');
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$result = bcsub($n, $temp);
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if ($result[0] == \'-\') {
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$temp = \'1' . str_repeat('0', $m_length + 1) . '\';
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$result = bcadd($result, $temp);
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}
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while (bccomp($result, ' . $m . ') >= 0) {
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$result = bcsub($result, ' . $m . ');
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}
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return $result;';
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eval('$func = function ($n) { ' . $code . '};');
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self::$custom_reduction = $func;
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}
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} |