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**PHP Modular Exponentiation Engine PHP version 5 and 7**

Author: | Jim Wigginton |

Copyright: | 2017 Jim Wigginton |

License: | http://www.opensource.org/licenses/mit-license.html MIT License |

Link: | http://pear.php.net/package/Math_BigInteger |

File Size: | 143 lines (5 kb) |

Included or required: | 0 times |

Referenced: | 0 times |

Includes or requires: | 0 files |

Functions that are not part of a class:

isValidEngine() X-Ref |

Test for engine validityreturn: bool |

powModHelper(PHP $x, PHP $e, PHP $n, $class) X-Ref |

Performs modular exponentiation.The most naive approach to modular exponentiation has very unreasonable requirements, and and although the approach involving repeated squaring does vastly better, it, too, is impractical for our purposes. The reason being that division - by far the most complicated and time-consuming of the basic operations (eg. +,-,*,/) - occurs multiple times within it. Modular reductions resolve this issue. Although an individual modular reduction takes more time then an individual division, when performed in succession (with the same modulo), they're a lot faster. The two most commonly used modular reductions are Barrett and Montgomery reduction. Montgomery reduction, although faster, only works when the gcd of the modulo and of the base being used is 1. In RSA, when the base is a power of two, the modulo - a product of two primes - is always going to have a gcd of 1 (because the product of two odd numbers is odd), but what about when RSA isn't used? In contrast, Barrett reduction has no such constraint. As such, some bigint implementations perform a Barrett reduction after every operation in the modpow function. Others perform Barrett reductions when the modulo is even and Montgomery reductions when the modulo is odd. BigInteger.java's modPow method, however, uses a trick involving the Chinese Remainder Theorem to factor the even modulo into two numbers - one odd and the other, a power of two - and recombine them, later. This is the method that this modPow function uses. {@link http://islab.oregonstate.edu/papers/j34monex.pdf Montgomery Reduction with Even Modulus} elaborates. param: PHP $xparam: PHP $eparam: PHP $nparam: string $classreturn: PHP |

prepareReduce(array $x, array $n, $class) X-Ref |

Modular reduction preparationsee: self::slidingWindow()param: array $xparam: array $nparam: string $classreturn: array |

multiplyReduce(array $x, array $y, array $n, $class) X-Ref |

Modular multiplysee: self::slidingWindow()param: array $xparam: array $yparam: array $nparam: string $classreturn: array |

squareReduce(array $x, array $n, $class) X-Ref |

Modular squaresee: self::slidingWindow()param: array $xparam: array $nparam: string $classreturn: array |