fuchsia / third_party / dropbear / 1c5fda515f96c27d4e3b732d887f418453f1cb14 / . / bn_mp_reduce_2k_l.c

#include <tommath.h> | |

#ifdef BN_MP_REDUCE_2K_L_C | |

/* LibTomMath, multiple-precision integer library -- Tom St Denis | |

* | |

* LibTomMath is a library that provides multiple-precision | |

* integer arithmetic as well as number theoretic functionality. | |

* | |

* The library was designed directly after the MPI library by | |

* Michael Fromberger but has been written from scratch with | |

* additional optimizations in place. | |

* | |

* The library is free for all purposes without any express | |

* guarantee it works. | |

* | |

* Tom St Denis, tomstdenis@iahu.ca, http://math.libtomcrypt.org | |

*/ | |

/* reduces a modulo n where n is of the form 2**p - d | |

This differs from reduce_2k since "d" can be larger | |

than a single digit. | |

*/ | |

int mp_reduce_2k_l(mp_int *a, mp_int *n, mp_int *d) | |

{ | |

mp_int q; | |

int p, res; | |

if ((res = mp_init(&q)) != MP_OKAY) { | |

return res; | |

} | |

p = mp_count_bits(n); | |

top: | |

/* q = a/2**p, a = a mod 2**p */ | |

if ((res = mp_div_2d(a, p, &q, a)) != MP_OKAY) { | |

goto ERR; | |

} | |

/* q = q * d */ | |

if ((res = mp_mul(&q, d, &q)) != MP_OKAY) { | |

goto ERR; | |

} | |

/* a = a + q */ | |

if ((res = s_mp_add(a, &q, a)) != MP_OKAY) { | |

goto ERR; | |

} | |

if (mp_cmp_mag(a, n) != MP_LT) { | |

s_mp_sub(a, n, a); | |

goto top; | |

} | |

ERR: | |

mp_clear(&q); | |

return res; | |

} | |

#endif |