## Mercurial > dropbear

### view libtommath/bn_mp_montgomery_calc_normalization.c @ 709:abd99ecd7ec2

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use an empty writebuf rather than a NULL one

author | Matt Johnston <matt@ucc.asn.au> |
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date | Wed, 20 Mar 2013 22:31:07 +0800 |

parents | 5ff8218bcee9 |

children | 60fc6476e044 |

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#include <tommath.h> #ifdef BN_MP_MONTGOMERY_CALC_NORMALIZATION_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, [email protected], http://math.libtomcrypt.com */ /* * shifts with subtractions when the result is greater than b. * * The method is slightly modified to shift B unconditionally upto just under * the leading bit of b. This saves alot of multiple precision shifting. */ int mp_montgomery_calc_normalization (mp_int * a, mp_int * b) { int x, bits, res; /* how many bits of last digit does b use */ bits = mp_count_bits (b) % DIGIT_BIT; if (b->used > 1) { if ((res = mp_2expt (a, (b->used - 1) * DIGIT_BIT + bits - 1)) != MP_OKAY) { return res; } } else { mp_set(a, 1); bits = 1; } /* now compute C = A * B mod b */ for (x = bits - 1; x < (int)DIGIT_BIT; x++) { if ((res = mp_mul_2 (a, a)) != MP_OKAY) { return res; } if (mp_cmp_mag (a, b) != MP_LT) { if ((res = s_mp_sub (a, b, a)) != MP_OKAY) { return res; } } } return MP_OKAY; } #endif /* $Source: /cvs/libtom/libtommath/bn_mp_montgomery_calc_normalization.c,v $ */ /* $Revision: 1.3 $ */ /* $Date: 2006/03/31 14:18:44 $ */