## Mercurial > dropbear

### annotate bn_mp_montgomery_setup.c @ 282:91fbc376f010 libtommath-orig libtommath-0.35

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Import of libtommath 0.35
From ltm-0.35.tar.bz2 SHA1 of 3f193dbae9351e92d02530994fa18236f7fde01c

author | Matt Johnston <matt@ucc.asn.au> |
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date | Wed, 08 Mar 2006 13:16:18 +0000 |

parents | |

children | 97db060d0ef5 |

rev | line source |
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282 | 1 #include <tommath.h> |

2 #ifdef BN_MP_MONTGOMERY_SETUP_C | |

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

4 * | |

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

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

7 * | |

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

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

10 * additional optimizations in place. | |

11 * | |

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

13 * guarantee it works. | |

14 * | |

15 * Tom St Denis, [email protected], http://math.libtomcrypt.org | |

16 */ | |

17 | |

18 /* setups the montgomery reduction stuff */ | |

19 int | |

20 mp_montgomery_setup (mp_int * n, mp_digit * rho) | |

21 { | |

22 mp_digit x, b; | |

23 | |

24 /* fast inversion mod 2**k | |

25 * | |

26 * Based on the fact that | |

27 * | |

28 * XA = 1 (mod 2**n) => (X(2-XA)) A = 1 (mod 2**2n) | |

29 * => 2*X*A - X*X*A*A = 1 | |

30 * => 2*(1) - (1) = 1 | |

31 */ | |

32 b = n->dp[0]; | |

33 | |

34 if ((b & 1) == 0) { | |

35 return MP_VAL; | |

36 } | |

37 | |

38 x = (((b + 2) & 4) << 1) + b; /* here x*a==1 mod 2**4 */ | |

39 x *= 2 - b * x; /* here x*a==1 mod 2**8 */ | |

40 #if !defined(MP_8BIT) | |

41 x *= 2 - b * x; /* here x*a==1 mod 2**16 */ | |

42 #endif | |

43 #if defined(MP_64BIT) || !(defined(MP_8BIT) || defined(MP_16BIT)) | |

44 x *= 2 - b * x; /* here x*a==1 mod 2**32 */ | |

45 #endif | |

46 #ifdef MP_64BIT | |

47 x *= 2 - b * x; /* here x*a==1 mod 2**64 */ | |

48 #endif | |

49 | |

50 /* rho = -1/m mod b */ | |

51 *rho = (((mp_word)1 << ((mp_word) DIGIT_BIT)) - x) & MP_MASK; | |

52 | |

53 return MP_OKAY; | |

54 } | |

55 #endif |