Botan: src/math/numbertheory/reducer.cpp Source File

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reducer.cpp
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1 /*
2 * Modular Reducer
3 * (C) 1999-2011 Jack Lloyd
4 *
5 * Distributed under the terms of the Botan license
6 */
7
8 #include <botan/reducer.h>
9 #include <botan/internal/mp_core.h>
10
11 namespace Botan {
12
13 /*
14 * Modular_Reducer Constructor
15 */
16 Modular_Reducer::Modular_Reducer(const BigInt& mod)
17 {
18 if(mod <= 0)
19 throw Invalid_Argument("Modular_Reducer: modulus must be positive");
20
21 modulus = mod;
22 mod_words = modulus.sig_words();
23
24 modulus_2 = Botan::square(modulus);
25
26 mu = BigInt(BigInt::Power2, 2 * MP_WORD_BITS * mod_words) / modulus;
27 }
28
29 /*
30 * Barrett Reduction
31 */
32 BigInt Modular_Reducer::reduce(const BigInt& x) const
33 {
34 if(mod_words == 0)
35 throw Invalid_State("Modular_Reducer: Never initalized");
36
37 if(x.cmp(modulus, false) < 0)
38 {
39 if(x.is_negative())
40 return x + modulus; // make positive
41 return x;
42 }
43 else if(x.cmp(modulus_2, false) < 0)
44 {
45 BigInt t1 = x;
46 t1.set_sign(BigInt::Positive);
47 t1 >>= (MP_WORD_BITS * (mod_words - 1));
48 t1 *= mu;
49
50 t1 >>= (MP_WORD_BITS * (mod_words + 1));
51 t1 *= modulus;
52
53 t1.mask_bits(MP_WORD_BITS * (mod_words + 1));
54
55 BigInt t2 = x;
56 t2.set_sign(BigInt::Positive);
57 t2.mask_bits(MP_WORD_BITS * (mod_words + 1));
58
59 t2 -= t1;
60
61 if(t2.is_negative())
62 {
63 BigInt b_to_k1(BigInt::Power2, MP_WORD_BITS * (mod_words + 1));
64 t2 += b_to_k1;
65 }
66
67 while(t2 >= modulus)
68 t2 -= modulus;
69
70 if(x.is_positive())
71 return t2;
72 else
73 return (modulus - t2);
74 }
75 else
76 {
77 // too big, fall back to normal division
78 return (x % modulus);
79 }
80 }
81
82 }
Botan::BigInt::sig_words
size_t sig_words() const
Definition: bigint.h:290
Botan::BigInt
Definition: bigint.h:22
Botan::BigInt::Positive
Definition: bigint.h:33
Botan::BigInt::cmp
s32bit cmp(const BigInt &n, bool check_signs=true) const
Definition: bigint.cpp:132
Botan::Invalid_State
Definition: exceptn.h:25
Botan::BigInt::is_negative
bool is_negative() const
Definition: bigint.h:245
Botan::Invalid_Argument
std::invalid_argument Invalid_Argument
Definition: exceptn.h:20
Botan::BigInt::mask_bits
void mask_bits(size_t n)
Definition: bigint.cpp:227
Botan
Definition: algo_base.h:14
Botan::Modular_Reducer::reduce
BigInt reduce(const BigInt &x) const
Definition: reducer.cpp:32
Botan::Modular_Reducer::Modular_Reducer
Modular_Reducer()
Definition: reducer.h:52
Botan::square
BigInt square(const BigInt &x)
Definition: mp_numth.cpp:18
Botan::BigInt::is_positive
bool is_positive() const
Definition: bigint.h:251
Botan::BigInt::Power2
Definition: bigint.h:38
Botan::BigInt::set_sign
void set_sign(Sign sign)
Definition: bigint.cpp:291
Botan::MP_WORD_BITS
const size_t MP_WORD_BITS
Definition: mp_core.h:18