Coverage Report

Created: 2020-06-30 13:58

/src/botan/src/lib/math/numbertheory/reducer.cpp
Line
Count
Source (jump to first uncovered line)
1
/*
2
* Modular Reducer
3
* (C) 1999-2011,2018 Jack Lloyd
4
*
5
* Botan is released under the Simplified BSD License (see license.txt)
6
*/
7
8
#include <botan/reducer.h>
9
#include <botan/internal/ct_utils.h>
10
#include <botan/internal/mp_core.h>
11
#include <botan/divide.h>
12
13
namespace Botan {
14
15
/*
16
* Modular_Reducer Constructor
17
*/
18
Modular_Reducer::Modular_Reducer(const BigInt& mod)
19
99.2k
   {
20
99.2k
   if(mod < 0)
21
22
      throw Invalid_Argument("Modular_Reducer: modulus must be positive");
22
99.2k
23
99.2k
   // Left uninitialized if mod == 0
24
99.2k
   m_mod_words = 0;
25
99.2k
26
99.2k
   if(mod > 0)
27
98.9k
      {
28
98.9k
      m_modulus = mod;
29
98.9k
      m_mod_words = m_modulus.sig_words();
30
98.9k
31
98.9k
      // Compute mu = floor(2^{2k} / m)
32
98.9k
      m_mu.set_bit(2 * BOTAN_MP_WORD_BITS * m_mod_words);
33
98.9k
      m_mu = ct_divide(m_mu, m_modulus);
34
98.9k
      }
35
99.2k
   }
36
37
BigInt Modular_Reducer::reduce(const BigInt& x) const
38
6.61M
   {
39
6.61M
   BigInt r;
40
6.61M
   secure_vector<word> ws;
41
6.61M
   reduce(r, x, ws);
42
6.61M
   return r;
43
6.61M
   }
44
45
namespace {
46
47
/*
48
* Like if(cnd) x.rev_sub(...) but in const time
49
*/
50
void cnd_rev_sub(bool cnd, BigInt& x, const word y[], size_t y_sw, secure_vector<word>& ws)
51
6.61M
   {
52
6.61M
   if(x.sign() != BigInt::Positive)
53
0
      throw Invalid_State("BigInt::sub_rev requires this is positive");
54
6.61M
55
6.61M
   const size_t x_sw = x.sig_words();
56
6.61M
57
6.61M
   const size_t max_words = std::max(x_sw, y_sw);
58
6.61M
   ws.resize(std::max(x_sw, y_sw));
59
6.61M
   clear_mem(ws.data(), ws.size());
60
6.61M
   x.grow_to(max_words);
61
6.61M
62
6.61M
   const int32_t relative_size = bigint_sub_abs(ws.data(), x.data(), x_sw, y, y_sw);
63
6.61M
64
6.61M
   x.cond_flip_sign((relative_size > 0) && cnd);
65
6.61M
   bigint_cnd_swap(cnd, x.mutable_data(), ws.data(), max_words);
66
6.61M
   }
67
68
}
69
70
void Modular_Reducer::reduce(BigInt& t1, const BigInt& x, secure_vector<word>& ws) const
71
6.61M
   {
72
6.61M
   if(&t1 == &x)
73
0
      throw Invalid_State("Modular_Reducer arguments cannot alias");
74
6.61M
   if(m_mod_words == 0)
75
5
      throw Invalid_State("Modular_Reducer: Never initalized");
76
6.61M
77
6.61M
   const size_t x_sw = x.sig_words();
78
6.61M
79
6.61M
   if(x_sw > 2*m_mod_words)
80
364
      {
81
364
      // too big, fall back to slow boat division
82
364
      t1 = ct_modulo(x, m_modulus);
83
364
      return;
84
364
      }
85
6.61M
86
6.61M
   t1 = x;
87
6.61M
   t1.set_sign(BigInt::Positive);
88
6.61M
   t1 >>= (BOTAN_MP_WORD_BITS * (m_mod_words - 1));
89
6.61M
90
6.61M
   t1.mul(m_mu, ws);
91
6.61M
   t1 >>= (BOTAN_MP_WORD_BITS * (m_mod_words + 1));
92
6.61M
93
6.61M
   // TODO add masked mul to avoid computing high bits
94
6.61M
   t1.mul(m_modulus, ws);
95
6.61M
   t1.mask_bits(BOTAN_MP_WORD_BITS * (m_mod_words + 1));
96
6.61M
97
6.61M
   t1.rev_sub(x.data(), std::min(x_sw, m_mod_words + 1), ws);
98
6.61M
99
6.61M
   /*
100
6.61M
   * If t1 < 0 then we must add b^(k+1) where b = 2^w. To avoid a
101
6.61M
   * side channel perform the addition unconditionally, with ws set
102
6.61M
   * to either b^(k+1) or else 0.
103
6.61M
   */
104
6.61M
   const word t1_neg = t1.is_negative();
105
6.61M
106
6.61M
   if(ws.size() < m_mod_words + 2)
107
132k
      ws.resize(m_mod_words + 2);
108
6.61M
   clear_mem(ws.data(), ws.size());
109
6.61M
   ws[m_mod_words + 1] = t1_neg;
110
6.61M
111
6.61M
   t1.add(ws.data(), m_mod_words + 2, BigInt::Positive);
112
6.61M
113
6.61M
   // Per HAC this step requires at most 2 subtractions
114
6.61M
   t1.ct_reduce_below(m_modulus, ws, 2);
115
6.61M
116
6.61M
   cnd_rev_sub(t1.is_nonzero() && x.is_negative(), t1, m_modulus.data(), m_modulus.size(), ws);
117
6.61M
   }
118
119
}