Botan  1.10.9
Public Member Functions | List of all members
Botan::Modular_Reducer Class Reference

#include <reducer.h>

Public Member Functions

BigInt cube (const BigInt &x) const
 
const BigIntget_modulus () const
 
bool initialized () const
 
 Modular_Reducer ()
 
 Modular_Reducer (const BigInt &mod)
 
BigInt multiply (const BigInt &x, const BigInt &y) const
 
BigInt reduce (const BigInt &x) const
 
BigInt square (const BigInt &x) const
 

Detailed Description

Modular Reducer (using Barrett's technique)

Definition at line 18 of file reducer.h.

Constructor & Destructor Documentation

Botan::Modular_Reducer::Modular_Reducer ( )
inline

Definition at line 52 of file reducer.h.

52 { mod_words = 0; }
Botan::Modular_Reducer::Modular_Reducer ( const BigInt mod)

Definition at line 16 of file reducer.cpp.

References mod, Botan::MP_WORD_BITS, Botan::BigInt::Power2, Botan::BigInt::sig_words(), and Botan::square().

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  }
size_t sig_words() const
Definition: bigint.h:290
std::invalid_argument Invalid_Argument
Definition: exceptn.h:20
GMP_MPZ mod
Definition: gmp_powm.cpp:29
BigInt square(const BigInt &x)
Definition: mp_numth.cpp:18
const size_t MP_WORD_BITS
Definition: mp_core.h:18

Member Function Documentation

BigInt Botan::Modular_Reducer::cube ( const BigInt x) const
inline

Cube mod p

Parameters
x
Returns
(x * x * x) % p

Definition at line 47 of file reducer.h.

References Botan::square().

48  { return multiply(x, this->square(x)); }
BigInt multiply(const BigInt &x, const BigInt &y) const
Definition: reducer.h:31
BigInt square(const BigInt &x) const
Definition: reducer.h:39
const BigInt& Botan::Modular_Reducer::get_modulus ( ) const
inline
bool Botan::Modular_Reducer::initialized ( ) const
inline

Definition at line 50 of file reducer.h.

Referenced by Botan::Blinder::blind(), and Botan::Blinder::unblind().

50 { return (mod_words != 0); }
BigInt Botan::Modular_Reducer::multiply ( const BigInt x,
const BigInt y 
) const
inline
BigInt Botan::Modular_Reducer::reduce ( const BigInt x) const

Definition at line 32 of file reducer.cpp.

References Botan::BigInt::cmp(), Botan::BigInt::is_negative(), Botan::BigInt::is_positive(), Botan::BigInt::mask_bits(), Botan::MP_WORD_BITS, Botan::BigInt::Positive, Botan::BigInt::Power2, and Botan::BigInt::set_sign().

Referenced by Botan::NR_Signature_Operation::sign(), Botan::DSA_Signature_Operation::sign(), Botan::RW_Signature_Operation::sign(), Botan::ECDSA_Signature_Operation::sign(), Botan::DSA_Verification_Operation::verify(), and Botan::NR_Verification_Operation::verify_mr().

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  }
void mask_bits(size_t n)
Definition: bigint.cpp:227
const size_t MP_WORD_BITS
Definition: mp_core.h:18
BigInt Botan::Modular_Reducer::square ( const BigInt x) const
inline

Square mod p

Parameters
x
Returns
(x * x) % p

Definition at line 39 of file reducer.h.

References Botan::square().

Referenced by Botan::Blinder::blind(), Botan::Fixed_Window_Exponentiator::execute(), and Botan::ressol().

40  { return reduce(Botan::square(x)); }
BigInt reduce(const BigInt &x) const
Definition: reducer.cpp:32
BigInt square(const BigInt &x)
Definition: mp_numth.cpp:18

The documentation for this class was generated from the following files: