blinding.cpp

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/*
* Blinding for public key operations
* (C) 1999-2010,2015 Jack Lloyd
*
* Botan is released under the Simplified BSD License (see license.txt)
*/

#include <botan/blinding.h>

namespace Botan {

Blinder::Blinder(const BigInt& modulus,
                 RandomNumberGenerator& rng,
                 std::function<BigInt (const BigInt&)> fwd,
                 std::function<BigInt (const BigInt&)> inv) :
      m_reducer(modulus),
      m_rng(rng),
      m_fwd_fn(fwd),
      m_inv_fn(inv),
      m_modulus_bits(modulus.bits()),
      m_e{},
      m_d{},
      m_counter{}
   {
   const BigInt k = blinding_nonce();
   m_e = m_fwd_fn(k);
   m_d = m_inv_fn(k);
   }

BigInt Blinder::blinding_nonce() const
   {
   return BigInt(m_rng, m_modulus_bits - 1);
   }

BigInt Blinder::blind(const BigInt& i) const
   {
   if(!m_reducer.initialized())
      throw Exception("Blinder not initialized, cannot blind");

   ++m_counter;

   if((BOTAN_BLINDING_REINIT_INTERVAL > 0) && (m_counter > BOTAN_BLINDING_REINIT_INTERVAL))
      {
      const BigInt k = blinding_nonce();
      m_e = m_fwd_fn(k);
      m_d = m_inv_fn(k);
      m_counter = 0;
      }
   else
      {
      m_e = m_reducer.square(m_e);
      m_d = m_reducer.square(m_d);
      }

   return m_reducer.multiply(i, m_e);
   }

BigInt Blinder::unblind(const BigInt& i) const
   {
   if(!m_reducer.initialized())
      throw Exception("Blinder not initialized, cannot unblind");

   return m_reducer.multiply(i, m_d);
   }

}