fpe_fe1.cpp

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/*
* Format Preserving Encryption (FE1 scheme)
* (C) 2009,2018 Jack Lloyd
*
* Botan is released under the Simplified BSD License (see license.txt)
*/
 
#include <botan/fpe_fe1.h>
#include <botan/internal/loadstor.h>
#include <botan/numthry.h>
#include <botan/internal/divide.h>
#include <botan/internal/fmt.h>
#include <botan/reducer.h>
#include <botan/mac.h>
 
namespace Botan {
 
namespace {
 
// Normally FPE is for SSNs, CC#s, etc, nothing too big
const size_t MAX_N_BYTES = 128/8;
 
/*
* Factor n into a and b which are as close together as possible.
* Assumes n is composed mostly of small factors which is the case for
* typical uses of FPE (typically, n is a power of 10)
*/
void factor(BigInt n, BigInt& a, BigInt& b)
   {
   a = BigInt::one();
   b = BigInt::one();
 
   size_t n_low_zero = low_zero_bits(n);
 
   a <<= (n_low_zero / 2);
   b <<= n_low_zero - (n_low_zero / 2);
   n >>= n_low_zero;
 
   for(size_t i = 0; i != PRIME_TABLE_SIZE; ++i)
      {
      while(n % PRIMES[i] == 0)
         {
         a *= PRIMES[i];
         if(a > b)
            std::swap(a, b);
         n /= BigInt::from_word(PRIMES[i]);
         }
      }
 
   if(a > b)
      std::swap(a, b);
   a *= n;
 
   if(a <= 1 || b <= 1)
      throw Internal_Error("Could not factor n for use in FPE");
   }
 
}
 
FPE_FE1::FPE_FE1(const BigInt& n,
                 size_t rounds,
                 bool compat_mode,
                 std::string_view mac_algo) :
   m_rounds(rounds)
   {
   if(m_rounds < 3)
      throw Invalid_Argument("FPE_FE1 rounds too small");
 
   m_mac = MessageAuthenticationCode::create_or_throw(mac_algo);
 
   m_n_bytes = BigInt::encode(n);
 
   if(m_n_bytes.size() > MAX_N_BYTES)
      throw Invalid_Argument("N is too large for FPE encryption");
 
   factor(n, m_a, m_b);
 
   if(compat_mode)
      {
      if(m_a < m_b)
         std::swap(m_a, m_b);
      }
   else
      {
      if(m_a > m_b)
         std::swap(m_a, m_b);
      }
 
   mod_a = std::make_unique<Modular_Reducer>(m_a);
   }
 
FPE_FE1::~FPE_FE1() = default;
 
void FPE_FE1::clear()
   {
   m_mac->clear();
   }
 
std::string FPE_FE1::name() const
   {
   return fmt("FPE_FE1({},{})", m_mac->name(), m_rounds);
   }
 
Key_Length_Specification FPE_FE1::key_spec() const
   {
   return m_mac->key_spec();
   }
 
bool FPE_FE1::has_keying_material() const
   {
   return m_mac->has_keying_material();
   }
 
void FPE_FE1::key_schedule(const uint8_t key[], size_t length)
   {
   m_mac->set_key(key, length);
   }
 
BigInt FPE_FE1::F(const BigInt& R, size_t round,
                  const secure_vector<uint8_t>& tweak_mac,
                  secure_vector<uint8_t>& tmp) const
   {
   tmp = BigInt::encode_locked(R);
 
   m_mac->update(tweak_mac);
   m_mac->update_be(static_cast<uint32_t>(round));
 
   m_mac->update_be(static_cast<uint32_t>(tmp.size()));
   m_mac->update(tmp.data(), tmp.size());
 
   tmp = m_mac->final();
   return BigInt(tmp.data(), tmp.size());
   }
 
secure_vector<uint8_t> FPE_FE1::compute_tweak_mac(const uint8_t tweak[], size_t tweak_len) const
   {
   m_mac->update_be(static_cast<uint32_t>(m_n_bytes.size()));
   m_mac->update(m_n_bytes.data(), m_n_bytes.size());
 
   m_mac->update_be(static_cast<uint32_t>(tweak_len));
   if(tweak_len > 0)
      m_mac->update(tweak, tweak_len);
 
   return m_mac->final();
   }
 
BigInt FPE_FE1::encrypt(const BigInt& input, const uint8_t tweak[], size_t tweak_len) const
   {
   const secure_vector<uint8_t> tweak_mac = compute_tweak_mac(tweak, tweak_len);
 
   BigInt X = input;
 
   secure_vector<uint8_t> tmp;
 
   BigInt L, R, Fi;
   for(size_t i = 0; i != m_rounds; ++i)
      {
      ct_divide(X, m_b, L, R);
      Fi = F(R, i, tweak_mac, tmp);
      X = m_a * R + mod_a->reduce(L + Fi);
      }
 
   return X;
   }
 
BigInt FPE_FE1::decrypt(const BigInt& input, const uint8_t tweak[], size_t tweak_len) const
   {
   const secure_vector<uint8_t> tweak_mac = compute_tweak_mac(tweak, tweak_len);
 
   BigInt X = input;
   secure_vector<uint8_t> tmp;
 
   BigInt W, R, Fi;
   for(size_t i = 0; i != m_rounds; ++i)
      {
      ct_divide(X, m_a, R, W);
 
      Fi = F(R, m_rounds-i-1, tweak_mac, tmp);
      X = m_b * mod_a->reduce(W - Fi) + R;
      }
 
   return X;
   }
 
BigInt FPE_FE1::encrypt(const BigInt& x, uint64_t tweak) const
   {
   uint8_t tweak8[8];
   store_be(tweak, tweak8);
   return encrypt(x, tweak8, sizeof(tweak8));
   }
 
BigInt FPE_FE1::decrypt(const BigInt& x, uint64_t tweak) const
   {
   uint8_t tweak8[8];
   store_be(tweak, tweak8);
   return decrypt(x, tweak8, sizeof(tweak8));
   }
 
namespace FPE {
 
BigInt fe1_encrypt(const BigInt& n, const BigInt& X,
                   const SymmetricKey& key,
                   const std::vector<uint8_t>& tweak)
   {
   FPE_FE1 fpe(n, 3, true, "HMAC(SHA-256)");
   fpe.set_key(key);
   return fpe.encrypt(X, tweak.data(), tweak.size());
   }
 
BigInt fe1_decrypt(const BigInt& n, const BigInt& X,
                   const SymmetricKey& key,
                   const std::vector<uint8_t>& tweak)
   {
   FPE_FE1 fpe(n, 3, true, "HMAC(SHA-256)");
   fpe.set_key(key);
   return fpe.decrypt(X, tweak.data(), tweak.size());
   }
 
}
 
}