idea.cpp

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/*
* IDEA
* (C) 1999-2010 Jack Lloyd
*
* Distributed under the terms of the Botan license
*/
 
#include <botan/idea.h>
#include <botan/loadstor.h>
 
namespace Botan {
 
namespace {
 
/*
* Multiplication modulo 65537
*/
inline u16bit mul(u16bit x, u16bit y)
   {
   const u32bit P = static_cast<u32bit>(x) * y;
 
   // P ? 0xFFFF : 0
   const u16bit P_mask = !P - 1;
 
   const u32bit P_hi = P >> 16;
   const u32bit P_lo = P & 0xFFFF;
 
   const u16bit r_1 = (P_lo - P_hi) + (P_lo < P_hi);
   const u16bit r_2 = 1 - x - y;
 
   return (r_1 & P_mask) | (r_2 & ~P_mask);
   }
 
/*
* Find multiplicative inverses modulo 65537
*
* 65537 is prime; thus Fermat's little theorem tells us that
* x^65537 == x modulo 65537, which means
* x^(65537-2) == x^-1 modulo 65537 since
* x^(65537-2) * x == 1 mod 65537
*
* Do the exponentiation with a basic square and multiply: all bits are
* of exponent are 1 so we always multiply
*/
u16bit mul_inv(u16bit x)
   {
   u16bit y = x;
 
   for(size_t i = 0; i != 15; ++i)
      {
      y = mul(y, y); // square
      y = mul(y, x);
      }
 
   return y;
   }
 
/**
* IDEA is involutional, depending only on the key schedule
*/
void idea_op(const byte in[], byte out[], size_t blocks, const u16bit K[52])
   {
   const size_t BLOCK_SIZE = 8;
 
   for(size_t i = 0; i != blocks; ++i)
      {
      u16bit X1 = load_be<u16bit>(in, 0);
      u16bit X2 = load_be<u16bit>(in, 1);
      u16bit X3 = load_be<u16bit>(in, 2);
      u16bit X4 = load_be<u16bit>(in, 3);
 
      for(size_t j = 0; j != 8; ++j)
         {
         X1 = mul(X1, K[6*j+0]);
         X2 += K[6*j+1];
         X3 += K[6*j+2];
         X4 = mul(X4, K[6*j+3]);
 
         u16bit T0 = X3;
         X3 = mul(X3 ^ X1, K[6*j+4]);
 
         u16bit T1 = X2;
         X2 = mul((X2 ^ X4) + X3, K[6*j+5]);
         X3 += X2;
 
         X1 ^= X2;
         X4 ^= X3;
         X2 ^= T0;
         X3 ^= T1;
         }
 
      X1  = mul(X1, K[48]);
      X2 += K[50];
      X3 += K[49];
      X4  = mul(X4, K[51]);
 
      store_be(out, X1, X3, X2, X4);
 
      in += BLOCK_SIZE;
      out += BLOCK_SIZE;
      }
   }
 
}
 
/*
* IDEA Encryption
*/
void IDEA::encrypt_n(const byte in[], byte out[], size_t blocks) const
   {
   idea_op(in, out, blocks, &EK[0]);
   }
 
/*
* IDEA Decryption
*/
void IDEA::decrypt_n(const byte in[], byte out[], size_t blocks) const
   {
   idea_op(in, out, blocks, &DK[0]);
   }
 
/*
* IDEA Key Schedule
*/
void IDEA::key_schedule(const byte key[], size_t)
   {
   for(size_t i = 0; i != 8; ++i)
      EK[i] = load_be<u16bit>(key, i);
 
   for(size_t i = 1, j = 8, offset = 0; j != 52; i %= 8, ++i, ++j)
      {
      EK[i+7+offset] = static_cast<u16bit>((EK[(i     % 8) + offset] << 9) |
                                           (EK[((i+1) % 8) + offset] >> 7));
      offset += (i == 8) ? 8 : 0;
      }
 
   DK[51] = mul_inv(EK[3]);
   DK[50] = -EK[2];
   DK[49] = -EK[1];
   DK[48] = mul_inv(EK[0]);
 
   for(size_t i = 1, j = 4, counter = 47; i != 8; ++i, j += 6)
      {
      DK[counter--] = EK[j+1];
      DK[counter--] = EK[j];
      DK[counter--] = mul_inv(EK[j+5]);
      DK[counter--] = -EK[j+3];
      DK[counter--] = -EK[j+4];
      DK[counter--] = mul_inv(EK[j+2]);
      }
 
   DK[5] = EK[47];
   DK[4] = EK[46];
   DK[3] = mul_inv(EK[51]);
   DK[2] = -EK[50];
   DK[1] = -EK[49];
   DK[0] = mul_inv(EK[48]);
   }
 
}