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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]);
}
}
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