protected override void InverseLayer(State state)
{
byte[] tempVector = new byte[4];
for (int col = 0; col < state.Columns; col++)
{
ByteMatrixColumn currentColumn = state.GetColumn(col);
// Multiply each column by d(x) which is the inverse of c(x). This means:
// c(x) * d(x) ≡ 1, which expanded is
// (03*x^3 ⊕ 01*x^2 ⊕ 01*x ⊕ 02) * d(x) ≡ 1
// After some derivation, we can calculate that
// d(x) = 0B*x^3 ⊕ 0D*x^2 ⊕ 09*x ⊕ 0E;
// This is the same as multiplication by this matrix:
// | 0E 0B 0D 09 | | x0 |
// | 09 0E 0B 0D | | x1 |
// | 0D 09 0E 0B | * | x2 |
// | 0B 0D 09 0E | | x3 |
// We can perform this multiply by starting with the top row of the matrix:
// [0E 0B 0D 09] and keep rotating it by 1 each column. Note that the multiply
// is in the Rijndael field.
for (int row = 0; row < 4; row++)
{
tempVector[row] = (byte)(
FiniteFieldMath.Multiply(0x0E, currentColumn[row]) ^
FiniteFieldMath.Multiply(0x0B, currentColumn[(row + 1) % 4]) ^
FiniteFieldMath.Multiply(0x0D, currentColumn[(row + 2) % 4]) ^
FiniteFieldMath.Multiply(0x09, currentColumn[(row + 3) % 4]));
}
// Now that we have the result of the multiply in tempVector, we
// copy it back to the state matrix:
for (int row = 0; row < 4; row++)
{
currentColumn[row] = tempVector[row];
}
}
}
protected override void ApplyLayer(State state)
{
byte[] tempVector = new byte[4];
for (int col = 0; col < state.Columns; col++)
{
ByteMatrixColumn currentColumn = state.GetColumn(col);
// Multiply each column by c(x) which is defined as
// c(x) = 03*x^3 ⊕ 01*x^2 ⊕ 01*x ⊕ 02;
// This is the same as multiplication by this matrix:
// | 02 03 01 01 | | x0 |
// | 01 02 03 01 | | x1 |
// | 01 01 02 03 | * | x2 |
// | 03 01 01 02 | | x3 |
// We can perform this multiply by starting with the top row of the matrix:
// [02 03 01 01] and keep rotating it by 1 each column. Note that the multiply
// is in the Rijndael field.
for (int row = 0; row < 4; row++)
{
tempVector[row] = (byte)(
FiniteFieldMath.Multiply(0x02, currentColumn[row]) ^
FiniteFieldMath.Multiply(0x03, currentColumn[(row + 1) % 4]) ^
FiniteFieldMath.Multiply(0x01, currentColumn[(row + 2) % 4]) ^
FiniteFieldMath.Multiply(0x01, currentColumn[(row + 3) % 4]));
}
// Now that we have the result of the multiply in tempVector, we
// copy it back to the state matrix:
for (int row = 0; row < 4; row++)
{
currentColumn[row] = tempVector[row];
}
}
}