private void ApplyFilePass(XlsBiffFilePass filePass)
{
RC4Key key = new RC4Key("VelvetSweatshop", filePass.Salt);
int blockNumber = 0;
RC4 rc4 = key.Create(blockNumber);
int position = 0;
while (position < _bytes.Length - 4)
{
uint id = BitConverter.ToUInt16(_bytes, position);
int length = BitConverter.ToUInt16(_bytes, position + 2) + 4;
int startDecrypt = 4;
switch ((BIFFRECORDTYPE)id)
{
case BIFFRECORDTYPE.BOF:
case BIFFRECORDTYPE.FILEPASS:
case BIFFRECORDTYPE.INTERFACEHDR:
startDecrypt = length;
break;
case BIFFRECORDTYPE.BOUNDSHEET:
startDecrypt += 4; // For some reason the sheet offset is not encrypted
break;
}
for (int i = 0; i < length; i++)
{
int currentBlock = position / 1024;
if (blockNumber != currentBlock)
{
blockNumber = currentBlock;
rc4 = key.Create(blockNumber);
}
byte mask = rc4.Output();
if (i >= startDecrypt)
{
_bytes[position] = (byte)(_bytes[position] ^ mask);
}
position++;
}
}
}