internal static string DecodeFileName(Span <byte> encodedName)
{
for (int i = 0; i < encodedName.Length; i++)
{
// swap nibbles
encodedName[i] = (byte)((encodedName[i] & 0x0F) << 4 | (encodedName[i] & 0xF0) >> 4);
}
for (int i = 0; i < encodedName.Length / DataEncryptionStandard.BlockSize; i++)
{
DataEncryptionStandard.DecryptBlock(encodedName.Slice(
i * DataEncryptionStandard.BlockSize,
DataEncryptionStandard.BlockSize));
}
var fileName = string.Empty;
for (int i = 0; i < encodedName.Length; i++)
{
if ((char)encodedName[i] == 0)
{
break;
}
fileName += (char)encodedName[i];
}
return(fileName);
}
private static void DecodeHeader(Span <byte> data)
{
int blocks = data.Length / DataEncryptionStandard.BlockSize;
// first 20 blocks are DES-encrypted
for (int i = 0; i < 20 && i < blocks; ++i)
{
DataEncryptionStandard.DecryptBlock(data.Slice(i * DataEncryptionStandard.BlockSize, DataEncryptionStandard.BlockSize));
}
}
private static void DecodeFull(Span <byte> source, uint compressedSize)
{
DecodeHeader(source);
int blocks = source.Length / DataEncryptionStandard.BlockSize;
int digits = compressedSize.ToString().Length;
int gapBetweenEncryptedBlocks = (digits < 3) ? 1
: (digits < 5) ? digits + 1
: (digits < 7) ? digits + 9
: digits + 15;
int gapBetweenShuffledBlocks = 7;
int j = -1;
for (int i = 20; i < blocks; ++i)
{
if ((i % gapBetweenEncryptedBlocks) == 0)
{
// DES-encrypted
DataEncryptionStandard.DecryptBlock(
source.Slice(
i * DataEncryptionStandard.BlockSize,
DataEncryptionStandard.BlockSize));
continue;
}
++j;
if ((j % gapBetweenShuffledBlocks) == 0 && j != 0)
{
DeshuffleBlock(
source.Slice(
i * DataEncryptionStandard.BlockSize,
DataEncryptionStandard.BlockSize));
continue;
}
}
}
