void EncryptionModeXTS::DecryptBufferXTS (const Cipher &cipher, const Cipher &secondaryCipher, byte *buffer, uint64 length, uint64 startDataUnitNo, unsigned int startCipherBlockNo) const
{
byte finalCarry;
byte whiteningValues [ENCRYPTION_DATA_UNIT_SIZE];
byte whiteningValue [BYTES_PER_XTS_BLOCK];
byte byteBufUnitNo [BYTES_PER_XTS_BLOCK];
uint64 *whiteningValuesPtr64 = (uint64 *) whiteningValues;
uint64 *whiteningValuePtr64 = (uint64 *) whiteningValue;
uint64 *bufPtr = (uint64 *) buffer;
uint64 *dataUnitBufPtr;
unsigned int startBlock = startCipherBlockNo, endBlock, block;
uint64 *const finalInt64WhiteningValuesPtr = whiteningValuesPtr64 + sizeof (whiteningValues) / sizeof (*whiteningValuesPtr64) - 1;
uint64 blockCount, dataUnitNo;
startDataUnitNo += SectorOffset;
// Convert the 64-bit data unit number into a little-endian 16-byte array.
// Note that as we are converting a 64-bit number into a 16-byte array we can always zero the last 8 bytes.
dataUnitNo = startDataUnitNo;
*((uint64 *) byteBufUnitNo) = Endian::Little (dataUnitNo);
*((uint64 *) byteBufUnitNo + 1) = 0;
if (length % BYTES_PER_XTS_BLOCK)
TC_THROW_FATAL_EXCEPTION;
blockCount = length / BYTES_PER_XTS_BLOCK;
// Process all blocks in the buffer
while (blockCount > 0)
{
if (blockCount < BLOCKS_PER_XTS_DATA_UNIT)
endBlock = startBlock + (unsigned int) blockCount;
else
endBlock = BLOCKS_PER_XTS_DATA_UNIT;
whiteningValuesPtr64 = finalInt64WhiteningValuesPtr;
whiteningValuePtr64 = (uint64 *) whiteningValue;
// Encrypt the data unit number using the secondary key (in order to generate the first
// whitening value for this data unit)
*whiteningValuePtr64 = *((uint64 *) byteBufUnitNo);
*(whiteningValuePtr64 + 1) = 0;
secondaryCipher.EncryptBlock (whiteningValue);
// Generate subsequent whitening values for blocks in this data unit. Note that all generated 128-bit
// whitening values are stored in memory as a sequence of 64-bit integers in reverse order.
for (block = 0; block < endBlock; block++)
{
if (block >= startBlock)
{
*whiteningValuesPtr64-- = *whiteningValuePtr64++;
*whiteningValuesPtr64-- = *whiteningValuePtr64;
}
else
whiteningValuePtr64++;
// Derive the next whitening value
#if BYTE_ORDER == LITTLE_ENDIAN
// Little-endian platforms
finalCarry =
(*whiteningValuePtr64 & 0x8000000000000000ULL) ?
135 : 0;
*whiteningValuePtr64-- <<= 1;
if (*whiteningValuePtr64 & 0x8000000000000000ULL)
*(whiteningValuePtr64 + 1) |= 1;
*whiteningValuePtr64 <<= 1;
#else
// Big-endian platforms
finalCarry =
(*whiteningValuePtr64 & 0x80) ?
135 : 0;
*whiteningValuePtr64 = Endian::Little (Endian::Little (*whiteningValuePtr64) << 1);
whiteningValuePtr64--;
if (*whiteningValuePtr64 & 0x80)
*(whiteningValuePtr64 + 1) |= 0x0100000000000000ULL;
*whiteningValuePtr64 = Endian::Little (Endian::Little (*whiteningValuePtr64) << 1);
#endif
whiteningValue[0] ^= finalCarry;
}
dataUnitBufPtr = bufPtr;
whiteningValuesPtr64 = finalInt64WhiteningValuesPtr;
// Decrypt blocks in this data unit
for (block = startBlock; block < endBlock; block++)
{
*bufPtr++ ^= *whiteningValuesPtr64--;
*bufPtr++ ^= *whiteningValuesPtr64--;
}
cipher.DecryptBlocks ((byte *) dataUnitBufPtr, endBlock - startBlock);
bufPtr = dataUnitBufPtr;
whiteningValuesPtr64 = finalInt64WhiteningValuesPtr;
for (block = startBlock; block < endBlock; block++)
{
*bufPtr++ ^= *whiteningValuesPtr64--;
*bufPtr++ ^= *whiteningValuesPtr64--;
}
blockCount -= endBlock - startBlock;
startBlock = 0;
dataUnitNo++;
*((uint64 *) byteBufUnitNo) = Endian::Little (dataUnitNo);
}
FAST_ERASE64 (whiteningValue, sizeof (whiteningValue));
FAST_ERASE64 (whiteningValues, sizeof (whiteningValues));
}
