XTS mode, shown in Figure 14.14, is XEX supplemented with the so-called ciphertext stealing , that extends the plaintext so it can be properly processed by a block cipher. More precisely, cipher stealing is an efficient way for using a block cipher to process plaintexts whose size is not an integer multiple of the block size.
Cipher stealing works by processing the last two plaintext blocks in a special way: the second-to-last block’s ciphertext is re-used or stolen for padding the last plaintext block. The last block is now padded, which means it has the required length, and can be encrypted in the standard way.