Ciphertext Stealing - Ciphertext Stealing Mode Description

Ciphertext Stealing Mode Description

In order to encrypt or decrypt data, use the standard block cipher mode of operation on all but the last two blocks of data.

The following steps describe how to handle the last two blocks of the plaintext, called P_n−1 and P_n, where the length of P_n−1 equals the block size of the cipher in bits, B, the length of the last block, P_n, is M bits, and K is the key that is in use. M can range from 1 to B, inclusive, so P_n could possibly be a complete block. The CBC mode description also makes use of the ciphertext block just previous to the blocks concerned, C_n−2, which may in fact be the IV if the plaintext fits within two blocks.

For this description, the following functions and operators are used:

• Head (data, a): returns the first a bits of the 'data' string.
• Tail (data, a): returns the last a bits of the 'data' string.
• Encrypt (K, data): use the underlying block cipher in encrypt mode on the 'data' string using the key K.
• Decrypt (K, data): use the underlying block cipher in decrypt mode on the 'data' string using the key K.
• XOR: Bitwise Exclusive-OR. Equivalent to bitwise addition without use of a carry bit.
• ||: Concatenation operator. Combine the strings on either side of the operator.
• 0a: a string of a 0 bits.