CBC-Pad is a block cipher mode of operation used in the RCS block cipher, but it could be used in any block cipher. CBC-Pad handles plain text of any length. The ciphertext is longer than the plaintext by at most the size of a single block. Padding is used to ensure that the plaintext input is a multiple of the block length. Assume that the original plaintext is an integer number of bytes. This plaintext is padded at the end by from 1 to bb bytes, where bb equals the block size in bytes. The pad bytes are all the same and set to a byte that represents the number of bytes of padding. For example, if there are 8 bytes of padding, each byte has the bit pattern 00001000.

Why not allow zero bytes of padding? That is, if the original plaintext is an integer multiple of the block size, why not refrain from padding?