Padding refers to filling in empty space in a block of data so that it reaches a specific size.

When encrypting using symmetric encryption algorithms, the resulting encrypted stream will always be a multiple of the internal block size used (128 bits for Rijndael and Twofish). Unlike asymmetric encryption (RSA), one full block of data to encrypt will always produce one block of the same of encrypted data. Hence, an encrypted stream will always be of the same size as the plain text (before encryption) if the latter has a size that is a multiple of the symmetric algorithm's block size. If the plain text is not a multiple of a block size, then the last block will have to be padded before encryption. There are different ways of doing that. The Xceed Encryption library supports some popular ones. See the PaddingMethod property.