Specifies the type of padding to apply when the message data block is shorter than the full number of bytes needed for a cryptographic operation.
Most plain text messages do not consist of a number of bytes that completely fill blocks. Often, there are not enough bytes to fill the last block. When this happens, a padding string is added to the text. For example, if the block length is 64 bits and the last block contains only 40 bits, 24 bits of padding are added.
Some encryption standards specify a particular padding scheme. The following example shows how these modes work. Given a blocklength of 8, a data length of 9, the number of padding octets equal to 7, and the data equal to FF FF FF FF FF FF FF FF FF:
Data: FF FF FF FF FF FF FF FF FF
X923 padding: FF FF FF FF FF FF FF FF FF 00 00 00 00 00 00 07
PKCS7 padding: FF FF FF FF FF FF FF FF FF 07 07 07 07 07 07 07
ISO10126 padding: FF FF FF FF FF FF FF FF FF 7D 2A 75 EF F8 EF 07