Most plaintext messages will not consist of an even number of blocks. Often, the last block is short, making it necessary to add a padding string. For example, if the block length is 64 bits and the last block contains only 40 bits, then 24 bits of padding must be added.
This padding string can consist of all zeros, alternating zeros and ones, or some other pattern. Some encryption standards specify a particular padding scheme, such as the one described in the next section.
Applications using the CryptoAPI need not add padding to their plaintext before it is encrypted, nor do they have to remove it after decrypting. This is all handled automatically by the CryptoAPI.
This padding scheme is defined by RSA Data Security, Inc. and is documented in Public-Key Cryptography Standards (PKCS), PKCS #5, section 6.2.
When this method is used, a padding string is always added, even if the plaintext message divides evenly into blocks. The padding string consists of a sequence of bytes, each of which is equal to the total number of bytes in the padding string. If 24 bits of padding need to be added, then the padding string is "03 03 03" (3 bytes times 8 bits equals 24 bits). Therefore, if 64 bits of padding needs to be added, then the string is "08 08 08 08 08 08 08 08."