1. Padding is the process of adding unused data to the end of a message in order to make it conform to a certain length. For example, block-ciphers often work on blocks that are 64-bits (8-bytes) long. Therefore, if you have a message that is 77-bytes long, you will need to "pad" it with an extra 3-bytes to make it an even 80-bytes in size (10-blocks). Key point: Padding is a regular feature of all crypto algorithms, including hashing and encryption. Some algorithms have been broken due to poor choices for padding. Most importantly, however, the size of the message can often reveal details about its contents. For example, let's assume a protocol whereby somebody accepts something with a simple message of "yes", but when it declines, it says "no" along with a reason why it was rejected. Therefore, even though the messages are encrypted, the "yes" will be a short message but the "no" will be a long message. From Hacking-Lexicon