Editing Padding (cryptography) (section)

====Bit padding====
Bit padding can be applied to messages of any size.

A single '1' bit is added to the message and then as many '0' bits as required (possibly none) are added.  The number of '0' bits added will depend on the block boundary to which the message needs to be extended.  In bit terms this is "1000 ... 0000".

This method can be used to pad messages which are any number of bits long, not necessarily a whole number of bytes long. For example, a message of 23 bits that is padded with 9 bits in order to fill a 32-bit block:

 ... | 1011 1001 1101 0100 0010 011'''1 0000 0000''' |

This padding is the first step of a two-step padding scheme used in many [[hash functions]] including [[MD5]] and [[Secure Hash Algorithm (disambiguation)|SHA]]. In this context, it is specified by [http://www.faqs.org/rfcs/rfc1321.html RFC1321] step 3.1.

This padding scheme is defined by [[ISO/IEC 9797-1]] as Padding Method 2.