HTTP: The Definitive Guide

Base-64 Padding

Base-64 encoding takes a sequence of 8-bit bytes and segments the bit stream into 6-bit chunks. It is unlikely that the sequence of bits will divide evenly into 6-bit pieces. When the bit sequence does not divide evenly into 6-bit pieces, the bit sequence is padded with zero bits at the end to make the length of the bit sequence a multiple of 24 (the least common multiple of 6 and 8 bits).

When encoding the padded bit string, any group of 6 bits that is completely padding (containing no bits from the original data) is represented by a special 65th symbol: " =”. If a group of 6 bits is partially padded, the padding bits are set to zero.

Table E-2 shows examples of padding. The initial input string “a:a” is 3 bytes long, or 24 bits. 24 is a multiple of 6 and 8, so no padding is required. The resulting base 64-encoded string is “YTph”.

Table E-2. Base-64 padding examples

Input data	Binary sequence (padding noted as “x”)	Encoded data
a:a	011000 010011 101001 100001	YTph
a:aa	011000 010011 101001 100001 011000 01xxxx xxxxxx xxxxxx	YTphYQ==
a:aaa	011000 010011 101001 100001 011000 010110 0001xx xxxxxx	YTphYWE=
a:aaaa	011000 010011 101001 100001 011000 010110 000101 100001	YTphYWFh

However, when another character is added, the input string grows to 32 bits long. The next smallest multiple of 6 and 8 is 48 bits, so 16 bits of padding are added. The first 4 bits of padding are mixed with data bits. The resulting 6-bit group, 01xxxx, is treated as 010000, ...