HTTP: The Definitive Guide

Eight Bits to Six Bits

Base-64 encoding takes a sequence of 8-bit bytes, breaks the sequence into 6-bit pieces, and assigns each 6-bit piece to one of 64 characters comprising the base-64 alphabet. The 64 possible output characters are common and safe to place in HTTP header fields. The 64 characters include upper- and lowercase letters, numbers, +, and /. The special character = also is used. The base-64 alphabet is shown in Table E-1.

Note that because the base-64 encoding uses 8-bit characters to represent 6 bits of information, base 64-encoded strings are about 33% larger than the original values.

Table E-1. Base-64 alphabet

0	A	8	I	16	Q	24	Y	32	g	40	o	48	w	56	4
1	B	9	J	17	R	25	Z	33	h	41	p	49	x	57	5
2	C	10	K	18	S	26	a	34	i	42	q	50	y	58	6
3	D	11	L	19	T	27	b	35	j	43	r	51	z	59	7
4	E	12	M	20	U	28	c	36	k	44	s	52	0	60	8
5	F	13	N	21	V	29	d	37	l	45	t	53	1	61	9
6	G	14	O	22	W	30	e	38	m	46	u	54	2	62	+
7	H	15	P	23	X	31	f	39	n	47	v	55	3	63	/

Figure E-1 shows a simple example of base-64 encoding. Here, the three-character input value “Ow!” is base 64-encoded, resulting in the four-character base 64-encoded value “T3ch”. It works like this:

1. The string “Ow!” is broken into 3 8-bit bytes (0x4F, 0x77, 0x21).

2. The 3 bytes create the 24-bit binary value 010011110111011100100001.

3. These bits are segmented into the 6-bit sequences 010011, 110111, 01110, 100001.

4. Each of these ...