Base-64 Encoding

Transmitting binary data such as keys or digital certificates in printable textual form is the goal of Base-64 encoding. This type of encoding is necessary if these objects are sent in the body of an email message, through a Web page, or as part of an XML message.

A 65-character subset of US-ASCII is used, enabling 6 bits to be represented per printable character. (The extra 65th character, =, is used to signify a special processing function. See Table A.1.)

The encoding process represents 24-bit groups of input bits as output strings of four encoded characters . Proceeding from left to right, a 24-bit input group is formed by concatenating three 8-bit input groups. These 24 bits are then treated as four concatenated 6-bit groups, each of which is translated into a single digit in the Base-64 alphabet.

Each 6-bit group is used as an index into an array of 64 printable characters. The character referenced by the index is placed in the output string.

Table A.1. The 65-Character Subset of US-ASCII

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