Introduction to Network Security, 2nd Edition

Appendix D

Base64 Encoding

Base64 encoding represents a 6-bit binary string using a printable character (see Table D.1), where a 6-bit value of 0–25 represents an upper-case letter A–Z correspondingly; a 6-bit value of 26–51 represents a lower-case letter a–z correspondingly; a 6-bit value of 52–61 represents a digit 0–9 correspondingly; and the last two 6-bit values of 62 and 63 represent “” and “”, respectively. Transmitted in ASCII format, this means that every 6-bit string is replaced with an 8-bit string.

Table D.1 Base64 encoding

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

In addition, Base64 encoding uses character “=” as a special indicator. Using Base64 encoding, a binary string is converted to a character string as follows:

Case 1: The binary ...