Base64 Radix64

Base64 is a group of similar binary-to-text encoding.

The Base64 index table:

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

Encoding example. 3 char( Byte ) 씩 끊어서 읽고 4 char로 encoding(3Byte -> 4Byte)한다.

Text content	M								a								n
ASCII	77 (0x4d)								97 (0x61)								110 (0x6e)
Bit pattern	0	1	0	0	1	1	0	1	0	1	1	0	0	0	0	1	0	1	1	0	1	1	1	0
Index	19						22						5						46
Base64-encoded	T						W						F						u

만약 3 Byte 중 1 byte가 없을 경우,

Text content	M								a
ASCII	77 (0x4d)								97 (0x61)								0 (0x00)
Bit pattern	0	1	0	0	1	1	0	1	0	1	1	0	0	0	0	1	0	0	0	0	0	0	0	0
Index	19						22						4						0
Base64-encoded	T						W						E						=

만약 3 Byte 중 2 byte가 없을 경우,

Text content	M
ASCII	77 (0x4d)								0 (0x00)								0 (0x00)
Bit pattern	0	1	0	0	1	1	0	1	0	0	0	0	0	0	0	0	0	0	0	0	0	0	0	0
Index	19						16						0						0
Base64-encoded	T						Q						=						=

1. 맨 끝에 '='나 '=='가 들어가는 것 이외에도, 그 앞의 char가 바뀐다.

2. 앞에서부터 3Byte 씩 encoding 하기 때문에, 문자열의 위치에 따라 output이 변경될 수 있다.

Radix-64

Radix-64 encoding, also known as "ASCII Armor". Radix-64 is identical to the "Base64" encoding described from MIME, with the addition of an optional 24-bit CRC. The checksum is calculated on the input data before encoding; the checksum is then encoded with the same Base64 algorithm and, using an additional "=" symbol as separator, appended to the encoded output data.

즉 Base64 포맷에 맨 끝쪽에 = 이후 24-bit CRC 있으면. Radix-64