Base64 编码

Posted 2020-08-01 RushoutAsia

换行：

The encoded output stream must be represented in lines of no more than 76 characters each。

MIME规定，base64编码中，每76个字符后，要加一个\n

避免换行：

android:

通过指定标志位：Base64.NO_WRAP

Base64编码表及示例：

	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	/

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						=						=

If there were two significant input bytes (say ‘Ma‘), the first three base64 digits are picked (18 bits). ‘=‘ characters might be added to make the last block contain four base64 characters.

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						=

As a result, when the last input group contains only one octet, the four least significant bits of the last used 6-bit block are set to zero:

Bit pattern	0	1	0	0	0	0
Index	16
Base64-encoded	Q

And when the last input group contains two octets, the two least significant bits of the last used 6-bit block are set to zero:

Bit pattern	0	0	0	1	0	0
Index	4
Base64-encoded	E

Padding

The ‘==‘ sequence indicates that the last group contained only one byte, and ‘=‘ indicates that it contained two bytes. The example below illustrates how truncating the input of the whole of the above quote changes the output padding:

Length	Input	Length	Output	Padding
20	any carnal pleasure.	28	YW55IGNhcm5hbCBwbGVhc3VyZS4=	1
19	any carnal pleasure	28	YW55IGNhcm5hbCBwbGVhc3VyZQ==	2
18	any carnal pleasur	24	YW55IGNhcm5hbCBwbGVhc3Vy	0
17	any carnal pleasu	24	YW55IGNhcm5hbCBwbGVhc3U=	1
16	any carnal pleas	24	YW55IGNhcm5hbCBwbGVhcw==	2

The same characters will be encoded differently depending on their position within the three-octet group which is encoded to produce the four characters. For example:

Input	Output
pleasure.	cGxlYXN1cmUu
leasure.	bGVhc3VyZS4=
easure.	ZWFzdXJlLg==
asure.	YXN1cmUu
sure.	c3VyZS4=