Computer encoding rules: Base64 encoding

Introduction

We know that the files in the computer can be divided into two types, one is a text file that is readable by the human eye, and the other is a binary file that is not readable by the naked eye. Generally speaking, binary files will display garbled characters if they are opened with a text editor, and the storage and transmission methods of binary files and text files are different. Is there any way to convert binary files into text files for transmission or storage? The answer is yes.

This encoding method is the Base64 encoding we are going to talk about today.

Base64 and its encoding principle

Base64 is a form of converting binary encoding format to text encoding. We know that binary encoding is in the form of 0 and 1, and its unit is usually one byte, which is 8 bits, and each bit represents 0 or 1.

There are many types of text encoding formats. The earliest and simplest encoding format is ASCII encoding. The full name of ASCII encoding is American Standard Code for Information Interchange, which is the American Standard Code for Information Interchange. It mainly represents some commonly used Western European character.

The encoding range of ASCII is 0x00-0x7F, which is 0-127 in decimal, a total of 128 characters, which is exactly the range represented by 7bits.

The ASCII encoding contains 33 control characters and 95 printable characters, as follows:

ASCII code			meaning	ASCII code			meaning
hexadecimal	10 hex	binary		hexadecimal	10 hex	binary
0x00	0	0	NUL empty	0x40	64	1000000	@
0x01	1	1	SOH title starts	0x41	65	1000001	A
0x02	2	10	STX text starts	0x42	66	1000010	B
0x03	3	11	End of ETX body	0x43	67	1000011	C
0x04	4	100	End of EOT transfer	0x44	68	1000100	D
0x05	5	101	ENQ query character	0x45	69	1000101	E
0x06	6	110	ACK acknowledgement	0x46	70	1000110	F
0x07	7	111	BEL alarm	0x47	71	1000111	G
0x08	8	1000	BS back one space	0x48	72	1001000	H
0x09	9	1001	HT horizontal tab	0x49	73	1001001	I
0x0A	10	1010	LF line feed	0x4A	74	1001010	J
0x0B	11	1011	VT vertical tab	0x4B	75	1001011	K
0x0C	12	1100	FF paper feed control	0x4C	76	1001100	L
0x0D	13	1101	CR Enter	0x4D	77	1001101	M
0x0E	14	1110	SO shift output	0x4E	78	1001110	N
0x0F	15	1111	SI shift input	0x4F	79	1001111	O
0x10	16	10000	DLE data link escape	0x50	80	1010000	P
0x11	17	10001	DC1 Device Control 1	0x51	81	1010001	Q
0x12	18	10010	DC2 Device Control 2	0x52	82	1010010	R
0x13	19	10011	DC3 Device Control 3	0x53	83	1010011	S
0x14	20	10100	DC4 Device Control 4	0x54	84	1010100	T
0x15	twenty one	10101	NAK Negative	0x55	85	1010101	U
0x16	twenty two	10110	SYN idle synchronization	0x56	86	1010110	V
0x17	twenty three	10111	End of ETB packet transfer	0x57	87	1010111	W
0x18	twenty four	11000	CAN void	0x58	88	1011000	X
0x19	25	11001	EM paper out	0x59	89	1011001	Y
0x1A	26	11010	SUB permutation	0x5A	90	1011010	Z
0x1B	27	11011	ESC escape	0x5B	91	1011011	[
0x1C	28	11100	FS literal separator	0x5C	92	1011100	\
0x1D	29	11101	GS group separator	0x5D	93	1011101	]
0x1E	30	11110	RS record separator	0x5E	94	1011110	^
0x1F	31	11111	US unit separator	0x5F	95	1011111	_
0x20	32	100000	(space)	0x60	96	1100000	`
0x21	33	100001	!	0x61	97	1100001	a
0x22	34	100010	"	0x62	98	1100010	b
0x23	35	100011	#	0x63	99	1100011	c
0x24	36	100100	$	0x64	100	1100100	d
0x25	37	100101	%	0x65	101	1100101	e
0x26	38	100110	&	0x66	102	1100110	f
0x27	39	100111	'	0x67	103	1100111	g
0x28	40	101000	(	0x68	104	1101000	h
0x29	41	101001	)	0x69	105	1101001	i
0x2A	42	101010	*	0x6A	106	1101010	j
0x2B	43	101011	+	0x6B	107	1101011	k
0x2C	44	101100	,	0x6C	108	1101100	l
0x2D	45	101101	-	0x6D	109	1101101	m
0x2E	46	101110	.	0x6E	110	1101110	n
0x2F	47	101111	/	0x6F	111	1101111	o
0x30	48	110000	0	0x70	112	1110000	p
0x31	49	110001	1	0x71	113	1110001	q
0x32	50	110010	2	0x72	114	1110010	r
0x33	51	110011	3	0x73	115	1110011	s
0x34	52	110100	4	0x74	116	1110100	t
0x35	53	110101	5	0x75	117	1110101	u
36	54	110110	6	0x76	118	1110110	v
0x37	55	110111	7	0x77	119	1110111	w
0x38	56	111000	8	0x78	120	1111000	x
0x39	57	111001	9	0x79	121	1111001	y
0x3A	58	111010	:	0x7A	122	1111010	z
0x3B	59	111011	;	0x7B	123	1111011	{
0x3C	60	111100	<	0x7C	124	1111100	\
0x3D	61	111101	=	0x7D	125	1111101	}
0x3E	62	111110	>	0x7E	126	1111110	~
0x3F	63	111111	?	0x7F	127	1111111	DEL delete

Base64 is to select 64 characters from ASCII encoding and map one byte 8bits in binary, which is the meaning of 64 in Base64. Why choose ASCII encoding? This is because ASCII encoding is the earliest form of encoding, which is fully supported by almost all computer applications, and there is no content conversion during data transmission, which is very safe.

Of course, Base64 encoding also has a variety of encoding forms. For example, in MIME, Base64 selects AZ, az, and 0-9 for a total of 62 characters, plus two other optional characters to form 64 encoded characters.

64 characters are represented by 6bits in binary, and the commonly used binary is represented by one byte, that is, 8bits, then the question is, how to represent 8bits binary with 6bits Base64 characters?

Very simple, we only need to connect 3 8bits to become 24bits, so that we can use 4 Base64 to represent.

Why must binary conversion be performed? This is because some transmission protocols in the Internet only support some specific character sets, and other character sets are not supported. For example, it is commonly used to send email attachments. Because the SMTP protocol was originally designed to support 7-bit ASCII characters, if we want to transmit a file, we need to encode the file and then transmit it.

Another use of Base64 is to embed images into web pages in HTML to display images.

Although Base64 is very useful, because it can only use 6bits character mapping set, it will cause the loss of data mapping, which will lead to the disadvantage that the size of the binary file becomes larger after encoding.

Variants of Base64

Base64 is simply the mapping from bit to bit, so there must be more than one mapping method. Let's take a look at the various variants of Base64 encoding. Generally speaking, the first 62 bits are basically the same, the difference is The last two characters, and the character used for padding (this may be mandatory in some protocols, or may be removed in others).

The following table is a common Base64 encoding variant:

encoding name	coded character	coded character	coded character
	62nd	63rd	Completion
RFC 1421: Base64 for Privacy-Enhanced Mail (deprecated)	+	/	= mandatory
RFC 2045: Base64 transfer encoding for MIME	+	/	= mandatory
RFC 2152: Base64 for UTF-7	+	/	No
RFC 3501: Base64 encoding for IMAP mailbox names	+	,	No
RFC 4648: base64 (standard)	+	/	= optional
RFC 4648: base64url (URL- and filename-safe standard)	-	_	= optional
RFC 4880: Radix-64 for OpenPGP	+	/	= mandatory

Base64 encoding details

In the previous section, we talked about the basic principles of Base64 encoding and some common variants, so how exactly is it mapped?

In this section, we will take the standard form of Base64, RFC 4648, as an example to explain in detail.

RFC 4648 chooses the two characters + and / as bits 62 and 63 in the encoding, and chooses = as the completion character.

Let's first observe the mapping table of RFC 4648:

index	binary	character	index	binary	Char	index	binary	Char	index	binary	Char
0	000000	A	16	010000	Q	32	100000	g	48	110000	w
1	000001	B	17	010001	R	33	100001	h	49	110001	x
2	000010	C	18	010010	S	34	100010	i	50	110010	y
3	000011	D	19	010011	T	35	100011	j	51	110011	z
4	000100	E	20	010100	U	36	100100	k	52	110100	0
5	000101	F	twenty one	010101	V	37	100101	l	53	110101	1
6	000110	G	twenty two	010110	W	38	100110	m	54	110110	2
7	000111	H	twenty three	010111	X	39	100111	n	55	110111	3
8	001000	I	twenty four	011000	Y	40	101000	o	56	111000	4
9	001001	J	25	011001	Z	41	101001	p	57	111001	5
10	001010	K	26	011010	a	42	101010	q	58	111010	6
11	001011	L	27	011011	b	43	101011	r	59	111011	7
12	001100	M	28	011100	c	44	101100	s	60	111100	8
13	001101	N	29	011101	d	45	101101	t	61	111101	9
14	001110	O	30	011110	e	46	101110	u	62	111110	+
15	001111	P	31	011111	f	47	101111	v	63	111111	/
Completion	=

Let's take the word man as an example to observe the encoding process of Base64.

The word man is represented by 77, 97, and 110 in ASCII, which translates to 01001101, 01100001, and 01101110 in binary.

Combining the above three binaries together becomes: 010011010110000101101110, a total of 24-bit, select the corresponding characters from the above table, so we can get man after base64 encoding and get: TWFu.

In the above example, man is exactly 3 characters, that is, 24 bits, which can be fully represented by base64. If we only have the two characters ma, how should we encode it?

As above, the binary of ma is 01001101, 01100001 respectively, and the combination is 0100110101100001.

But the above bits are only 16 bits, because a base64 is 6 bits, so it can be represented by 3 base64, because the original bits are two less, so it is filled with 0:

0100110101100001+00 = 010011010110000100.

Converting 010011010110000100 to base64 is TWE, because base64 encoding requires 4 characters, so the last character is completed with =, which means that me becomes TWE= after base64.

Summarize

The above is the basic meaning and conversion rules of Base64. In fact, the protocol is very simple. Convert the data to be converted into binary, and then convert and complete it according to the conversion table.

This article has been included in http://www.flydean.com/18-base64-encoding/

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