Base64 encoding

Basic concepts

Base64 The term was originally used in “MIME Content transfer coding specification ” Proposed in .Base64 It's not an encryption algorithm , Although the encoded string looks a bit encrypted . It's actually a kind of “ Binary to text ” The coding method of , It can convert any given binary data （ mapping ） by ASCII String form , So that binary data can be transferred smoothly in a text only environment . For example, support MIME Email app for , Or need to be in XML Store complex data in （ For example, pictures ） when .

Base64 Is a kind of use 64 A method of representing arbitrary binary data with characters . It's a coding method , Instead of encryption . It converts binary data into 64 individual “ Printable characters ”, Completed the data in HTTP Transmission over the Protocol .

Under what circumstances will we use Base64 Well ？Base64 Generally used in HTTP Transfer binary data under the protocol , because HTTP The protocol is hypertext , So in HTTP To transmit binary data under the protocol, we need to convert binary data into character data . However, direct conversion is not possible . You can only transmit characters over the Internet .

What are printable characters ？ stay ASCII The code says ,0-31、127 this 33 Characters are control characters ,32-126 this 95 Characters are printable （ See ASCII Code comparison table ）, That is to say, network transmission can only transmit this 95 Characters , Characters that are not in this range cannot be transferred . So how can I transfer other characters ？ One way is to use Base64.

Base64, Is the use of 64 A printable character to represent binary data . this 64 Characters include upper and lower case letters 、 Numbers 、＋ and ／, There are also special characters used to fill gaps ＝.

Be careful ： because base64 Coding used 8 Bit character to represent... In the message 6 bits , therefore base64 The encoded string is approximately larger than the original value 33%.

Base64 Encoding table

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

in other words , At most if the index is converted to the corresponding binary data 6 individual Bit. However ASCII Code needs 8 individual Bit To express , So how to use 6 individual Bit To express 8 individual Bit What about the data? ？6 individual Bit Of course, you can't store 8 individual Bit The data of , however 4×6 individual Bit Can be stored 3×8 individual Bit The data of ！ As shown in the following table ：

 

You can see “Son” adopt Base64 The code was converted to “U29u”. This is just a good situation ,3 individual ASCII The character just translates to the corresponding 4 individual Base64 character . however , When the number of characters to be converted is not 3 What to do in the case of multiple of ？Base64 Regulations , When the character to be converted is not 3 A multiple of the , Make up for all 0 It's a good way 3 Multiple , The details are shown in the table below ：

Every time 6 individual Bit For a group , The first group is converted to characters “U”, Fill in at the end of the second group 4 individual 0 Convert to character “w”. The rest of the use “=” replace . That is, the characters “S” adopt Base64 After coding is “Uw==”. This is it. Base64 The coding process .

If the binary data to be encoded is not 3 Multiple , In the end, there will be 1 Or 2 What about a byte ？Base64 use \x00 Byte after complement at the end , Add... At the end of the code 1 Or 2 individual = Number , Indicates how many bytes are filled , When decoding , Will automatically remove .

This is the total number of bits in bytes, not 6 In the case of multiples of , When the rest 4 When a , We need to 2 individual = Come together 8 Multiple ; When the rest is 2 When a , We need to make up 1 individual = Come together 8 Multiple .

 

To achieve Base64, First of all, we need to choose the appropriate 64 Characters form a character set . A general rule is to choose from some common character set 64 Printable characters , In this way, data loss during transmission can be avoided （ Unprintable characters may be treated as special characters during transmission , Which leads to loss ）. for example ,MIME Of Base64 The implementation uses capital letters 、 Lowercase letters and 0~9 As the first 62 Characters . Other implementations usually follow MIME In this way , And only in the end 2 Different characters , for example UTF-7 code .

Base64 Complete example

The following text ：

Man is distinguished, not only by his reason, but by this singular passion from

other animals, which is a lust of the mind, that by a perseverance of delight

in the continued and indefatigable generation of knowledge, exceeds the short

vehemence of any carnal pleasure.

adopt MIME Base64 After conversion, it becomes ：

TWFuIGlzIGRpc3Rpbmd1aXNoZWQsIG5vdCBvbmx5IGJ5IGhpcyByZWFzb24sIGJ1dCBieSB0aGlz

IHNpbmd1bGFyIHBhc3Npb24gZnJvbSBvdGhlciBhbmltYWxzLCB3aGljaCBpcyBhIGx1c3Qgb2Yg

dGhlIG1pbmQsIHRoYXQgYnkgYSBwZXJzZXZlcmFuY2Ugb2YgZGVsaWdodCBpbiB0aGUgY29udGlu

dWVkIGFuZCBpbmRlZmF0aWdhYmxlIGdlbmVyYXRpb24gb2Yga25vd2xlZGdlLCBleGNlZWRzIHRo

ZSBzaG9ydCB2ZWhlbWVuY2Ugb2YgYW55IGNhcm5hbCBwbGVhc3VyZS4=

Transformation method

Beginning with an example “Man” Converted to “TWFu” For example , Let's see Base64 Basic conversion process ：

1. M、a and n Of ASCII The codes are 01001101、01100001 and 01101110, Merge to get one 24 Binary string of bits 010011010110000101101110

2. Per click 6 Divide them into 4 Group ：010011、010110、000101、101110

3. Finally, take it out of the character set according to the corresponding relationship 4 Characters （ namely T、W、F、u） As a result （ Later in this article, we list MIME Defined character set ）.

Base64 The basic idea of is so simple ： It will every 3 Bytes （24 position ） Convert to 4 Characters . because 6 Bit binary numbers can represent 64 A different number , So as long as the character set is determined （ contain 64 Characters ）, And determine a unique code for each character , Binary bytes can be converted into Base64 Code or vice versa .

Zero filling

By constantly turning every 3 Bytes to 4 individual Base64 After the character , Finally, the following may appear 3 One of the three situations ：

1. No bytes left

2. There is still left 1 Bytes

3. There is still left 2 Bytes

1 There's nothing to say . hinder 2 and 3 How to deal with it ？

In this case , You need to fill in zeros after the remaining bytes , Until its digits can be 6 to be divisible by （ because Base64 Yes, for every 6 Bit encoded ）. If there is still 1 Bytes , namely 8 position , Then it needs to be supplemented 4 individual 0 Make it a 12 position , In this way, it can be divided into 2 Group ; If there is left 2 Bytes , namely 16 position , Then we just need to make up 2 individual 0（18 position ） Can be divided into 3 Group . Finally, use the common method to map .

When restoring , Turn each 4 Characters are restored to 3 Bytes , At the end 3 One of the three situations ：

1. No characters left

2. There is still left 2 Characters

3. There is still left 3 Characters

this 3 This situation is similar to the above 3 Each situation corresponds one by one , As long as the process of zero filling is handled in reverse , You can restore it as it is .

fill

We often Base64 In the encoded string, you can see that there is “=” character , This is generated by filling . Padding is what happens when encoding occurs 2 and 3 when , Fill in the back “=” character , Make the number of characters after encoding 4 Multiple .

So we can easily think of , situation 2, That is, there is still 1 Bytes , Need to be supplemented 2 individual “=”, Because the last byte is encoded as 2 Characters , Fill up 2 individual “=” Just enough 4 individual . situation 3 Empathy , Need to be supplemented 1 individual “=”.

Filling is not necessary , Because the missing bytes can be calculated from the encoded content without filling . So it is necessary to fill in some implementations , Some are not . One occasion where filling must be used is when multiple Base64 When coding files are merged into one file .

Extended Topic ： utilize Base64 Encryption and decryption

Although it has been mentioned at the beginning of this article ,Base64 It's not an encryption algorithm , But in fact, we can use Base64 To encrypt data .

We all know , Encryption is the process of changing plaintext into ciphertext . Algorithm plays a key role in this process , The second is the key . Algorithm is equivalent to manufacturing process or machining process , The key is the recipe . The manufacturing process can be disclosed , But the recipe must be kept secret , Otherwise everyone can produce Yunnan Baiyao .

Easy to think of ,Base64 The recipe of is character set . Different character sets are selected , Even just change the order of characters in the character set （ Number ）, The same processing process will produce different Base64 code .

for example , If I don't tell you the character set used in encoding , Can you know the original text corresponding to the following code ？

TWl+Im1DImR5sHR5r2tFqXN4pWQ8ImZ/tih/r2BZImJZImx5sChCpWlDrGY8ImJFtihyuSh
Eqm1DInN5r2tFrmlCInhxsHN5rGYwp3J/rSh/tmx1syhxr219oWBDLihHqm1zqih5sChxImB
FsHQwrGowtmx1ImF5r2Q8InR4oXQwo30woShApXJDpXp1s2l+oGUwrGowpmV8qWt4t
ih5ryhEqmUwoGd+tm1+tWV0Iml+pih5r2R1p2lEqWtxo2B1Imt1r2VCoXR5rGYwrGowqGZ
/tGB1pmt1Lih1umN1pWRDInR4pShDqmdCtihGpWx1rWV+oGUwrGowoWZZImNxs2Zxrih
ArmVxsHVCpSY=

Since we use Base64 To encrypt and decrypt is completely feasible , Why is it said that it is not an encryption algorithm ？

This is because ：

1. Development Base64 The purpose of is not to encrypt , But to facilitate the transmission of binary data in the text environment

2. therefore , Different from developing an encryption algorithm , Security is not Base64 The goal of , Just a by-product of it .

actually ,Base64 The security of is very poor , This is the reason why it is not used for encryption in practical applications . If you know some common encryption methods , You should know that there is an old encryption method , be called “ Character substitution ”. That is, specify a rule , Replace each character with another character , For example a Turn into c、b Turn into d etc. , The result of this replacement is the ciphertext . When decrypting, you only need to operate in reverse , take c Turn into a、 take d Turn into b That's all right. . Encrypt with different replacement rules , The generated ciphertext is also different .

use Base64 To encrypt is actually equivalent to character substitution , It just makes some changes to bytes first , And then replace it , For the encryption process , It's essentially the same thing .

Base64 usage
Java It has been written for us Base64 Implementation details , Call directly when using . The specific code is as follows ：

package com.first;
 
import org.junit.Test;
 
import java.io.UnsupportedEncodingException;
import java.util.Base64;
 
public class Test {
 
    @Test
    public void test() throws UnsupportedEncodingException {
        // code
        String encode = Base64.getEncoder().encodeToString("So".getBytes("UTF-8"));
        System.out.println(encode);
        // decode
        byte[] decode = Base64.getDecoder().decode(encode);
        System.out.println(new String(decode, "UTF-8"));
    }
}

Base64 Characteristics
1、 First, the algorithm is coding , Not compression , After encoding, only the number of bytes will be increased ; The number of bytes will become the original number of bytes 4/3;
2、 Method is simple , Little impact on efficiency ;
3、 Algorithm reversible , Decoding is very convenient , Not used for private information communication ;
4、 Although decoding is convenient , But after all, it's coded , The naked eye still can't see the original content directly ;
5、 The encrypted string has only [0-9a-zA-Z+/=], Non printable characters ( Including transfer characters ) It can also transmit ;