Base64 algorithm principle, as well as encoding, decoding [encryption, decryption] introduction_PHP tutorial

Base64 encoding is an encoding method often used in our program development. It is a representation method based on using 64 printable characters to represent binary data. It is usually used as a encoding method for storing and transmitting some binary data! It is also a common encoding method for binary data represented by printable characters in MIME (Multipurpose Internet Mail Extensions, mainly used as an email standard)! It actually just defines a method of transmitting content using printable characters, and does not create a new character set! Sometimes, after we learn the idea of ​​conversion, we can actually construct some of our own interface definition coding methods based on our own actual needs. Okay, let’s take a look at its conversion ideas!

Base64 implementation conversion principle

It is a method that uses 64 printable characters to represent all binary data. Since 2 to the 6th power is equal to 64, every 6 bits can be used as a unit, corresponding to a certain printable character. We know that three bytes have 24 bits, which can correspond to 4 Base64 units, that is, 3 bytes need to be represented by 4 Base64 printable characters. Printable characters in Base64 include letters A-Z, a-z, and numbers 0-9, so there are 62 characters in total. In addition, the two printable symbols are generally different in different systems. However, the other two characters of Base64 that we often refer to are: "+/". The corresponding table of these 64 characters is as follows.

编号	字符		编号	字符		编号	字符		编号	字符
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	/

During conversion, three bytes of data are put into a 24-bit buffer one after another, and the byte that comes first occupies the high bit. If the data is less than 3 bytes, the remaining bits in the buffer will be filled with 0s. Then, 6 bits are taken out each time, and the characters in <br>ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+/ are selected according to their values ​​as the encoded output. Continue until all input data is converted.

If there are two input data left at the end, add 1 "=" after the encoding result; if there is one input data left at the end, add 2 "=" after the encoding result; if there is no data left, just what Do not add them, so as to ensure the accuracy of data restoration.

The encoded data is slightly longer than the original data, 4/3 of the original. No matter what kind of characters, all characters will be encoded, so unlike Quoted-printable encoding, some printable characters are retained. Therefore, it is not as readable as Quoted-printable encoding!

文本	M								a								n
ASCII编码	77								97								110
二进制位	0	1	0	0	1	1	0	1	0	1	1	0	0	0	0	1	0	1	1	0	1	1	1	0
索引	19						22						5						46
Base64编码	T						W						F						u

The Ascii code of M is 77, the first six digits correspond to 19, the corresponding base64 character is T, and so on. Other character encodings can be automatically converted! Let's look at the other situation where it's not exactly 3 bytes!

文本（1 Byte）	A
二进制位	0	1	0	0	0	0	0	1
二进制位（补0）	0	1	0	0	0	0	0	1	0	0	0	0
Base64编码	Q						Q						=						=
文本（2 Byte）	B								C
二进制位	0	1	0	0	0	0	1	0	0	1	0	0	0	0	1	1			x	x	x	x	x	x
二进制位（补0）	0	1	0	0	0	0	1	0	0	1	0	0	0	0	1	1	0	0	x	x	x	x	x	x
Base64编码	Q						k						M						=

Base64 conversion code implementation

Now that we know the method, it seems to be very easy if we want to write a simple conversion ourselves! Below, I wrote down the php code I do the conversion!

<span><?php </span></span>

/**

*base64编码方法、本方法只是做base64转换过程代码举例说明，通过该例子可以任意改造不同语言版

*@author 程默

*@copyright http://blog.chacuo.net

*@param $src 原字符串

*@return string base64字符串*

*/

function c_base64_encode($src)

{

static $base="ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+/";

////将原始的3个字节转换为4个字节

$slen=strlen($src);

$smod = ($slen%3);

<code>$snum = floor($slen/3);

$desc = array();

for($i=0;$i<code>$snum;$i++)

{

////读取3个字节

$_arr = array_map('ord',str_split(substr($src,$i*3,3)));

///计算每一个base64值

$_dec0= $_arr[0]>>2;

$_dec1= (($_arr[0]&3)<code>$_arr[1]>>4);

$_dec2= (($_arr[1]&0xF)<code>$_arr[2]>>6); 

$_dec3= $_arr[2]&63;

$desc = array_merge($desc,array($base[$_dec0],$base[$_dec1],$base[$_dec2],$base[$_dec3]));

}

if($smod==0) return implode('',$desc);

///计算非3倍数字节

$_arr = array_map('ord',str_split(substr($src,<code>$snum*3,3)));

$_dec0= $_arr[0]>>2;

///只有一个字节

if(!isset($_arr[1]))

{

$_dec1= (($_arr[0]&3)

$_dec2=$_dec3="=";

}

else

{

///2个字节

$_dec1= (($_arr[0]&3)<code>$_arr[1]>>4);

$_dec2= $base[($_arr[1]&7)

$_dec3="=";

}

$desc = array_merge($desc,array($base[$_dec0],$base[$_dec1],$_dec2,$_dec3));

return implode('',$desc);

}

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Okay, through this example, I think I have some understanding of the base64 encoding conversion principle and algorithm! The conversion process is very simple. You only need to make a mapping table and then do some shift operations on the original one! Through this example, can we make our own base32 encoding? Welcome friends to communicate!

Author: Cheng Mo’s blog QQ:8292669
Original URL: http://blog.chacuo.net/719.html
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