Base64 encoding and decoding

Base64 is one of the most common encoding methods used to transmit 8-bit code on the network, base64 requires that each three 8-bit bytes be converted into four 6-bit bytes (3*8 = 4*6 = 24), and then 6-bit bytes be added with two more high 0 values, it consists of four 8-bit bytes. That is to say, the converted string is theoretically 1/3 longer than the original one. After conversion, a base64 code table can be used to obtain base64 encoding. If the character string is not a multiple of 3, '=' is used in the case of insufficient bytes during conversion.

 

Base64 code table:

 

Index	Character	Index	Character	Index	Character	Index	Character
0	A	17	R	34	I	51	Z
1	B	18	S	35	J	52	0
2	C	19	T	36	K	53	1
3	D	20	U	37	L	54	2
4	E	21	V	38	M	55	3
5	F	22	W	39	N	56	4
6	G	23	X	40	O	57	5
7	H	24	Y	41	P	58	6
8	I	25	Z	42	Q	59	7
9	J	26	A	43	R	60	8
10	K	27	B	44	S	61	9
11	L	28	C	45	T	62	+
12	M	29	D	46	U	63	/
13	N	30	E	47	V
14	O	31	F	48	W
15	P	32	G	49	X
16	Q	33	H	50	Y

 

For example, if the source string is

00110110

11001100

11011010, it is divided into 4 bytes according to each 6 bits, and in the high 2 is zero (6 bits are insufficient, so in the high 2 bits are zero, constitute one byte), the result is as follows:

00001101

00101100

00110011

00011010

The corresponding decimal values are:

13

44

51

26

According to the code table, it can be base64 encoded as nsza. Decoding is only the inverse process of encoding.

 

The base64 encoding implemented by VC is as follows:

// Define a code table

Const char * jbase64: pszencodetable = "abcdefghijklmnopqrstuvwxyzabcdefghijklmnopqrstuvwxyz0123456789 + /";

// Decoding function

Bool jbase64: encode (<br/> const byte * pbytein, <br/> int ninlength, <br/> byte * pbyteout, <br/> Int & noutlength <br/>) <br/> {<br/> bool breturn = false; <br/> assert (pbytein! = NULL); <br/> assert (pbyteout! = NULL); <br/> assert (ninlength> 0); <br/> If (pbytein = NULL | pbyteout = NULL | ninlength <= 0) <br/>{< br/> goto exit0; <br/>}</P> <p> int ncount = ninlength/3; <br/> int nmode = ninlength % 3; <br/> byte * pout = pbyteout; <br/> for (INT I = 0; I <ncount; I ++) <br/>{< br/> * pout ++ = pszencodetable [(pbytein [0]> 2)]; <br/> * pout ++ = pszencodetable [(pbytein [0] & 3) <4) | (pbytein [1]> 4)]; <br/> * pout ++ = pszencodetable [(pbytein [1] & 0xf) <2 | (pbytein [2]> 6)]; <br/> * pout ++ = pszencodetable [(pbytein [2] & 0x3f)]; <br/> pbytein + = 3; <br/>}< br/> If (nmode! = 0) <br/>{< br/> * pout ++ = pszencodetable [(pbytein [0]> 2)]; <br/> * pout ++ = pszencodetable [(pbytein [0] & 3) <4) | (nmode = 1 )? 0: (pbytein [1]> 4)]; <br/> * pout ++ = (nmode = 1 )? '=': Pszencodetable [(pbytein [1] & 0xf) <2)]; <br/> * pout ++ = '; <br/>}< br/> * pout = '/0'; <br/> noutlength = static_cast <int> (pout-pbyteout ); <br/> breturn = true; <br/> exit0: <br/> return breturn; <br/>}

 

// Define the decoding table

Const unsigned char jbase64: pszdecodetable [256] =
{
255,255,255,255,255,255,255,255,255,255,255,255,
255,255,255,255,255,255,255,255,255,255,255,255,
255,255,255,255,255,255,255,255,255,255,255,255,
255,255,255,255,255,255,255, 62,255,255,255, 63,
52, 53, 54, 55, 56, 57, 58, 59, 60, 61,255,255,
255,254,255,255,255, 0, 1, 2, 3, 4, 5, 6,
7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18,
19, 20, 21, 22, 23, 24, 25,255,255,255,255,255,
255, 26, 27, 28, 29, 30, 31, 32, 33, 34, 35, 36,
37, 38, 39, 40, 41, 42, 43, 44, 45, 46, 47, 48,
49, 50, 51,255,255,255,255,255,255,255,255,255,
255,255,255,255,255,255,255,255,255,255,255,255,
255,255,255,255,255,255,255,255,255,255,255,255,
255,255,255,255,255,255,255,255,255,255,255,255,
255,255,255,255,255,255,255,255,255,255,255,255,
255,255,255,255,255,255,255,255,255,255,255,255,
255,255,255,255,255,255,255,255,255,255,255,255,
255,255,255,255,255,255,255,255,255,255,255,255,
255,255,255,255,255,255,255,255,255,255,255,255,
255,255,255,255,255,255,255,255,255,255,255,255,
255,255,255,255,255,255,255,255,255,255,255,255,
255,255,255,255
};

// Decoding function

Bool jbase64: Decode (<br/> const byte * pbytein, <br/> int ninlength, <br/> byte * pbyteout, <br/> Int & noutlength <br/>) <br/> {<br/> bool breturn = false; <br/> assert (pbytein! = NULL); <br/> assert (pbyteout! = NULL); <br/> assert (ninlength> 0); <br/> assert (ninlength % 4 = 0 ); <br/> If (pbytein = NULL | pbyteout = NULL | ninlength % 4! = 0) <br/>{< br/> goto exit0; <br/>}< br/> byte TMP [4]; <br/> byte * pout = pbyteout; <br/> int nlength = ninlength/4; <br/> for (INT I = 0; I <nlength; I ++) <br/> {<br/> TMP [0] = pszdecodetable [pbytein [0]; <br/> TMP [1] = pszdecodetable [pbytein [1]; <br/> TMP [2] = pszdecodetable [pbytein [2]; <br/> TMP [3] = pszdecodetable [pbytein [3]; </P> <p> * pout ++ = (TMP [0] <2) | (TMP [1]> 4); <br/> * Po Ut ++ = (TMP [1] <4) | (TMP [2] = 254? 0: TMP [2]> 2); <br/> * pout ++ = (TMP [2] = 254? 0: TMP [2] <6) | (TMP [3] = 254? 0: TMP [3]); <br/> pbytein + = 4; <br/>}< br/> * pout = '/0 '; <br/> noutlength = static_cast <int> (pout-pbyteout); <br/> breturn = true; <br/> exit0: <br/> return breturn; <br/>}