Base64 encoding

xiaoxiao2021-03-06  68

These days, use Base64 encoding, find a good, collection

Base64 is one of the commonly used coding methods in MIME messages. Its main idea is to encode the input string or data into only {'A' - 'Z', 'A' - 'Z', '0' - '9', ' ', '/'} 64 strings that printable characters are called "Base64".

The Base64 encoding method is to take the input data stream each time 6 bit, use this 6 bit value (0-63) as an index to check the table, output the corresponding character. In this way, each 3 bytes are encoded to 4 characters (3 × 8 → 4 × 6); filled with '=' by dissatisfaction with 4 characters.

Const char enbase64tab [] = "Abcdefghijklmnopqrstuvwxyzabcdefghijklmnopqrstuvwxyz0123456789 /";

Int Encodebase64 (Const unsigned char * psrc, char * pdst, int nsrclen, int nmaxlinelen)

{

Unsigned CHAR C1, C2, C3; // Input buffer reading 3 bytes

INT ndstlen = 0; // output character count

INT NLINELEN = 0; // Output of long-term count

INT NDIV = nsrclen / 3; // Enter data length divided by 3

INT nMOD = nsrclen% 3; // Input data length divided by 3

/ / Take 3 bytes each time, encode 4 characters

For (int i = 0; i

{

// Take 3 bytes

C1 = * psrc ;

C2 = * psrc ;

C3 = * psrc ;

/ / Encode into 4 characters

* PDST = Enbase64TAB [C1 >> 2];

* PDST = eNBase64TAB [((C1 << 4) | (C2 >> 4)) & 0x3f];

* PDST = eNBase64TAB [((C2 << 2) | (C3 >> 6)) & 0x3f];

* PDST = Enbase64TAB [C3 & 0x3f];

NLINELEN = 4;

NDSTLEN = 4;

/ / Output wrap?

IF (nLinelen> NmaxLinelen - 4)

{

* PDST = '/ r';

* PDST = '/ n';

NLINELEN = 0;

NDSTLEN = 2;

}

}

// Code the remaining bytes

IF (nmod == 1)

{

C1 = * psrc ;

* PDST = Enbase64TAB [(C1 & 0xFC) >> 2];

* PDST = Enbase64TAB [((C1 & 0x03) << 4)];

* PDST = '=';

* PDST = '='; nLinelen = 4;

NDSTLEN = 4;

}

Else IF (nmod == 2)

{

C1 = * psrc ;

C2 = * psrc ;

* PDST = Enbase64TAB [(C1 & 0xFC) >> 2];

* PDST = eNBase64TAB [((C1 & 0x03) << 4) | ((C2 & 0xF0) >> 4)];

* PDST = Enbase64TAB [((C2 & 0x0f) << 2)];

* PDST = '=';

NDSTLEN = 4;

}

// Output add a end value

* PDST = '/ 0';

Return ndstlen;

}

In the Base64 decoding method, the simplest is also a checklist: the value of 64 printable characters as an index, the value obtained by the table (range is 0-63) is connected in turn, the patchwork is output, and the decoding is obtained. result.

Const char debase64tab [] =

{

0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0,

0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0,

62, // ' '

0, 0, 0,

63, // '/'

52, 53, 54, 55, 56, 57, 58, 59, 60, 61, // '0' - '9'

0, 0, 0, 0, 0, 0, 0,

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, // 'A' - 'Z'

0, 0, 0, 0, 0, 0,

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, // 'A' - 'Z'

}

INT decodebase64 (const char * psrc, unsigned char * pdst, int nsrclen)

{

INT ndstlen; // output character count

INT nvalue; // Decoding used long intensity

INT I;

i = 0;

Ndstlen = 0;

// Take 4 characters, decode to a long integer, then shift to get 3 bytes

While (i

{

IF (* psrc! = '/ r' && * psrc! = '/ n')

{

NValue = debase64tab [* psrc ] << 18;

NValue = debase64tab [* psrc ] << 12;

* PDST = (nvalue & 0x00ff0000) >> 16; NDSTLEN ;

IF (* psrc! = '=')

{

NValue = debase64tab [* psrc ] << 6;

* PDST = (nvalue & 0x0000FF00) >> 8;

NDSTLEN ;

IF (* psrc! = '=')

{

NValue = debase64tab [* psrc ];

* PDST = nvalue & 0x000000FF;

NDSTLEN ;

}

}

i = 4;

}

ELSE // Enter back, skip

{

PSRC ;

i ;

}

}

// Output add a end value

* PDST = '/ 0';

Return ndstlen;

}

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