BASE64 encoding and decoding

Base64 This is a binary encoding method that is converted to printable characters. Mainly used for mail transmission.

Base64 will be 64 characters (a-z,a-z,0-9,+,/) due to the basic character set. Converts all symbols into a character set.

Coding:
The encoding is converted to 4 bytes per 3 knots. If the input byte number is not a multiple of 3. The 0-byte padding at the end makes it a multiple of 3.

for 3 bytes. Remove 6 bits at a time and add 2 bits 0 to the front to make a byte. This byte is the subscript to find the Base64 code table (for example,) to output the corresponding characters.

Convert 3 bytes to 4 bytes (3*8=4*6) at a time until you get the encoding result for the entire input string. Finally, if a 0 byte is added to the input before, the last 1 bytes of the output will be replaced with "=", and if 2 0 bytes are added, the last 2 bytes of the output result will be replaced with "=".
Decoding:
Base64 decoding is the inverse of the encoding process, note that the decoding of each byte of the first 2 bits 0 is artificially populated. The latter 6 bits are the valid bits. Each time a 24-bit significant bit in 4 bytes is extracted to make up 3 bytes.

The core of the encoding and decoding process is bit manipulation.

Base64 Code Table

Code:

#include <stdio.h> #include <stdlib.h> #include <string.h>const char table[] = " abcdefghijklmnopqrstuvwxyzabcdefghijklmnopqrstuvwxyz0123456789+/"; int FindIndex (char c) {for (int i=0;i<64;i++) {if (C==table[i]) return i;} return-1;} void Base64enc (const CHAR*SRC,CHAR*&AMP;DST) {int Slen=strlen (SRC); int len=slen;if (len%3!=0) len= (len/3+1) *3;// The length is multiplied by 3 unsigned char*s= (unsigned char*) malloc (len*sizeof (unsigned char));//unsigned char is used here, otherwise the Chinese input memset cannot be processed (s,0 , Len); memcpy (s,src,slen); int dlen= (LEN/3) *4+1;dst= (char*) malloc (dlen*sizeof (char)); for (int i=0,j=0;i<len;i+=3 ) {//Core step//**************************************dst[j]=table[s[i]>>2];d st[j+1]=table[((s[i]&0X03) &LT;&LT;4) | (s[i+1]>>4)]; dst[j+2]=table[((s[i+1]&0x0f) <<2) | ( S[I+2]&GT;&GT;6)];d st[j+3]=table[s[i+2]&0x3f];//**************************************j+=4;} Free (s); if (slen%3==1) dst[dlen-2]=dst[dlen-3]= ' = '; if (slen%3==2) dst[dlen-2]= ' = ';d st[dlen-1]=0;} void Base64dec (const CHAR*SRC,CHAR*&AMP;DST) {int SLEn=strlen (SRC); if (slen%4!=0) return; int dlen= (SLEN/4) *3+1;char*d= (char*) malloc (dlen*sizeof (char)); int index[4]; for (int i=0,j=0;i<slen;i+=4) {for (int k=0;k<4;k++) {index[k]=findindex (src[i+k]);} Core Step//**************************************d[j]=index[0]<<2|index[1]>>4;d[j+1]=index[1]<< 4|index[2]>>2;d[j+2]=index[2]<<6|index[3];//**************************************j+=3;} Remove the effect of the src tail ' = ' if (src[slen-1]== ' = ' &&src[slen]!= ' = ') dlen-=1;if (src[slen-1]== ' = ' &&src[slen]== ' = ') ) dlen-=2;dst= (char*) malloc (dlen*sizeof (char)); memcpy (Dst,d,dlen); free (d);d st[dlen-1]=0;} int main () {char src[]= "Hello, Little Luo"; Char*enc,*dec;base64enc (Src,enc); Base64dec (Enc,dec);p rintf ("%s\n", SRC);p rintf (" %s\n ", enc);p rintf ("%s\n ", DEC); free (enc); free (dec); return 0;}

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BASE64 encoding and decoding