base64是一種基於64個可打印字符來表示二進製數據的表示方法。由於26=64，所以每6位為一個單位，對應某個可打印字符。三個字節共24位，對應於4個base64單位，即3個字節需要用4個可打印字符來表示。它常用來作為電子郵件的傳輸編碼。在base64中的可打印字符包括大寫英文字母A-Z，小寫英文字母a-z、阿拉伯數字0-9，這樣共有62個字符，此外兩個可打印符號在不同的係統中而不同，通常用加號（+）和正斜杠（/）。外加“補全符號”，通常用等號（=）。
完整的base64定義可見RFC 1421和RFC 2045。編碼後的數據比原始數據略長，為原來的4/3。在電子郵件中，根據RFC 822的規定，每76個字符，還需要加上回車符和換行符。可以估算編碼後數據長度大約為原長的135.1%。
base64編碼的時候，將三個自己的數據，先後放入一個24位的緩衝區中，先來的自己占高位。數據不足3個字節的話，在緩衝區中剩下的位用0補足。然後，每次取出6（因為26=64）位，按照其值選擇 ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+/ 中的字符作為編碼後的輸出。不斷進行，直到全部輸入數據轉換完成。如果最後剩下兩個輸入數據，在編碼結果後加1個“=”；如果最後剩下一個輸入數據，編碼結果後加2個“=”；如果沒有剩下任何數據，就什麼都不要加。這樣才可以保證資料還原的正確性。
C語言源代碼如下：
/**
* base64編碼、解碼實現
* C語言源代碼
*
* 注意：請使用gcc編譯
*
* 葉劍飛
*
*
*
* 使用說明：
* 命令行參數說明：若有“-d”參數，則為base64解碼，否則為base64編碼。
* 若有“-o”參數，後接文件名，則輸出到標準輸出文件。
* 輸入來自標準輸入stdin，輸出為標準輸出stdout。可重定向輸入輸出流。
*
* base64編碼：輸入任意二進製流，讀取到文件讀完了為止（鍵盤輸入則遇到文件結尾符為止）。
* 輸出純文本的base64編碼。
*
* base64解碼：輸入純文本的base64編碼，讀取到文件讀完了為止（鍵盤輸入則遇到文件結尾符為止）。
* 輸出原來的二進製流。
* */#include <stdio.h>#include <stdlib.h>#include <string.h>#include <unistd.h>#include <io.h>#include <fcntl.h>#include <stdbool.h>#ifndef MAX_PATH#define MAX_PATH 256#endifconst char * base64char = "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+/";char * base64_encode( const unsigned char * bindata, char * base64, int binlength )
{ int i, j;
unsigned char current; for ( i = 0, j = 0 ; i < binlength ; i += 3 )
{
current = (bindata[i] >> 2) ;
current &= (unsigned char)0x3F;
base64[j++] = base64char[(int)current];
current = ( (unsigned char)(bindata[i] << 4 ) ) & ( (unsigned char)0x30 ) ; if ( i + 1 >= binlength )
{
base64[j++] = base64char[(int)current];
base64[j++] = '=';
base64[j++] = '='; break;
}
current |= ( (unsigned char)(bindata[i+1] >> 4) ) & ( (unsigned char) 0x0F );
base64[j++] = base64char[(int)current];
current = ( (unsigned char)(bindata[i+1] << 2) ) & ( (unsigned char)0x3C ) ; if ( i + 2 >= binlength )
{
base64[j++] = base64char[(int)current];
base64[j++] = '='; break;
}
current |= ( (unsigned char)(bindata[i+2] >> 6) ) & ( (unsigned char) 0x03 );
base64[j++] = base64char[(int)current];
current = ( (unsigned char)bindata[i+2] ) & ( (unsigned char)0x3F ) ;
base64[j++] = base64char[(int)current];
}
base64[j] = '\0'; return base64;
}int base64_decode( const char * base64, unsigned char * bindata )
{ int i, j;
unsigned char k;
unsigned char temp[4]; for ( i = 0, j = 0; base64[i] != '\0' ; i += 4 )
{
memset( temp, 0xFF, sizeof(temp) ); for ( k = 0 ; k < 64 ; k ++ )
{ if ( base64char[k] == base64[i] )
temp[0]= k;
} for ( k = 0 ; k < 64 ; k ++ )
{ if ( base64char[k] == base64[i+1] )
temp[1]= k;
} for ( k = 0 ; k < 64 ; k ++ )
{ if ( base64char[k] == base64[i+2] )
temp[2]= k;
} for ( k = 0 ; k < 64 ; k ++ )
{ if ( base64char[k] == base64[i+3] )
temp[3]= k;
}
bindata[j++] = ((unsigned char)(((unsigned char)(temp[0] << 2))&0xFC)) |
((unsigned char)((unsigned char)(temp[1]>>4)&0x03)); if ( base64[i+2] == '=' ) break;
bindata[j++] = ((unsigned char)(((unsigned char)(temp[1] << 4))&0xF0)) |
((unsigned char)((unsigned char)(temp[2]>>2)&0x0F)); if ( base64[i+3] == '=' ) break;
bindata[j++] = ((unsigned char)(((unsigned char)(temp[2] << 6))&0xF0)) |
((unsigned char)(temp[3]&0x3F));
} return j;
}void encode(FILE * fp_in, FILE * fp_out)
{
unsigned char bindata[2050]; char base64[4096];
size_t bytes; while ( !feof( fp_in ) )
{
bytes = fread( bindata, 1, 2049, fp_in );
base64_encode( bindata, base64, bytes );
fprintf( fp_out, "%s", base64 );
}
}void decode(FILE * fp_in, FILE * fp_out)
{ int i;
unsigned char bindata[2050]; char base64[4096];
size_t bytes; while ( !feof( fp_in ) )
{ for ( i = 0 ; i < 2048 ; i ++ )
{
base64[i] = fgetc(fp_in); if ( base64[i] == EOF ) break; else if ( base64[i] == '\n' || base64[i] == '\r' )
i --;
}
bytes = base64_decode( base64, bindata );
fwrite( bindata, bytes, 1, fp_out );
}
}void help(const char * filepath)
{
fprintf( stderr, "Usage: %s [-d] [input_filename] [-o output_filepath]\n", filepath );
fprintf( stderr, "\t-d\tdecode data\n" );
fprintf( stderr, "\t-o\toutput filepath\n\n" );
}int main(int argc, char * argv[])
{
FILE * fp_input = NULL;
FILE * fp_output = NULL; bool isencode = true; bool needHelp = false; int opt = 0; char input_filename[MAX_PATH] = ""; char output_filename[MAX_PATH] = "";
opterr = 0; while ( (opt = getopt(argc, argv, "hdo:")) != -1 )
{ switch(opt)
{ case 'd':
isencode = false; break; case 'o':
strncpy(output_filename, optarg, sizeof(output_filename));
output_filename[sizeof(output_filename)-1] = '\0'; break; case 'h':
needHelp = true; break; default:
fprintf(stderr, "%s: invalid option -- %c\n", argv[0], optopt);
needHelp = true; break;
}
} if ( optind < argc )
{
strncpy(input_filename, argv[optind], sizeof(input_filename));
input_filename[sizeof(input_filename)-1] = '\0';
} if (needHelp)
{
help(argv[0]); return EXIT_FAILURE;
} if ( !strcmp(input_filename, "") )
{
fp_input = stdin; if (isencode)
_setmode( _fileno(stdin), _O_BINARY );
} else
{ if (isencode)
fp_input = fopen(input_filename, "rb"); else
fp_input = fopen(input_filename, "r");
} if ( fp_input == NULL )
{
fprintf(stderr, "Input file open error\n"); return EXIT_FAILURE;
} if ( !strcmp(output_filename, "") )
{
fp_output = stdout; if (!isencode)
_setmode( _fileno(stdout), _O_BINARY );
} else
{ if (isencode)
fp_output = fopen(output_filename, "w"); else
fp_output = fopen(output_filename, "wb");
} if ( fp_output == NULL )
{
fclose(fp_input);
fp_input = NULL;
fprintf(stderr, "Output file open error\n"); return EXIT_FAILURE;
} if (isencode)
encode(fp_input, fp_output); else
decode(fp_input, fp_output);
fclose(fp_input);
fclose(fp_output);
fp_input = fp_output = NULL; return EXIT_SUCCESS;
}
