base64码简介
Base64是网络上最常见的用于传输8Bit字节代码的编码方式之一，大家可以查看RFC2045～RFC2049，上面有MIME的详细规范。Base64编码可用于在HTTP环境下传递较长的标识信息。例如，在Java Persistence系统Hibernate中，就采用了Base64来将一个较长的唯一标识符（一般为128-bit的UUID）编码为一个字符串，用作HTTP表单和HTTP GET URL中的参数。在其他应用程序中，也常常需要把二进制数据编码为适合放在URL（包括隐藏表单域）中的形式。此时，采用Base64编码具有不可读性，即所编码的数据不会被人用肉眼所直接看到。
0. 源数据都是8位位宽的数据；
1. 相当于分组码，将源数据分为3个一组，每一组共24bits，采用每6位对应一个编码码字，那么3*8bits = 4*6its, 将3个数据映射成4个数据，由于编码的码字都是6位长度，换位10进制就是0-63，总共有64中可能性，这也是base64名字的来历；
2. 6bits对应10进制数对应的码字如最后的表；
C代码编码
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#include <stdio.h>
#include <string.h>
// 全局常量定义
const char * base64char = "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+/";
const char padding_char = '=';
/*编码代码
* const unsigned char * sourcedata， 源数组
* char * base64 ，码字保存
*/
int base64_encode(const unsigned char * sourcedata, char * base64)
{
int i=0, j=0;
unsigned char trans_index=0; // 索引是8位，但是高两位都为0
const int datalength = strlen((const char*)sourcedata);
for (; i < datalength; i += 3){
// 每三个一组，进行编码
// 要编码的数字的第一个
trans_index = ((sourcedata[i] >> 2) & 0x3f);
base64[j++] = base64char[(int)trans_index];
// 第二个
trans_index = ((sourcedata[i] << 4) & 0x30);
if (i + 1 < datalength){
trans_index |= ((sourcedata[i + 1] >> 4) & 0x0f);
base64[j++] = base64char[(int)trans_index];
}else{
base64[j++] = base64char[(int)trans_index];
base64[j++] = padding_char;
base64[j++] = padding_char;
break; // 超出总长度，可以直接break
}
// 第三个
trans_index = ((sourcedata[i + 1] << 2) & 0x3c);
if (i + 2 < datalength){ // 有的话需要编码2个
trans_index |= ((sourcedata[i + 2] >> 6) & 0x03);
base64[j++] = base64char[(int)trans_index];
trans_index = sourcedata[i + 2] & 0x3f;
base64[j++] = base64char[(int)trans_index];
}
else{
base64[j++] = base64char[(int)trans_index];
base64[j++] = padding_char;
break;
}
}
base64[j] = '\0';
return 0;
}
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解码
包括两个函数：
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/** 在字符串中查询特定字符位置索引
* const char *str ，字符串
* char c，要查找的字符
*/
inline int num_strchr(const char *str, char c) //
{
const char *pindex = strchr(str, c);
if (NULL == pindex){
return -1;
}
return pindex - str;
}
/* 解码
* const char * base64 码字
* unsigned char * dedata， 解码恢复的数据
*/
int base64_decode(const char * base64, unsigned char * dedata)
{
int i = 0, j=0;
int trans[4] = {0,0,0,0};
for (;base64[i]!='\0';i+=4){
// 每四个一组，译码成三个字符
trans[0] = num_strchr(base64char, base64[i]);
trans[1] = num_strchr(base64char, base64[i+1]);
// 1/3
dedata[j++] = ((trans[0] << 2) & 0xfc) | ((trans[1]>>4) & 0x03);
if (base64[i+2] == '='){
continue;
}
else{
trans[2] = num_strchr(base64char, base64[i + 2]);
}
// 2/3
dedata[j++] = ((trans[1] << 4) & 0xf0) | ((trans[2] >> 2) & 0x0f);
if (base64[i + 3] == '='){
continue;
}
else{
trans[3] = num_strchr(base64char, base64[i + 3]);
}
// 3/3
dedata[j++] = ((trans[2] << 6) & 0xc0) | (trans[3] & 0x3f);
}
dedata[j] = '\0';
return 0;
}
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下面是其他网友的补充可以参考一下
核心代码
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#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <ctype.h>
static const char b64_table[65] = "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+/";
static const char reverse_table[128] =
{
64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64,
64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64,
64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 62, 64, 64, 64, 63,
52, 53, 54, 55, 56, 57, 58, 59, 60, 61, 64, 64, 64, 64, 64, 64,
64, 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, 64, 64, 64, 64, 64,
64, 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, 64, 64, 64, 64, 64
};
unsigned char *base64_encode(unsigned char *bindata,size_t inlen,unsigned char **out,size_t *outlen)
{
size_t _outlen = *outlen;
unsigned char *_out = NULL;
size_t out_pos = 0;
if(NULL == *out)
{
_outlen = (inlen / 3 + (inlen%3 != 0)) * 4 + 1;
_out = malloc(_outlen);
}
else
{
_outlen = *outlen;
_out = *out;
}
memset(_out,'=',_outlen);
_out[_outlen-1] = 0;
unsigned int bits_collected = 0;
unsigned int accumulator = 0;
for(int i = 0; i < inlen; i++)
{
accumulator = (accumulator << 8) | (bindata[i] & 0xffu);
bits_collected += 8;
while (bits_collected >= 6)
{
bits_collected -= 6;
_out[out_pos++] = b64_table[(accumulator >> bits_collected) & 0x3fu];
}
}
if(bits_collected >= 6)
{
if(NULL == *out)
{
free(_out);
}
return NULL;
}
if (bits_collected > 0)
{
// Any trailing bits that are missing.
accumulator <<= 6 - bits_collected;
_out[out_pos++] = b64_table[accumulator & 0x3fu];
}
*outlen = _outlen;
*out = _out;
return _out;
}
unsigned char *base64_decode(unsigned char *bindata,size_t inlen,unsigned char **out,size_t *outlen)
{
size_t _outlen = *outlen;
unsigned char *_out = NULL;
int bits_collected = 0;
unsigned int accumulator = 0;
size_t out_pos = 0;
if(NULL == *out)
{
_outlen = inlen;
_out = malloc(_outlen);
}
else
{
_outlen = *outlen;
_out = *out;
}
int c = 0;
for(int i = 0; i < inlen; i++)
{
c = bindata[i];
if (isspace(c) || c == '=')
{
// Skip whitespace and padding. Be liberal in what you accept.
continue;
}
if ((c > 127) || (c < 0) || (reverse_table[c] > 63))
{
return NULL;
}
accumulator = (accumulator << 6) | reverse_table[c];
bits_collected += 6;
if (bits_collected >= 8)
{
bits_collected -= 8;
_out[out_pos++] = (char)((accumulator >> bits_collected) & 0xffu);
}
}
*outlen = _outlen;
*out = _out;
return _out;
}
int main(int argc,char *argv[])
{
unsigned char *str = argv[1];
unsigned char *out = 0;
size_t len = 0;
printf("%s\n",base64_encode(str,strlen(str),&out,&len));
unsigned char *_out = 0;
size_t _len = 0;
printf("%s\n",base64_decode(out,strlen(out),&_out,&_len));
return 0;
}
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这篇C语言base64编解码的文章就介绍到这了，希望大家以后多多支持服务器之家。