Base64编解码

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#include<stdio.h>
#include<string.h>
/*
此程序只适用于UTF-8
*/
const char base64_map[]="ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+/";
//base64的编码表

/*编码模块的两个函数encrypt64和base64_encrypt*/
void encrypt64(char *en_three_bits,char *en_four_bits)
{   //编码函数
    union{ unsigned char a[3];unsigned int b;} en;
    int i;
    for(i=0;i<3;i++)    //把读取的3个字节赋给联合体en
    en.a[2-i]=en_three_bits[i];
    for(i=0;i<4;i++)
    {
        en_four_bits[3-i]=base64_map[en.a[0]&63];
        en.b=en.b>>6;
    }
    en_four_bits[4]='\0';
}
void base64_encrypt(char *en_source,char *encrypted)
{           //通过此函数来将原字符串进行分割，并进行转换
     int len=strlen(en_source),i;
     char en_four_bits[5];
     for(i=0;i<len-2;i+=3)
     {
            encrypt64(&en_source[i],en_four_bits);
            strcat(encrypted,en_four_bits);
     }
     if(len%3==1)
        {
            encrypt64(&en_source[len-1],en_four_bits);
            en_four_bits[2]='\0';
            strcat(encrypted,en_four_bits);
            strcat(encrypted,"==");
        }
    if(len%3==2)
        {
            encrypt64(&en_source[len-2],en_four_bits);
            en_four_bits[3]='\0';
            strcat(encrypted,en_four_bits);
            strcat(encrypted,"=");
        }
}
/*解码模块的三个函数index64和decrypted64和base64_decrypt*/
unsigned int index64(char singel_char)
{       //求对应字符的下标
    int j;
    for(j=0;j<64;j++)
        if(singel_char==base64_map[j])
            return j;
}

void decrypted64(const char *de_four_bits,char *de_three_bits)
{   //解码主模块
    union {unsigned int b;char a[4];} de;    //和解码一样继续通过共用体来进行特定位的读取
    int i,flag=3;       //通过flag标记来去除后面的若干等号
    if(de_four_bits[2]=='=')
        flag=1;
    else if(de_four_bits[3]=='=')
        flag=2;
    for(de.b=0,i=0;i<4;i++)  //通过四次移位操作把四个字符的下标加起来
        de.b+=index64(de_four_bits[3-i])<<(i*6);
    for(i=0;i<3;i++)
           de_three_bits[i]=de.a[2-i];
    de_three_bits[flag]='\0';
}
void base64_decrypt(const char *de_source,char *decrypted)
{   // 分割待解码的字串
    int i,len=strlen(de_source);
    char de_three_bits[4];
    for(i=0;i<=len-4;i +=4)
     {
         decrypted64(&de_source[i],de_three_bits);
         strcat(decrypted,de_three_bits);
     }
}
void main()
{
     char  b1[20]="",b2[20]="";

     //s1为需要编码的字符串，s2为需要解码的字串
     char s1[]="hello world !",s2[]="aGVsbG8gd29ybGQgIQ==";        
    base64_encrypt(s1,b1);
    base64_decrypt(s2,b2);
    printf("字串 \"%s\" 编码后：\n%s\n",s1,b1);
    printf("字串 \"%s\" 解码后：\n%s\n",s2,b2);
    //printf("%s\n",b2);

}