Base64算法实现

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
 
// base64 转换表, 共64个
static const char base64_alphabet[] = "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+/";
 
// 解码时使用
static const unsigned char base64_suffix_map[256] = {
    255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 253, 255,
    255, 253, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255,
    255, 255, 255, 255, 255, 255, 255, 255, 253, 255, 255, 255,
    255, 255, 255, 255, 255, 255, 255,  62, 255, 255, 255,  63,
    52,  53,  54,  55,  56,  57,  58,  59,  60,  61, 255, 255,
    255, 254, 255, 255, 255,   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, 255, 255, 255, 255, 255,
    255,  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, 255, 255, 255, 255, 255, 255, 255, 255, 255,
    255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255,
    255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255,
    255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255,
    255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255,
    255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255,
    255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255,
    255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255,
    255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255,
    255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255,
    255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255,
    255, 255, 255, 255 };

static char cmove_bits(unsigned char src, unsigned lnum, unsigned rnum) {
    src <<= lnum; // src = src << lnum;
    src >>= rnum; // src = src >> rnum;
    return src;
}

int base64_encode(const char *indata, int inlen, char *outdata, int *outlen) {
 
    int ret = 0; // return value
    if (indata == NULL || inlen == 0) {
        return ret = -1;
    }
    int i;
    int in_len = 0; // 源字符串长度, 如果in_len不是3的倍数, 那么需要补成3的倍数
    int pad_num = 0; // 需要补齐的字符个数, 这样只有2, 1, 0(0的话不需要拼接, )
    if (inlen % 3 != 0) {
        pad_num = 3 - inlen % 3;
    }
    in_len = inlen + pad_num; // 拼接后的长度, 实际编码需要的长度(3的倍数)
 
    int out_len = in_len * 8 / 6; // 编码后的长度
 
    char *p = outdata; // 定义指针指向传出data的首地址
 
    //编码, 长度为调整后的长度, 3字节一组
    for (i = 0; i < in_len; i+=3) {
        int value = *indata >> 2; // 将indata第一个字符向右移动2bit(丢弃2bit)
        char c = base64_alphabet[value]; // 对应base64转换表的字符
        *p = c; // 将对应字符(编码后字符)赋值给outdata第一字节
 
        //处理最后一组(最后3字节)的数据
        if (i == inlen + pad_num - 3 && pad_num != 0) {
            if(pad_num == 1) {
                *(p + 1) = base64_alphabet[(int)(cmove_bits(*indata, 6, 2) + cmove_bits(*(indata + 1), 0, 4))];
                *(p + 2) = base64_alphabet[(int)cmove_bits(*(indata + 1), 4, 2)];
                *(p + 3) = '=';
            } else if (pad_num == 2) { // 编码后的数据要补两个 '='
                *(p + 1) = base64_alphabet[(int)cmove_bits(*indata, 6, 2)];
                *(p + 2) = '=';
                *(p + 3) = '=';
            }
        } else { // 处理正常的3字节的数据
            *(p + 1) = base64_alphabet[cmove_bits(*indata, 6, 2) + cmove_bits(*(indata + 1), 0, 4)];
            *(p + 2) = base64_alphabet[cmove_bits(*(indata + 1), 4, 2) + cmove_bits(*(indata + 2), 0, 6)];
            *(p + 3) = base64_alphabet[*(indata + 2) & 0x3f];
        }
 
        p += 4;
        indata += 3;
    }
 
    if(outlen != NULL) {
        *outlen = out_len;
    }
 
    return ret;
}

int base64_decode(const char *indata, int inlen, char *outdata, int *outlen) {
 
    int ret = 0;
    if (indata == NULL || inlen <= 0 || outdata == NULL || outlen == NULL) {
        return ret = -1;
    }
    if (inlen % 4 != 0) { // 需要解码的数据不是4字节倍数
        return ret = -2;
    }
 
    int t = 0, x = 0, y = 0, i = 0;
    unsigned char c = 0;
    int g = 3;
 
    while (indata[x] != 0) {
        // 需要解码的数据对应的ASCII值对应base64_suffix_map的值
        c = base64_suffix_map[indata[x++]];
        if (c == 255) return -1;// 对应的值不在转码表中
        if (c == 253) continue;// 对应的值是换行或者回车
        if (c == 254) { c = 0; g--; }// 对应的值是'='
        t = (t<<6) | c; // 将其依次放入一个int型中占3字节
        if (++y == 4) {
            outdata[i++] = (unsigned char)((t>>16)&0xff);
            if (g > 1) outdata[i++] = (unsigned char)((t>>8)&0xff);
            if (g > 2) outdata[i++] = (unsigned char)(t&0xff);
            y = t = 0;
        }
    }
    if (outlen != NULL) {
        *outlen = i;
    }
    return ret;
}

int main(){

    char str1[] = "enstr";
    char str2[] = "Y3liZXJwZWFjZXtXZWxjb21lX3RvX25ld19Xb3JsZCF9"; //注意长度要给够
    int len = 0;
    // base64_encode(str1,(int)strlen(str1),str2,&len);
    // printf("%s, len = %d\n", str2, len);
    char str3[64] = {0};
    base64_decode(str2, (int)strlen(str2), str3, &len);
    printf("%s, len = %d\n", str3, len);
    return 0;
}