Base64 转换工具

核心代码：

void EncodeBase64(unsigned char *ce, unsigned char *co)
{
    unsigned char *p;
    unsigned int ocl; // 原串长度
    unsigned int ecl; // 加密后长度
    unsigned int t;   // ocl 除以 3 后的余数
    unsigned char *oend;
    unsigned char *eend;
    unsigned char *p1, *p2, *p3;
    unsigned char *pe;
    unsigned char Base64Table[68] = "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+/=";

    for (p = co, ocl = 0; *p; ++p)
    {
        ++ocl;
    }

    ecl = ocl / 3 * 4; // 不用补0的部分
    t = ocl % 3;
    if (t)
        ecl += 4;      // 末尾4字节

    oend = co + ocl;
    eend = ce + ecl;
    *eend = '\0';

    p1 = co;
    p2 = co + 1;
    p3 = co + 2;
    pe = ce;
    while (p3 < oend) // 不用补0的部分
    {
        *pe++ = *p1 >> 2;
        *pe++ = (*p1 & 0x03) << 4 | *p2 >> 4;
        *pe++ = (*p2 & 0x0F) << 2 | *p3 >> 6;
        *pe++ = *p3 & 0x3F;

        p1 += 3;
        p2 += 3;
        p3 += 3;
    }


    /*
    末尾4字节
    t = 1:
    [1101 0101]
    00[110101] 00[010000] 40H pad    40H pad
    35H        10H        40H pad    40H pad
    31H '1'    51H 'Q'    3DH '='    3DH '='

    t = 2:
    [1101 0101] [1100 0101]
    00[110101] 00[011100] 00[010100] 40H pad
    35H        1CH        14H        40H pad
    31H '1'    62H 'c'    55H 'U'    3DH '='
    */
    switch (t)
    {
    case 1:
        *pe++ = *p1 >> 2;
        *pe++ = (*p1 & 0x03) << 4;
        *pe++ = 0x40;
        *pe = 0x40;
        break;

    case 2:
        *pe++ = *p1 >> 2;
        *pe++ = (*p1 & 0x03) << 4 | *p2 >> 4;
        *pe++ = (*p2 & 0x0F) << 2;
        *pe = 0x40;
        break;

    default:
        break;
    }

    // 已创建好索引值，开始转换

    /*
    Base 64 加密索引表
    一一对应关系，解密就是反过来

    000 ~ 025   026 ~ 051   052 ~ 061   062   063   064
    00H ~ 19H   1AH ~ 33H   34H ~ 3DH   3EH   3FH   40H
        ↓           ↓           ↓        ↓     ↓     ↓
    41H ~ 5AH   61H ~ 7AH   30H ~ 39H   2BH   2FH   3DH
    'A' ~ 'Z'   'a' ~ 'z'   '0' ~ '9'   '+'   '/'   '='

    for (p = ce; p < eend; ++p)
    {
        if (*p <= 0x19)
            *p += 0x41;
        else if (*p >= 0x1A && *p <= 0x33)
            *p += 0x61 - 0x1A;
        else if (*p >= 0x34 && *p <= 0x3D)
            *p -= 0x04;
        else if (*p == 0x3E)
            *p = 0x2B; // '+'
        else if (*p == 0x3F)
            *p = 0x2F; // '/'
        else if (*p == 0x40)
            *p = 0x3D; // '='
    }
    */
    /*
    查表法使用额外空间，减少代码长度，不再需要if判断分支
    */
    for (p = ce; p < eend; ++p)
    {
        *p = Base64Table[*p];
    }
}
void DecodeBase64(unsigned char *co, unsigned char *ce)
{
    unsigned char *p;
    unsigned int ocl; // 原串长度
    unsigned int ecl; // 加密串长度
    unsigned int t;   // '='个数

    unsigned char *oend;
    unsigned char *eend;
    unsigned char *eendf; // 加密串中每4个字节计算，不含'='的部分的末尾
    unsigned char *p1, *p2, *p3, *p4;
    unsigned char *po;

    for (p = ce, ecl = 0; *p; ++p)
    {
        ++ecl;
    }

    eendf = eend = ce + ecl;
    for (t = 0, p = eend - 1; *p == '='; --p)
    {
        ++t;
    }

    if (t)
        eendf -= 4;

    ocl = ecl / 4 * 3 - t; // 合法加密串长度总是4的倍数，函数不做合法性检查
    oend = co + ocl;
    *oend = '\0';

    /*
    还原为索引

    41H ~ 5AH   61H ~ 7AH   30H ~ 39H   2BH   2FH   3DH
    'A' ~ 'Z'   'a' ~ 'z'   '0' ~ '9'   '+'   '/'   '='
        ↓           ↓           ↓        ↓     ↓     ↓
    00H ~ 19H   1AH ~ 33H   34H ~ 3DH   3EH   3FH   40H
    */
    for (p = ce; p < eend; ++p)
    {
        if (*p >= 'A' && *p <= 'Z')
            *p -= 0x41;
        else if (*p >= 'a' && *p <= 'z')
            *p -= 0x61 - 0x1A;
        else if (*p >= '0' && *p <= '9')
            *p += 0x04;
        else if (*p == '+')
            *p = 0x3E;
        else if (*p == '/')
            *p = 0x3F;
        else if (*p == '=')
            *p = 0x40;
    }

    // 合并字节
    p1 = ce;
    p2 = ce + 1;
    p3 = ce + 2;
    p4 = ce + 3;
    po = co;
    while (p1 < eendf)
    {
        *po++ = *p1 << 2 | *p2 >> 4;
        *po++ = *p2 << 4 | *p3 >> 2;
        *po++ = *p3 << 6 | *p4;

        p1 += 4;
        p2 += 4;
        p3 += 4;
        p4 += 4;
    }
    /*
    while (p1 < eendf)
    {
        *po++ = *p1 << 2 | (*p2 & 0x30) >> 4;
        *po++ = *p2 << 4 | (*p3 & 0x3C) >> 2;
        *po++ = *p3 << 6 | *p4 & 0x3F;

        p1 += 4;
        p2 += 4;
        p3 += 4;
        p4 += 4;
    }
    */
    /*
    末尾4字节

    t = 1:
    31H '1'    62H 'c'    55H 'U'    3DH '='
    35H        1CH        14H        40H pad
    00[110101] 00[011100] 00[010100] 40H pad
    [1101 0101] [1100 0101]

    t = 2:
    31H '1'    51H 'Q'    3DH '='    3DH '='
    35H        10H        40H pad    40H pad
    00[110101] 00[010000] 40H pad    40H pad
    [1101 0101]
    */
    switch (t)
    {
    case 1:
        *po++ = *p1 << 2 | *p2 >> 4;
        *po = *p2 << 4 | *p3 >> 2;
        break;

    case 2:
        *po = *p1 << 2 | *p2 >> 4;
        break;

    default:
        break;
    }
}
int DecodeBase64_safe(unsigned char *co, unsigned char *ce)
{
    if (!IsBase64String(ce))
        return 0;
    else
    {
        DecodeBase64(co, ce);
        return 1;
    }
}

完整源码