C语言实现base64编码解码

以下是C语言实现base64编码解码的示例代码：

#include <stdio.h>
#include <string.h>
#include <stdlib.h>

// Base64编码表
static const char base64_table[] = "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+/";

// Base64解码表
static const char base64_decode_table[] = {
    62, -1, -1, -1, 63, // '+', '/', and 0x3D are special
    52, 53, 54, 55, 56, 57, 58, 59, 60, 61, // '0'-'9', '=', and 'A'-'Z'
    -1, -1, -1, -1, -1, -1, -1,
    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, -1, -1, -1, -1,
    -1, -1, 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
};

// Base64编码函数
char* base64_encode(const unsigned char *data, size_t input_length, size_t *output_length) {
    *output_length = 4 * ((input_length + 2) / 3); // 计算编码后字符串长度
    char *encoded_data = malloc(*output_length + 1); // 分配内存
    if (encoded_data == NULL) {
        return NULL;
    }

    // 编码
    for (size_t i = 0, j = 0; i < input_length;) {
        uint32_t octet_a = i < input_length ? (unsigned char)data[i++] : 0;
        uint32_t octet_b = i < input_length ? (unsigned char)data[i++] : 0;
        uint32_t octet_c = i < input_length ? (unsigned char)data[i++] : 0;

        uint32_t triple = (octet_a << 0x10) + (octet_b << 0x08) + octet_c;

        encoded_data[j++] = base64_table[(triple >> 3 * 6) & 0x3F];
        encoded_data[j++] = base64_table[(triple >> 2 * 6) & 0x3F];
        encoded_data[j++] = base64_table[(triple >> 1 * 6) & 0x3F];
        encoded_data[j++] = base64_table[(triple >> 0 * 6) & 0x3F];
    }

    // 添加填充字符
    for (size_t i = 0; i < (*output_length - input_length % 3) % 3; i++) {
        encoded_data[*output_length - 1 - i] = '=';
    }
    encoded_data[*output_length] = '\0';

    return encoded_data;
}

// Base64解码函数
unsigned char* base64_decode(const char *data, size_t input_length, size_t *output_length) {
    if (input_length % 4 != 0) {
        return NULL;
    }

    *output_length = input_length / 4 * 3; // 计算解码后字节数
    if (data[input_length - 1] == '=') {
        (*output_length)--;
    }
    if (data[input_length - 2] == '=') {
        (*output_length)--;
    }

    unsigned char *decoded_data = malloc(*output_length + 1); // 分配内存
    if (decoded_data == NULL) {
        return NULL;
    }

    // 解码
    for (size_t i = 0, j = 0; i < input_length;) {
        uint32_t sextet_a = data[i] == '=' ? 0 & i++ : base64_decode_table[(int)data[i++]];
        uint32_t sextet_b = data[i] == '=' ? 0 & i++ : base64_decode_table[(int)data[i++]];
        uint32_t sextet_c = data[i] == '=' ? 0 & i++ : base64_decode_table[(int)data[i++]];
        uint32_t sextet_d = data[i] == '=' ? 0 & i++ : base64_decode_table[(int)data[i++]];

        uint32_t triple = (sextet_a << 3 * 6)
                        + (sextet_b << 2 * 6)
                        + (sextet_c << 1 * 6)
                        + (sextet_d << 0 * 6);

        if (j < *output_length) {
            decoded_data[j++] = (triple >> 2 * 8) & 0xFF;
        }
        if (j < *output_length) {
            decoded_data[j++] = (triple >> 1 * 8) & 0xFF;
        }
        if (j < *output_length) {
            decoded_data[j++] = (triple >> 0 * 8) & 0xFF;
        }
    }

    decoded_data[*output_length] = '\0';

    return decoded_data;
}

int main() {
    char *data = "Hello, world!";
    size_t data_length = strlen(data);

    // 编码
    size_t encoded_length;
    char *encoded_data = base64_encode((const unsigned char*)data, data_length, &encoded_length);
    printf("Base64 encoded data: %s\n", encoded_data);

    // 解码
    size_t decoded_length;
    unsigned char *decoded_data = base64_decode(encoded_data, encoded_length, &decoded_length);
    printf("Base64 decoded data: %s\n", decoded_data);

    free(encoded_data);
    free(decoded_data);

    return 0;
}

上述代码中，`base64_encode`函数用于将二进制数据编码为Base64字符串，`base64_decode`函数用于将Base64字符串解码为原始的二进制数据。在编码时，函数会将每3个字节的二进制数据转换为4个Base64字符，如果遇到不足3个字节的末尾数据，则会用`=`字符进行填充。在解码时，函数会将每4个Base64字符转换为3个字节的二进制数据，如果遇到末尾的`=`字符，则会忽略它们。