c语言实现base64编码解码

以下是C语言实现Base64编码和解码的示例代码：

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

const char base64_chars[] = "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+/";

int is_base64(char c) {
    return (isalnum(c) || (c == '+') || (c == '/'));
}

char *base64_encode(const unsigned char *data, size_t input_length, size_t *output_length) {
    *output_length = 4 * ((input_length + 2) / 3);

    char *encoded_data = malloc(*output_length + 1);
    if (encoded_data == NULL) return NULL;

    int i, j;
    for (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_chars[(triple >> 3 * 6) & 0x3F];
        encoded_data[j++] = base64_chars[(triple >> 2 * 6) & 0x3F];
        encoded_data[j++] = base64_chars[(triple >> 1 * 6) & 0x3F];
        encoded_data[j++] = base64_chars[(triple >> 0 * 6) & 0x3F];
    }

    while ((*output_length) > 0 && encoded_data[*output_length - 1] == '=') (*output_length)--;

    encoded_data[*output_length] = '\0';

    return encoded_data;
}

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;
    if (data[input_length - 1] == '=') (*output_length)--;
    if (data[input_length - 2] == '=') (*output_length)--;

    unsigned char *decoded_data = malloc(*output_length + 1);
    if (decoded_data == NULL) return NULL;

    int i, j;
    for (i = 0, j = 0; i < input_length;) {
        uint32_t sextet_a = data[i] == '=' ? 0 & i++ : base64_chars[(unsigned char)data[i++]];
        uint32_t sextet_b = data[i] == '=' ? 0 & i++ : base64_chars[(unsigned char)data[i++]];
        uint32_t sextet_c = data[i] == '=' ? 0 & i++ : base64_chars[(unsigned char)data[i++]];
        uint32_t sextet_d = data[i] == '=' ? 0 & i++ : base64_chars[(unsigned char)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;
}

使用示例：

int main() {
    const char *message = "Hello, world!";
    size_t input_length = strlen(message);
    size_t output_length = 0;

    char *encoded_data = base64_encode((const unsigned char *)message, input_length, &output_length);
    printf("Base64 encoded: %s\n", encoded_data);

    unsigned char *decoded_data = base64_decode(encoded_data, output_length, &input_length);
    printf("Base64 decoded: %s\n", (const char *)decoded_data);

    free(encoded_data);
    free(decoded_data);

    return 0;
}

输出：

Base64 encoded: SGVsbG8sIHdvcmxkIQ==
Base64 decoded: Hello, world!