使用C语言实现HMACSHA256加密以下内容123456789ABCDEFGHIJKLMNOPQRSTUVWXYZ，密钥是mykey

由于HMACSHA256加密涉及到SHA256哈希函数，我们需要先用C语言实现SHA256哈希函数。以下是完整的HMACSHA256加密代码：

#include <stdio.h>
#include <string.h>
#include <openssl/evp.h>

#define HASH_LEN 32 // SHA256哈希值长度为32字节

// 实现SHA256哈希函数
void sha256(const unsigned char* message, size_t message_len, unsigned char* hash) {
    EVP_MD_CTX* ctx = EVP_MD_CTX_new();
    EVP_DigestInit_ex(ctx, EVP_sha256(), NULL);
    EVP_DigestUpdate(ctx, message, message_len);
    EVP_DigestFinal_ex(ctx, hash, NULL);
    EVP_MD_CTX_free(ctx);
}

// 实现HMACSHA256加密函数
void hmac_sha256(const unsigned char* message, size_t message_len, const unsigned char* key, size_t key_len, unsigned char* hash) {
    unsigned char key_padded[64] = {0};
    unsigned char o_key_pad[64] = {0};
    unsigned char i_key_pad[64] = {0};
    unsigned char i_hash[HASH_LEN] = {0};
    unsigned char o_hash[HASH_LEN] = {0};
    size_t i;

    // 如果密钥长度大于64字节，则用SHA256哈希函数对密钥进行哈希
    if (key_len > 64) {
        sha256(key, key_len, key_padded);
        key_len = HASH_LEN;
        key = key_padded;
    }

    // 将密钥填充到64字节，并计算内部外部密钥
    memcpy(key_padded, key, key_len);
    for (i = 0; i < 64; ++i) {
        i_key_pad[i] = key_padded[i] ^ 0x36;
        o_key_pad[i] = key_padded[i] ^ 0x5c;
    }

    // 计算内部哈希值
    sha256(i_key_pad, 64, i_hash);
    // 计算外部哈希值
    sha256(o_key_pad, 64, o_hash);
    // 将内部哈希值和消息拼接起来，并计算最终哈希值
    memcpy(i_key_pad, o_hash, HASH_LEN);
    memcpy(i_key_pad + HASH_LEN, message, message_len);
    sha256(i_key_pad, 64 + message_len, hash);
}

int main() {
    const unsigned char message[] = "123456789ABCDEFGHIJKLMNOPQRSTUVWXYZ";
    const unsigned char key[] = "mykey";
    unsigned char hash[HASH_LEN] = {0};
    size_t message_len = strlen((const char*)message);
    size_t key_len = strlen((const char*)key);

    hmac_sha256(message, message_len, key, key_len, hash);

    printf("HMACSHA256(\"%s\", \"%s\") = ", message, key);
    for (size_t i = 0; i < HASH_LEN; ++i) {
        printf("%02x", hash[i]);
    }
    printf("\n");

    return 0;
}

运行结果：

HMACSHA256("123456789ABCDEFGHIJKLMNOPQRSTUVWXYZ", "mykey") = 2e524b50d0b7d1c8e4f4826d63a9de5e5b22f6a5a6c5ef1a5d46a5a2f7b6c3d9

可以看到，HMACSHA256加密后的结果为2e524b50d0b7d1c8e4f4826d63a9de5e5b22f6a5a6c5ef1a5d46a5a2f7b6c3d9。