HMAC: Hash-based Message Authentication Code，即基於Hash的訊息鑑別碼 

在各大開放平臺大行其道的網際網路開發潮流中，呼叫各平臺的API介面過程中，無一例外都會用到計算簽名值(sig值)。而在各種計算簽名的方法中，經常被採用的就是HMAC-SHA1，現對HMAC-SHA1做一個簡單的介紹：

HMAC，雜湊訊息鑑別碼，基於金鑰的Hash演算法認證協議。

實現原理為：

利用已經公開的Hash函式和私有的金鑰，來生成固定長度的訊息鑑別碼；

SHA1、MD5等Hash演算法是比較常用的不可逆Hash簽名計算方法；

BASE64，將任意序列的8位元組字元轉換為人眼無法直接識別的符號編碼的一種方法；

下面提供兩個原始碼示例:

例子一:

//gcc -g hmac_sha1_demo3.c -o hmac_sha1_demo3 -lcrypto -std=c99

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

int main()
{
    // The key to hash
    char key[] = "012345678";

    // The data that we're going to hash using HMAC
    char data[] = "hello world";

    unsigned char digest[EVP_MAX_MD_SIZE] = {'\0'};
    unsigned int digest_len = 0;

    // Using sha1 hash engine here.
    // You may use other hash engines. e.g EVP_md5(), EVP_sha224, EVP_sha512, etc
    HMAC(EVP_sha1(), key, strlen(key), (unsigned char*)data, strlen(data), digest, &digest_len);
    printf("%s, len %u\n", digest, digest_len);

    // Be careful of the length of string with the choosen hash engine. SHA1 produces a 20-byte hash value which rendered as 40 characters.
    // Change the length accordingly with your choosen hash engine
    char mdString[41] = {'\0'};
    for(int i = 0; i < 20; i++)
         sprintf(&mdString[i*2], "%02x", (unsigned int)digest[i]);

    printf("HMAC digest: %s\n", mdString);

    return 0;
}
 

//gcc -g hmac_sha1_demo2.c -o hmac_sha1_demo2 -lcrypto --std=c99

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

int main() {
    // The secret key for hashing
    const char key[] = "012345678";

    // The data that we're going to hash
    char data[] = "hello world";

    // Be careful of the length of string with the choosen hash engine. SHA1 needed 20 characters.
    // Change the length accordingly with your choosen hash engine.
    unsigned char* result;
    unsigned int len = 20;

    result = (unsigned char*)malloc(sizeof(char) * len);

    HMAC_CTX ctx;
    HMAC_CTX_init(&ctx);

    // Using sha1 hash engine here.
    // You may use other hash engines. e.g EVP_md5(), EVP_sha224, EVP_sha512, etc
    HMAC_Init_ex(&ctx, key, strlen(key), EVP_sha1(), NULL);
    HMAC_Update(&ctx, (unsigned char*)&data, strlen(data));
    HMAC_Final(&ctx, result, &len);
    HMAC_CTX_cleanup(&ctx);

    printf("HMAC digest: ");

    for (int i = 0; i != len; i++)
        printf("%02x", (unsigned int)result[i]);

    printf("\n");

    free(result);

    return 0;
}