HMAC: Hash-based Message Authentication Code，即基于Hash的消息鑒別碼 

在各大開放平臺大行其道的互聯網開發潮流中，調用各平臺的API接口過程中，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，經常被采用的就是HMAC-SHA1，現對HMAC-SHA1做一個簡單的介紹：
HMAC，散列消息鑒別碼，基于密鑰的Hash算法認證協議。
實現原理為：
利用已經公開的Hash函數和私有的密鑰，來生成固定長度的消息鑒別碼；
SHA1、MD5等Hash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字節字符轉換為人眼無法直接識別的符號編碼的一種方法；
相關依賴庫是openssl，安裝方法如下：apt-get install openssl ? //Ubuntu 14.04yum -y install openssl-devel ? ?//centos?
下面提供兩個源碼示例:
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 hashchar key[] = "012345678";// The data that we're going to hash using HMACchar 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, etcHMAC(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 enginechar 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;
}

例子二:
//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 hashingconst char key[] = "012345678";// The data that we're going to hashchar 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, etcHMAC_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;
}
運行截圖:


參考文獻:
[1].http://www.askyb.com/cpp/openssl-hmac-hasing-example-in-cpp/? 

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