HMAC: Hash-based Message Authentication Code，即基于Hash的消息鉴别码 

在各大开放平台大行其道的互联网开发潮流中，调用各平台的API接口过程中，无一例外都会用到计算签名值(sig值)。而在各种计算签名的方法中，经常被采用的就是HMAC-SHA1，现对HMAC-SHA1做一个简单的介绍：
HMAC，散列消息鉴别码，基于密钥的Hash算法认证协议。
实现原理为：
利用已经公开的Hash函数和私有的密钥，来生成固定长度的消息鉴别码；
SHA1、MD5等Hash算法是比较常用的不可逆Hash签名计算方法；
BASE64，将任意序列的8字节字符转换为人眼无法直接识别的符号编码的一种方法；
相关依赖库是openssl，安装方法如下：
apt-get install openssl   //Ubuntu 14.04
yum -y install openssl-devel    //centos 
下面提供两个源码示例:
例子一:
//gcc -g hmac_sha1_demo3.c -o hmac_sha1_demo3 -lcrypto -std=c99#include 
#include 
#include 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] = {''};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
", 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] = {''};for(int i = 0; i < 20; i++)sprintf(&mdString[i*2], "%02x", (unsigned int)digest[i]);printf("HMAC digest: %s
", mdString);return 0;
}


例子二:
//gcc -g hmac_sha1_demo2.c -o hmac_sha1_demo2 -lcrypto --std=c99#include 
#include 
#include 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("
");free(result);return 0;
}
运行截图:





参考文献:
[1].http://www.askyb.com/cpp/openssl-hmac-hasing-example-in-cpp/