OpenSSL HMAC Hasing Example in C++

This tutorial will guide you on how to hash a string by using OpenSSL’s HMAC hash function. HMAC hashing allow user to hash with a secret key. HMAC can take most of the hash engine in order to hash your data with the secret key. This tutorial will create two C++ example files which will compile and run in Ubuntu environment.

1. Here are the openssl HMAC sample source code.
   
   Example #1: hmac_sample1.cpp
   C++

   #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; // Using sha1 hash engine here. // You may use other hash engines. e.g EVP_md5(), EVP_sha224, EVP_sha512, etc digest = HMAC(EVP_sha1(), key, strlen(key), (unsigned char*)data, strlen(data), NULL, NULL); // 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 char mdString[20]; for(int i = 0; i < 20; i++) sprintf(&mdString[i*2], "%02x", (unsigned int)digest[i]); printf("HMAC digest: %s\n", mdString); return 0; }

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   #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;          // Using sha1 hash engine here.     // You may use other hash engines. e.g EVP_md5(), EVP_sha224, EVP_sha512, etc     digest = HMAC(EVP_sha1(), key, strlen(key), (unsigned char*)data, strlen(data), NULL, NULL);           // 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     char mdString[20];     for(int i = 0; i < 20; i++)          sprintf(&mdString[i*2], "%02x", (unsigned int)digest[i]);       printf("HMAC digest: %s\n", mdString);       return 0; }

   
   Example #2: hmac_sample2.cpp

   C++

   #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; }

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   #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; }

2. Let’s try to compile both sample cpp files and you should observe the following output screenshot.
   ~$ gcc hmac_sample1.cpp -o sample1 -lcrypto
   ~$ ./sample1
   HMAC digest: e19e220122b37b708bfb95aca2577905acabf0c0
   ~$ gcc hmac_sample2.cpp -o sample2 -lcrypto
   ~$ ./sample2
   HMAC digest: e19e220122b37b708bfb95aca2577905acabf0c0
   Note: -lcrypto will include the crypto library from openssl