This tutorial will guide you on how to hash a string by using OpenSSL’s MD5 hash function. This tutorial will create two C++ example files which will compile and run in Ubuntu environment.

  1. Make sure that we have both OpenSSL & libssl-dev installed in the system.
    ~$ dpkg --get-selections | grep openssl
    openssl     install
    ~$ dpkg --get-selections | grep libssl
    libssl-dev    install
    libssl-doc    install
    libssl1.0.0   install
  2. Install the modules in case there are not available in your system:
    ~$ sudo apt-get install openssl
    ~$ sudo apt-get install libssl-dev
  3. Here are the openssl md5 sample source code.

    Example #1: md5_sample1.cpp 
    #include <stdio.h>
#include <string.h>
#include <openssl/md5.h>

int main()
{
    unsigned char digest[MD5_DIGEST_LENGTH];
    char string[] = "happy";
    
    MD5((unsigned char*)&string, strlen(string), (unsigned char*)&digest);    

    char mdString[33];

    for(int i = 0; i < 16; i++)
         sprintf(&mdString[i*2], "%02x", (unsigned int)digest[i]);

    printf("md5 digest: %s\n", mdString);

    return 0;
}

    Example #2: md5_sample2.cpp 
    #include <stdio.h>
#include <string.h>
#include <openssl/md5.h>

int main() {
    unsigned char digest[16];
    const char* string = "happy";

    printf("string length: %d\n", strlen(string)); 

    MD5_CTX ctx;
    MD5_Init(&ctx);
    MD5_Update(&ctx, string, strlen(string));
    MD5_Final(digest, &ctx);

    char mdString[33];
    for (int i = 0; i < 16; i++)
        sprintf(&mdString[i*2], "%02x", (unsigned int)digest[i]);

    printf("md5 digest: %s\n", mdString);

    return 0;
}
  4. Let’s try to compile both sample cpp files and you should observe the following output screenshot.
    ~$ gcc md5_sample1.cpp -o sample1 -lcrypto
    ~$ ./sample1
    md5 digest: 5eb53bbbe01eeed093cb22bb8f5acdc3
    ~$ gcc md5_sample2.cpp -o sample2 -lcrypto
    ~$ ./sample2
    md5 digest: 5eb53bbbe01eeed093cb22bb8f5acdc3
    Note: -lcrypto will include the crypto library from openssl