How can I use openssl/md5 in C++ to hash a string? - c

Question

I need to hash with md5 algorithm a string in my program.
There is the lib openssl but I'm a newbie about it.
How is possible hash a string using that and where I can find a good doc that teach me how to use this lib, also with other function like aes?
I've tried this code:
int main()
{
unsigned char result[MD5_DIGEST_LENGTH];
const unsigned char* str;
str = (unsigned char*)"hello";
unsigned int long_size = 100;
MD5(str,long_size,result);
}
But the compiler told me:
undefined reference to MD5.
Why is there and undefined reference to MD5?

Answer 1

You should take a look at the documentation. An option is to use this function:
#include <openssl/md5.h>
unsigned char *MD5(const unsigned char *d,
unsigned long n,
unsigned char *md);
To which they state:
MD2(), MD4(), and MD5() compute the MD2, MD4, and MD5 message digest of the n bytes at d and place it in md (which must have space for MD2_DIGEST_LENGTH == MD4_DIGEST_LENGTH == MD5_DIGEST_LENGTH == 16 bytes of output). If md is NULL, the digest is placed in a static array.
As for AES, if you also want to use OpenSSL, then take a look at EVP doc and this example of how to use it. Just note that you have to add
#define AES_BLOCK_SIZE 16
In the top of the file for it to work, though.
Btw. I can really recommend the Crypto++ library since it's great and has all kinds of cryptographic primitives; AES, Elliptic Curves, MAC, public-key crypto and so on.

Answer 2

The MD5 function is now deprecated, so if you want to avoid all those nasty warnings in your code...
Here's a simple example of how to use md5 with OpenSSL 3.0 and above with C++:
#include <openssl/evp.h>
#include <cstdio>
using namespace std;
string md5(const string& content)
{
EVP_MD_CTX* context = EVP_MD_CTX_new();
const EVP_MD* md = EVP_md5();
unsigned char md_value[EVP_MAX_MD_SIZE];
unsigned int md_len;
string output;
EVP_DigestInit_ex2(context, md, NULL);
EVP_DigestUpdate(context, content.c_str(), content.length());
EVP_DigestFinal_ex(context, md_value, &md_len);
EVP_MD_CTX_free(context);
output.resize(md_len * 2);
for (unsigned int i = 0 ; i < md_len ; ++i)
std::sprintf(&output[i * 2], "%02x", md_value[i]);
return output;
}
According to: https://www.openssl.org/docs/man3.0/man3/EVP_DigestInit_ex.html