Unable to get md5 hash

Question

I am unable to get the md5 hash using OpenSSL. I am using the following command for build :

gcc -Wall test_3.c -o test_3 -lcrypto -lssl

but getting the the following link errors :

undefined reference to `MD5Init'
undefined reference to `MD5Update'
undefined reference to `MD5Final'
collect2: ld returned 1 exit status

The program is present below :

#include<stdio.h>
#include<string.h>
#include <openssl/hmac.h>
#include <openssl/md5.h>

int main()
{
  char    digest[17];
  char input[] = "asdfljiahfbqhebfjcnajclgfeliuaef";
  int length = strlen(input);

  MD5_CTX md5;

  MD5Init(&md5);
  MD5Update(&md5,input, length);
  MD5Final(digest,&md5);
  printf("digest  is %s \n",digest);

  return 0;
}

Please let me know if you know the problem.Please help me out

Answer 1

You made some mistakes, I've corrected them. Also I've added hexadecimal output of the hash. Otherwise it will break your terminal.

#include <stdio.h>
#include <string.h>
#include <openssl/hmac.h>
#include <openssl/md5.h>

int main()
{
    // use unsigned char
    unsigned char    digest[16];
    char *input = "hek2mgl";
    int length = strlen(input);
    int i=0;

    // don't miss the underscore after MD5
    MD5_CTX md5;    
    MD5_Init(&md5);

    while (length > 0) {
        if (length > 512) {
            MD5_Update(&md5, input, 512);
        } else {
            MD5_Update(&md5, input, length);
        }
        length -= 512;
        input += 512;
    }

    MD5_Final(digest, &md5);
    printf("digest is: ");
    for(i = 0; i < 16; i++) {
        printf("%02x", digest[i]);
    }
    printf("\n");
    return 0;
}

Output:

digest is: 1ff8a3b2958ee3340ed88a2b980a8099

Answer 2

In addition to hek2mgl's answer, the OpenSSL wiki offers the following example at EVP Message Digests.

The EVP_* interfaces are recommended, especially for those who are not familiar with some of OpenSSL's features and quirks (which can be a good thing). Instead of EVP_sha256(), just use EVP_md5().

void digest_message(unsigned char *message, unsigned char **digest, unsigned int *digest_len)
{
    EVP_MD_CTX *mdctx;

    if((mdctx = EVP_MD_CTX_create()) == NULL)
        handleErrors();

    if(1 != EVP_DigestInit_ex(mdctx, EVP_sha256(), NULL))
        handleErrors();

    if(1 != EVP_DigestUpdate(mdctx, message, strlen(message)))
        handleErrors();

    if((*digest = (unsigned char *)OPENSSL_malloc(EVP_MD_size(EVP_sha256()))) == NULL)
        handleErrors();

    if(1 != EVP_DigestFinal_ex(mdctx, *digest, digest_len))
        handleErrors();

    EVP_MD_CTX_destroy(mdctx);
}