Saturday, February 4, 2012

How to calculate MD5 hash value using OpenSSL library

OpenSSL is an open source library which provides the basic cryptographic functions (see for details).

This post describes how to use cryptographic hash functions (MD5 as an example) provided by this library.

Actually, type "man EVP_get_digestbyname" and you will see description of the needed functions and a sample.

Here is the example with my comments:
#include <stdio.h>
#include <openssl/evp.h>

main(int argc, char *argv[])
  EVP_MD_CTX mdctx;
  const EVP_MD *md;
  char input[] = "md5";
  unsigned char output[EVP_MAX_MD_SIZE];
  int output_len, i;

  /* Initialize digests table */

  /* You can pass the name of another algorithm supported by your version of OpenSSL here */
  /* For instance, MD2, MD4, SHA1, RIPEMD160 etc. Check the OpenSSL documentation for details */
  md = EVP_get_digestbyname("MD5");

  if(!md) {
         printf("Unable to init MD5 digest\n");

  EVP_DigestInit_ex(&mdctx, md, NULL);
  EVP_DigestUpdate(&mdctx, input, strlen(input));
  /* to add more data to hash, place additional calls to EVP_DigestUpdate here */
  EVP_DigestFinal_ex(&mdctx, output, &output_len);

  /* Now output contains the hash value, output_len contains length of output, which is 128 bit or 16 byte in case of MD5 */

  printf("Digest is: ");
  for(i = 0; i < output_len; i++) printf("%02x", output[i]);

Link with crypto library (-lcrypto).

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