Difference between revisions of "EVP Message Digests"

From OpenSSLWiki
(An Example use of a Hash Function)
m (Add info on EVP_MD_CTX_create and EVP_MD_CTX_destroy from OpenSSL 1.0.2.)
 
Line 45: Line 45:
 
EVP_MD_CTX_free(mdctx);
 
EVP_MD_CTX_free(mdctx);
 
}
 
}
 +
</pre>
 +
 +
If you need to support both OpenSSL 1.0.x and OpenSSL 1.1.x, then use a <tt>define</tt> for <tt>EVP_MD_CTX_new</tt> and <tt>EVP_MD_CTX_free</tt> as shown below.
 +
 +
<pre>#if OPENSSL_VERSION_NUMBER < 0x10100000L
 +
#  define EVP_MD_CTX_new  EVP_MD_CTX_create
 +
#  define EVP_MD_CTX_free  EVP_MD_CTX_destroy
 +
#endif
 
</pre>
 
</pre>
  

Latest revision as of 01:51, 12 January 2022

Message Digests
Documentation
#include <openssl/evp.h>

A Message Digest or Hash Function takes any arbitrary message (with any content or length) as an input and provides a fixed size hash value as a result. Specifically the function exhibits the following properties:

  • It is simple to create a hash value for any given message
  • It is computationally infeasible to calculate a message from any given hash (i.e. the function is one-way)
  • It is infeasible to modify a message without also modifying the hash value
  • It is infeasible to find two messages that result in the same hash

The OpenSSL library supports a wide number of different hash functions including the popular Category:SHA-2 set of hash functions (i.e. SHA-224, SHA-256, SHA-384 and SHA-512).

An Example use of a Hash Function[edit]

Using an OpenSSL message digest/hash function, consists of the following steps:

  • Create a Message Digest context
  • Initialise the context by identifying the algorithm to be used (built-in algorithms are defined in evp.h)
  • Provide the message whose digest needs to be calculated. Messages can be divided into sections and provided over a number of calls to the library if necessary
  • Caclulate the digest
  • Clean up the context if no longer required

Message digest algorithms are identified using an EVP_MD object. These are built-in to the library and obtained through appropriate library calls (e.g. such as EVP_sha256() or EVP_sha512()).

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

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

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

	if(1 != EVP_DigestUpdate(mdctx, message, message_len))
		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_free(mdctx);
}

If you need to support both OpenSSL 1.0.x and OpenSSL 1.1.x, then use a define for EVP_MD_CTX_new and EVP_MD_CTX_free as shown below.

#if OPENSSL_VERSION_NUMBER < 0x10100000L
#  define EVP_MD_CTX_new   EVP_MD_CTX_create
#  define EVP_MD_CTX_free  EVP_MD_CTX_destroy
#endif

Refer to the OpenSSL manual page for further details Manual:EVP_DigestInit(3)

See also[edit]