Difference between revisions of "EVP Message Digests"

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{{DocInclude
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|Name=Message Digests
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|Url=http://wiki.openssl.org/index.php/Manual:Evp(3)
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|Include=evp.h}}
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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:
 
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 simple to create a hash value for any given message
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* It is infeasible to find two messages that result in the same hash
 
* 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 SHA-2 set of hash functions (i.e. SHA-224, SHA-256, SHA-384 and SHA-512).
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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==
 
==An Example use of a Hash Function==
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<pre>
 
<pre>
void digest_message(unsigned char *message, unsigned char **digest, unsigned int *digest_len)
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void digest_message(const unsigned char *message, size_t message_len, unsigned char **digest, unsigned int *digest_len)
 
{
 
{
 
EVP_MD_CTX *mdctx;
 
EVP_MD_CTX *mdctx;
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handleErrors();
 
handleErrors();
  
if(1 != EVP_DigestUpdate(mdctx, message, strlen(message)))
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if(1 != EVP_DigestUpdate(mdctx, message, message_len))
 
handleErrors();
 
handleErrors();
  
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</pre>
 
</pre>
  
Refer to the OpenSSL manual page for further details http://www.openssl.org/docs/crypto/EVP_DigestInit.html
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Refer to the OpenSSL manual page for further details [[Manual:EVP_DigestInit(3)]]
  
 
==See also==
 
==See also==
 
* [[EVP]]
 
* [[EVP]]
 
* [[Libcrypto API]]
 
* [[Libcrypto API]]
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* [[EVP Symmetric Encryption and Decryption]]
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* [[EVP Authenticated Encryption and Decryption]]
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* [[EVP Asymmetric Encryption and Decryption of an Envelope]]
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* [[EVP Key Agreement]]
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* [[EVP Key and Parameter Generation]]
  
 
[[Category:Crypto API]]
 
[[Category:Crypto API]]
 
[[Category:C level]]
 
[[Category:C level]]
 
[[Category:Examples]]
 
[[Category:Examples]]

Latest revision as of 23:00, 28 April 2017

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_create()) == 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_destroy(mdctx);
}

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

See also[edit]