Difference between revisions of "EVP Message Digests"
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+ | {{DocInclude | ||
+ | |Name=Message Digests | ||
+ | |Url=http://wiki.openssl.org/index.php/Manual:Evp(3) | ||
+ | |Include=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: | 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 | ||
Line 5: | Line 10: | ||
* 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). | + | 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== | ||
Line 19: | Line 24: | ||
<pre> | <pre> | ||
− | void digest_message(unsigned char *message, unsigned char **digest, unsigned int *digest_len) | + | 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, | + | if(1 != EVP_DigestUpdate(mdctx, message, message_len)) |
handleErrors(); | handleErrors(); | ||
Line 41: | Line 46: | ||
} | } | ||
</pre> | </pre> | ||
+ | |||
+ | Refer to the OpenSSL manual page for further details [[Manual:EVP_DigestInit(3)]] | ||
==See also== | ==See also== | ||
* [[EVP]] | * [[EVP]] | ||
* [[Libcrypto API]] | * [[Libcrypto API]] | ||
+ | * [[EVP Symmetric Encryption and Decryption]] | ||
+ | * [[EVP Authenticated Encryption and Decryption]] | ||
+ | * [[EVP Asymmetric Encryption and Decryption of an Envelope]] | ||
+ | * [[EVP Key Agreement]] | ||
+ | * [[EVP Key and Parameter Generation]] | ||
+ | |||
+ | [[Category:Crypto API]] | ||
+ | [[Category:C level]] | ||
+ | [[Category:Examples]] |
Revision as of 23:00, 28 April 2017
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
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)