View source for EVP Signing and Verifying

There are two APIs available for performing sign and verify operations. The older EVP_Sign* functions, and the newer and more flexible EVP_DigestSign* functions. They are very similar, but for new applications the EVP_DigestSign* versions should be preferred.

==Signing a Message==

Signing a message is a three stage process:
* Initialise the operation and identify the message digest/hash function that will be used during the sign operation. 
* Add message data (this step can be repeated as many times as necessary to add more message data)
* Create the signature

In order to initialise the operation, you need to have first set up a EVP_PKEY object containing a public key for an algorithm that supports signing (this includes MAC codes). Refer to [[EVP#Working with EVP_PKEYs|Working with EVP_PKEYs]] for further information. You also need to provide a message digest algorithm (refer to [[EVP#Working with Algorithms and Modes|Working with Algorithms and Modes]]).

See the following for an example of signing a message:

 EVP_MD_CTX *mdctx = NULL;
 int ret = 0;
 
 *sig = NULL;
 
 /* Create the Message Digest Context */
 if(!(mdctx = EVP_MD_CTX_create())) goto err;
 
 /* Initialise the DigestSign operation - SHA-256 has been selected as the message digest function in this example */
 if(!EVP_DigestSignInit(mdctx, NULL, EVP_sha256(), NULL, key)) goto err;
 
 /* Call update with the message */
 if(!EVP_DigestSignUpdate(mdctx, msg, strlen(msg))) goto err;
 
 /* Finalise the DigestSign operation */
 if(!EVP_DigestSignFinal(mdctx, *sig, slen)) goto err;
 if(!(*sig = OPENSSL_malloc(sizeof(unsigned char) * (*slen)))) goto err;
 if(!EVP_DigestSignFinal(mdctx, *sig, slen)) goto err;
 
 /* Success */
 ret = 1;
 
 err:
 if(ret != 1)
 {
   /* Do some error handling */
 }
 
 /* Clean up */
 if(*sig && !ret) OPENSSL_free(*sig);
 if(mdctx) EVP_MD_CTX_destroy(mdctx);

Note: There is no difference in the API between signing using an asymmetric algorithm, and generating a MAC code. In the case of CMAC no message digest function is required (NULL can be passed). Signing using the EVP_Sign* functions is very similar to the above example, except there is no support for MAC codes.

Refer to http://www.openssl.org/docs/crypto/EVP_DigestSignInit.html for further details on the EVP_DigestSign* functions, and http://www.openssl.org/docs/crypto/EVP_SignInit.html for the EVP_Sign* functions.

==Verifying a Message==

Verifying a message is very similar to signing except the EVP_DigestVerify* functions (or EVP_Verify* functions) are used instead. Clearly only a public key is required for a verify operation:

 if(!EVP_DigestVerifyInit(mdctx, NULL, EVP_sha256(), NULL, key)) goto err;
 if(!EVP_DigestVerifyUpdate(mdctx, msg, strlen(msg))) goto err;
 if(EVP_DigestVerifyFinal(mdctx, sig, slen))
 {
   /* Success */
 }
 else
 {
   /* Failure */
 }

Note: MAC operations do not support the verify operation. Verifying a MAC code is done by calling the sign operations and confirming that the generated code is identical to the one provided.

Refer to http://www.openssl.org/docs/crypto/EVP_DigestVerifyInit.html and http://www.openssl.org/docs/crypto/EVP_VerifyInit.html for further information on the verify functions.

==See also==
* [[EVP]]
* [[Libcrypto API]]