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.
• 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 Working with EVP_PKEYs for further information. You also need to provide a message digest algorithm (refer to 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 */
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 = 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) 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. 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)) goto err;

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.