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 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 - SHA-256 has been selected as the message digest function in this example */
if(1 != EVP_DigestSignInit(mdctx, NULL, EVP_sha256(), NULL, key)) goto err;

/* Call update with the message */
if(1 != EVP_DigestSignUpdate(mdctx, msg, strlen(msg))) goto err;

/* Finalise the DigestSign operation */
/* First call EVP_DigestSignFinal with a NULL sig parameter to obtain the length of the
 * signature. Length is returned in slen */
if(1 != EVP_DigestSignFinal(mdctx, NULL, slen)) goto err;
/* Allocate memory for the signature based on size in slen */
if(!(*sig = OPENSSL_malloc(sizeof(unsigned char) * (*slen)))) goto err;
/* Obtain the signature */
if(1 != 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. Note that CMAC is only supported in the (as yet unreleased) version 1.1.0 of OpenSSL.

Refer to Manual:EVP_DigestSignInit(3) for further details on the EVP_DigestSign* functions, and Manual:EVP_SignInit(3) 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(1 != EVP_DigestVerifyInit(mdctx, NULL, EVP_sha256(), NULL, key)) goto err;
	if(1 != EVP_DigestVerifyUpdate(mdctx, msg, strlen(msg))) goto err;
	if(1 == EVP_DigestVerifyFinal(mdctx, sig, slen))
	{
		/* Success */
	}
	else
	{
		/* Failure */
	}

Note that 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. It is important that when comparing a supplied MAC with an expected MAC that the comparison takes a constant time whether the comparison returns a match or not. Failure to do this can expose your code to timing attacks, which could (for example) enable an attacker to forge MAC codes. To do this use the CRYPTO_memcmp function as shown in the code example below. Never use memcmp for this test:

	if(!(mdctx = EVP_MD_CTX_create())) goto err;

	/* Create a buffer to store the MAC for the received message */
	if(!(sigtmp = OPENSSL_malloc(sizeof(unsigned char) * EVP_PKEY_size(key)))) goto err;
	sigtmplen = EVP_PKEY_size(key);

	/* Calculate the MAC for the received message */
	if(1 != EVP_DigestSignInit(mdctx, NULL, EVP_sha256(), NULL, key)) goto err;
	if(1 != EVP_DigestSignUpdate(mdctx, msg, strlen(msg))) goto err;
	if(1 != EVP_DigestSignFinal(mdctx, sigtmp, &sigtmplen)) goto err;

	/* Check the lengths of the calculated and supplied MACs are the same */ 
	if(sigtmplen != slen) goto err;

	/* Calculated MAC is in sigtmp. Supplied MAC is in sig. Compare the two */
	if(CRYPTO_memcmp(sig, sigtmp, slen))	
		/* Verify failure */ goto failure;
	else
		/* Verify success */;

Refer to Manual:EVP_DigestVerifyInit(3) and Manual:EVP_VerifyInit(3) for further information on the verify functions.

See also

• EVP
• Libcrypto API