Problem - Kevin Tafuro

Otherwise, you can use the HMAC implementation provided with this recipe in
combination with any cryptographic hash function you have handy.
Discussion
Be sure to look at our generic recommendations for using a MAC
(Recipe 6.9).
Here’s an example of using OpenSSL’s incremental interface to hash two messages
using SHA1:
#include
#include
void spc_incremental_hmac(unsigned char *key, size_t keylen) {
int i;
HMAC_CTX ctx;
unsigned int len;
unsigned char out[20];
HMAC_Init(&ctx, key, keylen, EVP_sha1( ));
HMAC_Update(&ctx, "fred", 4);
HMAC_Final(&ctx, out, &len);
for (i = 0; i < len; i++) printf("%02x", out[i]);
printf("\n");
HMAC_Init(&ctx, 0, 0, 0);
HMAC_Update(&ctx, "fred", 4);
HMAC_Final(&ctx, out, &len);
for (i = 0; i < len; i++) printf("%02x", out[i]);
printf("\n");
HMAC_cleanup(&ctx); /* Remove key from memory */
}
To reset the HMAC context object, we call HMAC_Init( ), passing in zeros (NULLs) in
place of the key, key length, and digest type to use. The NULL argument when initializing
in OpenSSLgenerally means “I’m not supplying this value right now; use what
you already have.”
The following example shows an implementation of the same code provided for
OpenSSL, this time using CryptoAPI (with the exception of resetting the context,
because CryptoAPI actually requires a new one to be created). This implementation
requires the use of the code in Recipe 5.26 to convert raw key data into an HCRYPTKEY
object as required by CryptCreateHash( ). Note the difference in the arguments
required between spc_incremental_hmac( ) as implemented for OpenSSL, and
SpcIncrementalHMAC( ) as implemented for CryptoAPI. The latter requires an additional
argument that specifies the encryption algorithm for the key. Although the
information is never really used, CryptoAPI insists on tying an encryption algorithm
This is the Title of the Book, eMatter Edition
Copyright © 2007 O’Reilly & Associates, Inc. All rights reserved.
Using HMAC | 277