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Example C Program: Creating an HMAC

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A hashed message authentication checksum (HMAC) is typically used to verify that a message has not been changed during transit. Both parties to the message must have a shared secret key. The sender combines the key and the message into a string, creates a digest of the string by using an algorithm such as SHA-1 or MD5, and transmits the message and the digest. The receiver combines the shared key with the message, applies the appropriate algorithm, and compares the digest thus obtained with that transmitted by the sender. If the digests are exactly the same, the message was not tampered with.

This example demonstrates the following tasks and CryptoAPI functions:

#include <stdio.h>
#include <windows.h>
#include <wincrypt.h>

int main()
{
//--------------------------------------------------------------------
// Declare variables.
//
// hProv:           Handle to a cryptographic service provider (CSP). 
//                  This example retrieves the default provider for  
//                  the PROV_RSA_FULL provider type.  
// hHash:           Handle to the hash object needed to create a hash.
// hKey:            Handle to a symmetric key. This example creates a 
//                  key for the RC4 algorithm.
// hHmacHash:       Handle to an HMAC hash.
// pbHash:          Pointer to the hash.
// dwDataLen:       Length, in bytes, of the hash.
// Data1:           Password string used to create a symmetric key.
// Data2:           Message string to be hashed.
// HmacInfo:        Instance of an HMAC_INFO structure that contains 
//                  information about the HMAC hash.
// 
HCRYPTPROV  hProv       = NULL;
HCRYPTHASH  hHash       = NULL;
HCRYPTKEY   hKey        = NULL;
HCRYPTHASH  hHmacHash   = NULL;
PBYTE       pbHash      = NULL;
DWORD       dwDataLen   = 0;
BYTE        Data1[]     = {0x70,0x61,0x73,0x73,0x77,0x6F,0x72,0x64};
BYTE        Data2[]     = {0x6D,0x65,0x73,0x73,0x61,0x67,0x65};
HMAC_INFO   HmacInfo;

//--------------------------------------------------------------------
// Zero the HMAC_INFO structure and use the SHA1 algorithm for
// hashing.

ZeroMemory(&HmacInfo, sizeof(HmacInfo));
HmacInfo.HashAlgid = CALG_SHA1;

//--------------------------------------------------------------------
// Acquire a handle to the default RSA cryptographic service provider.

if (!CryptAcquireContext(
    &hProv,                   // handle of the CSP
    NULL,                     // key container name
    NULL,                     // CSP name
    PROV_RSA_FULL,            // provider type
    CRYPT_VERIFYCONTEXT))     // no key access is requested
{
   printf(" Error in AcquireContext 0x%08x \n",
          GetLastError());
   goto ErrorExit;
}

//--------------------------------------------------------------------
// Derive a symmetric key from a hash object by performing the
// following steps:
//    1. Call CryptCreateHash to retrieve a handle to a hash object.
//    2. Call CryptHashData to add a text string (password) to the 
//       hash object.
//    3. Call CryptDeriveKey to create the symmetric key from the
//       hashed password derived in step 2.
// You will use the key later to create an HMAC hash object. 

if (!CryptCreateHash(
    hProv,                    // handle of the CSP
    CALG_SHA1,                // hash algorithm to use
    0,                        // hash key
    0,                        // reserved
    &hHash))                  // address of hash object handle
{
   printf("Error in CryptCreateHash 0x%08x \n",
          GetLastError());
   goto ErrorExit;
}

if (!CryptHashData(
    hHash,                    // handle of the hash object
    Data1,                    // password to hash
    sizeof(Data1),            // number of bytes of data to add
    0))                       // flags
{
   printf("Error in CryptHashData 0x%08x \n", 
          GetLastError());
   goto ErrorExit;
}

if (!CryptDeriveKey(
    hProv,                    // handle of the CSP
    CALG_RC4,                 // algorithm ID
    hHash,                    // handle to the hash object
    0,                        // flags
    &hKey))                   // address of the key handle
{
   printf("Error in CryptDeriveKey 0x%08x \n", 
          GetLastError());
   goto ErrorExit;
}

//--------------------------------------------------------------------
// Create an HMAC by performing the following steps:
//    1. Call CryptCreateHash to create a hash object and retrieve 
//       a handle to it.
//    2. Call CryptSetHashParam to set the instance of the HMAC_INFO 
//       structure into the hash object.
//    3. Call CryptHashData to compute a hash of the message.
//    4. Call CryptGetHashParam to retrieve the size, in bytes, of
//       the hash.
//    5. Call malloc to allocate memory for the hash.
//    6. Call CryptGetHashParam again to retrieve the HMAC hash.

if (!CryptCreateHash(
    hProv,                    // handle of the CSP.
    CALG_HMAC,                // HMAC hash algorithm ID
    hKey,                     // key for the hash (see above)
    0,                        // reserved
    &hHmacHash))              // address of the hash handle
{
   printf("Error in CryptCreateHash 0x%08x \n", 
          GetLastError());
   goto ErrorExit;
}

if (!CryptSetHashParam(
    hHmacHash,                // handle of the HMAC hash object
    HP_HMAC_INFO,             // setting an HMAC_INFO object
    (BYTE*)&HmacInfo,         // the HMAC_INFO object
    0))                       // reserved
{
   printf("Error in CryptSetHashParam 0x%08x \n", 
          GetLastError());
   goto ErrorExit;
}

if (!CryptHashData(
    hHmacHash,                // handle of the HMAC hash object
    Data2,                    // message to hash
    sizeof(Data2),            // number of bytes of data to add
    0))                       // flags
{
   printf("Error in CryptHashData 0x%08x \n", 
          GetLastError());
   goto ErrorExit;
}

//--------------------------------------------------------------------
// Call CryptGetHashParam twice. Call it the first time to retrieve
// the size, in bytes, of the hash. Allocate memory. Then call 
// CryptGetHashParam again to retrieve the hash value.

if (!CryptGetHashParam(
    hHmacHash,                // handle of the HMAC hash object
    HP_HASHVAL,               // query on the hash value
    NULL,                     // filled on second call
    &dwDataLen,               // length, in bytes, of the hash
    0))
{
   printf("Error in CryptGetHashParam 0x%08x \n", 
          GetLastError());
   goto ErrorExit;
}

pbHash = (BYTE*)malloc(dwDataLen);
if(NULL == pbHash) 
{
   printf("unable to allocate memory\n");
   goto ErrorExit;
}
    
if (!CryptGetHashParam(
    hHmacHash,                 // handle of the HMAC hash object
    HP_HASHVAL,                // query on the hash value
    pbHash,                    // pointer to the HMAC hash value
    &dwDataLen,                // length, in bytes, of the hash
    0))
{
   printf("Error in CryptGetHashParam 0x%08x \n", GetLastError());
   goto ErrorExit;
}

// Print the hash to the console.

printf("The hash is:  ");
for(DWORD i = 0 ; i < dwDataLen ; i++) 
{
   printf("%2.2x ",pbHash[i]);
}
printf("\n");

// Free resources.
ErrorExit:
    if(hHmacHash)
        CryptDestroyHash(hHmacHash);
    if(hKey)
        CryptDestroyKey(hKey);
    if(hHash)
        CryptDestroyHash(hHash);    
    if(hProv)
        CryptReleaseContext(hProv, 0);
    if(pbHash)
        free(pbHash);
    return 0;
}