CryptGetHashParam

This function retrieves data that governs the operations of a hash object and retrieves the actual hash value.

BOOL CRYPTFUNC CryptGethashParam( HCRYPTHASHhHash,DWORDdwParam,BYTE* pbData,DWORD* pdwDataLen,DWORDdwFlags);

Parameters

• hHash
  [in] HCRYPTHASH handle to the hash object to query.

• dwParam
  [in] Specifies the query type.

  The following table shows the available query types. Cryptographic service providers (CSPs) can add values to the list.

  Query type	Description	pbData content
  HP_ALGID	Hash algorithm	ALG_ID structure indicating the algorithm specified when the hash object was created. For a list of hash algorithms, see CryptCreateHash.
  HP_HASHSIZE	Hash value size	DWORD value indicating the number of bytes in the hash value. This is usually 16 to 20, depending on the hash algorithm. Applications must retrieve this value just before the HP_HASHVAL value so the correct amount of memory can be allocated.
  HP_HASHVAL	Hash value	The hash value or message hash for the hash object specified by hHash. This value is generated based on the data supplied to the hash object earlier through the CryptHashData and CryptHashSessionKey functions. The CryptGetHashParam function completes the hash. After CryptGetHashParam is called, data cannot be added to the hash. Additional calls to CryptHashData or CryptHashSessionKey fail. After the application is done with the hash, CryptDestroyHash should be called to destroy the hash object.

• pbData
  [out] Pointer to a buffer that receives the specified parameter data. The form of this data varies depending on the dwParam query type.

  This parameter can be NULL to set the size of this buffer for memory allocation purposes.

• pdwDataLen
  [in, out] On input, pointer to a variable that specifies the size, in bytes, of the buffer pointed to by the pbData parameter. On output, the variable pointed to by the pdwDataLen parameter contains the number of bytes stored in the buffer.

  When processing the data returned in the buffer, applications must use the actual size of the data returned. The actual size may be slightly smaller than the size of the buffer specified on input. On input, buffer sizes are usually specified large enough to ensure that the largest possible output data will fit in the buffer. On output, the variable pointed to by this parameter is updated to reflect the actual size of the data copied to the buffer.

• dwFlags
  [in] Reserved for future use and must be set to zero.

Return Values

TRUE indicates success. FALSE indicates failure. To get extended error information, call the GetLastError function.

The following table shows the common values for GetLastError function. The error values prefaced by NTE are generated by the particular CSP you are using.

Example Code

#include <wincrypt.h>

HCRYPTPROV hProv = 0;
HCRYPTHASH hHash = 0;
BYTE *pbHash = NULL;
DWORD dwHashLen;
#define BUFFER_SIZE 256
BYTE pbBuffer[BUFFER_SIZE];
DWORD dwCount;
DWORD i;

// Get a handle to the default provider.
if(!CryptAcquireContext(&hProv, NULL, NULL, PROV_RSA_FULL, 0)) {
 printf("Error %x during CryptAcquireContext!\n", GetLastError());
 goto done;
}
// Create a hash object.
if(!CryptCreateHash(hProv, CALG_MD5, 0, 0, &hHash)) {
 printf("Error %x during CryptBeginHash!\n", GetLastError());
 goto done;
}

// Fill the buffer with test data.
for(i = 0 ; i < BUFFER_SIZE ; i++) {
 pbBuffer[i] = (BYTE)i;
}

// Put the hash in buffer.
if(!CryptHashData(hHash, pbBuffer, BUFFER_SIZE, 0)) {
 printf("Error %x during CryptHashData!\n", GetLastError());
 goto done;
}

// Read the hash value size and allocate memory.
dwCount = sizeof(DWORD);
if(!CryptGetHashParam(hHash, HP_HASHSIZE, (BYTE *)&dwHashLen, 
 &dwCount, 0)) {
 printf("Error %x during reading hash size!\n", GetLastError());
 goto done;
}
if((pbHash = malloc(dwHashLen)) == NULL) {
 printf("Out of memory!\n");
 goto done;
}

// Read the hash value.
if(!CryptGetHashParam(hHash, HP_HASHVAL, pbHash, &dwHashLen, 0)) {
 printf("Error %x during reading hash value!\n", GetLastError());
 goto done;
}

// Print the hash value.
for(i = 0 ; i < dwHashLen ; i++) {
 printf("%2.2x ",pbHash[i]);
}
printf("\n");

done:

// Free memory.
if(pbHash !=NULL) free(pbHash);

// Destroy the hash object.
if(hHash) CryptDestroyHash(hHash);
// Release the CSP handle.
if(hProv) CryptReleaseContext(hProv,0);
 

Requirements

OS Versions: Windows CE 2.10 and later.
Header: Wincrypt.h.
Link Library: Coredll.lib.

See Also

CryptCreateHash | CryptGetKeyParam | CryptHashData | CryptHashSessionKey | CryptSetHashParam | ALG_ID

 Last updated on Thursday, April 08, 2004

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