Beispiel C-Programm: Erstellen eines MD5-Hash aus Dateiinhalten

Im folgenden Beispiel wird die Verwendung von CryptoAPI zum Berechnen des MD5 Hash des Inhalts einer Datei veranschaulicht. In diesem Beispiel wird die Berechnung für den Inhalt einer zur Laufzeit angegebenen Datei ausgeführt.

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

#define BUFSIZE 1024
#define MD5LEN  16

DWORD main()
{
    DWORD dwStatus = 0;
    BOOL bResult = FALSE;
    HCRYPTPROV hProv = 0;
    HCRYPTHASH hHash = 0;
    HANDLE hFile = NULL;
    BYTE rgbFile[BUFSIZE];
    DWORD cbRead = 0;
    BYTE rgbHash[MD5LEN];
    DWORD cbHash = 0;
    CHAR rgbDigits[] = "0123456789abcdef";
    LPCWSTR filename=L"filename.txt";
    // Logic to check usage goes here.

    hFile = CreateFile(filename,
        GENERIC_READ,
        FILE_SHARE_READ,
        NULL,
        OPEN_EXISTING,
        FILE_FLAG_SEQUENTIAL_SCAN,
        NULL);

    if (INVALID_HANDLE_VALUE == hFile)
    {
        dwStatus = GetLastError();
        printf("Error opening file %s\nError: %d\n", filename, 
            dwStatus); 
        return dwStatus;
    }

    // Get handle to the crypto provider
    if (!CryptAcquireContext(&hProv,
        NULL,
        NULL,
        PROV_RSA_FULL,
        CRYPT_VERIFYCONTEXT))
    {
        dwStatus = GetLastError();
        printf("CryptAcquireContext failed: %d\n", dwStatus); 
        CloseHandle(hFile);
        return dwStatus;
    }

    if (!CryptCreateHash(hProv, CALG_MD5, 0, 0, &hHash))
    {
        dwStatus = GetLastError();
        printf("CryptCreateHash failed: %d\n", dwStatus); 
        CloseHandle(hFile);
        CryptReleaseContext(hProv, 0);
        return dwStatus;
    }

    while (bResult = ReadFile(hFile, rgbFile, BUFSIZE, 
        &cbRead, NULL))
    {
        if (0 == cbRead)
        {
            break;
        }

        if (!CryptHashData(hHash, rgbFile, cbRead, 0))
        {
            dwStatus = GetLastError();
            printf("CryptHashData failed: %d\n", dwStatus); 
            CryptReleaseContext(hProv, 0);
            CryptDestroyHash(hHash);
            CloseHandle(hFile);
            return dwStatus;
        }
    }

    if (!bResult)
    {
        dwStatus = GetLastError();
        printf("ReadFile failed: %d\n", dwStatus); 
        CryptReleaseContext(hProv, 0);
        CryptDestroyHash(hHash);
        CloseHandle(hFile);
        return dwStatus;
    }

    cbHash = MD5LEN;
    if (CryptGetHashParam(hHash, HP_HASHVAL, rgbHash, &cbHash, 0))
    {
        printf("MD5 hash of file %s is: ", filename);
        for (DWORD i = 0; i < cbHash; i++)
        {
            printf("%c%c", rgbDigits[rgbHash[i] >> 4],
                rgbDigits[rgbHash[i] & 0xf]);
        }
        printf("\n");
    }
    else
    {
        dwStatus = GetLastError();
        printf("CryptGetHashParam failed: %d\n", dwStatus); 
    }

    CryptDestroyHash(hHash);
    CryptReleaseContext(hProv, 0);
    CloseHandle(hFile);

    return dwStatus; 
}