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Programa C de ejemplo: Firmar un hash y comprobar la firma hash

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En el ejemplo siguiente se aplica un algoritmo hash a algunos datos y se firma ese hash. En una segunda fase, se comprueba el hash y su firma. El hash se firma con la clave privada del usuario y la clave pública del firmante se exporta para que se pueda comprobar la firma.

En este ejemplo se muestran las siguientes tareas y funciones CryptoAPI :

//--------------------------------------------------------------------
// Copyright (C) Microsoft.  All rights reserved.
// Example of signing a hash and 
// verifying the hash signature.
#pragma comment(lib, "crypt32.lib")

#include <stdio.h>
#include <windows.h>
#include <Wincrypt.h>
#define MY_ENCODING_TYPE  (PKCS_7_ASN_ENCODING | X509_ASN_ENCODING)
void MyHandleError(char *s);

void main(void)
{
//-------------------------------------------------------------------
// Declare and initialize variables.

HCRYPTPROV hProv;
BYTE *pbBuffer= (BYTE *)"The data that is to be hashed and signed.";
DWORD dwBufferLen = strlen((char *)pbBuffer)+1;
HCRYPTHASH hHash;
HCRYPTKEY hKey;
HCRYPTKEY hPubKey;
BYTE *pbKeyBlob;        
BYTE *pbSignature;
DWORD dwSigLen;
DWORD dwBlobLen;

//-------------------------------------------------------------------
// Acquire a cryptographic provider context handle.

if(CryptAcquireContext(
   &hProv, 
   NULL, 
   NULL, 
   PROV_RSA_FULL, 
   0)) 
{
     printf("CSP context acquired.\n");
}
else
{
     MyHandleError("Error during CryptAcquireContext.");
}
//-------------------------------------------------------------------
// Get the public at signature key. This is the public key
// that will be used by the receiver of the hash to verify
// the signature. In situations where the receiver could obtain the
// sender's public key from a certificate, this step would not be
// needed.

if(CryptGetUserKey(   
   hProv,    
   AT_SIGNATURE,    
   &hKey)) 
{
    printf("The signature key has been acquired. \n");
}
else
{
    MyHandleError("Error during CryptGetUserKey for signkey.");
}
//-------------------------------------------------------------------
// Export the public key. Here the public key is exported to a 
// PUBLICKEYBOLB so that the receiver of the signed hash can
// verify the signature. This BLOB could be written to a file and
// sent to another user.

if(CryptExportKey(   
   hKey,    
   NULL,    
   PUBLICKEYBLOB,
   0,    
   NULL, 
   &dwBlobLen)) 
{
     printf("Size of the BLOB for the public key determined. \n");
}
else
{
     MyHandleError("Error computing BLOB length.");
}
//-------------------------------------------------------------------
// Allocate memory for the pbKeyBlob.

if(pbKeyBlob = (BYTE*)malloc(dwBlobLen)) 
{
    printf("Memory has been allocated for the BLOB. \n");
}
else
{
    MyHandleError("Out of memory. \n");
}
//-------------------------------------------------------------------
// Do the actual exporting into the key BLOB.

if(CryptExportKey(   
   hKey, 
   NULL,    
   PUBLICKEYBLOB,    
   0,    
   pbKeyBlob,    
   &dwBlobLen))
{
     printf("Contents have been written to the BLOB. \n");
}
else
{
    MyHandleError("Error during CryptExportKey.");
}
//-------------------------------------------------------------------
// Create the hash object.

if(CryptCreateHash(
   hProv, 
   CALG_MD5, 
   0, 
   0, 
   &hHash)) 
{
     printf("Hash object created. \n");
}
else
{
    MyHandleError("Error during CryptCreateHash.");
}
//-------------------------------------------------------------------
// Compute the cryptographic hash of the buffer.

if(CryptHashData(
   hHash, 
   pbBuffer, 
   dwBufferLen, 
   0)) 
{
     printf("The data buffer has been hashed.\n");
}
else
{
     MyHandleError("Error during CryptHashData.");
}
//-------------------------------------------------------------------
// Determine the size of the signature and allocate memory.

dwSigLen= 0;
if(CryptSignHash(
   hHash, 
   AT_SIGNATURE, 
   NULL, 
   0, 
   NULL, 
   &dwSigLen)) 
{
     printf("Signature length %d found.\n",dwSigLen);
}
else
{
     MyHandleError("Error during CryptSignHash.");
}
//-------------------------------------------------------------------
// Allocate memory for the signature buffer.

if(pbSignature = (BYTE *)malloc(dwSigLen))
{
     printf("Memory allocated for the signature.\n");
}
else
{
     MyHandleError("Out of memory.");
}
//-------------------------------------------------------------------
// Sign the hash object.

if(CryptSignHash(
   hHash, 
   AT_SIGNATURE, 
   NULL, 
   0, 
   pbSignature, 
   &dwSigLen)) 
{
     printf("pbSignature is the hash signature.\n");
}
else
{
     MyHandleError("Error during CryptSignHash.");
}
//-------------------------------------------------------------------
// Destroy the hash object.

if(hHash) 
  CryptDestroyHash(hHash);

printf("The hash object has been destroyed.\n");
printf("The signing phase of this program is completed.\n\n");

//-------------------------------------------------------------------
// In the second phase, the hash signature is verified.
// This would most often be done by a different user in a
// separate program. The hash, signature, and the PUBLICKEYBLOB
// would be read from a file, an email message, 
// or some other source.

// Here, the original pbBuffer, pbSignature, szDescription. 
// pbKeyBlob, and their lengths are used.

// The contents of the pbBuffer must be the same data 
// that was originally signed.

//-------------------------------------------------------------------
// Get the public key of the user who created the digital signature 
// and import it into the CSP by using CryptImportKey. This returns
// a handle to the public key in hPubKey.

if(CryptImportKey(
   hProv,
   pbKeyBlob,
   dwBlobLen,
   0,
   0,
   &hPubKey))
{
     printf("The key has been imported.\n");
}
else
{
     MyHandleError("Public key import failed.");
}
//-------------------------------------------------------------------
// Create a new hash object.

if(CryptCreateHash(
   hProv, 
   CALG_MD5, 
   0, 
   0, 
   &hHash)) 
{
     printf("The hash object has been recreated. \n");
}
else
{
     MyHandleError("Error during CryptCreateHash.");
}
//-------------------------------------------------------------------
// Compute the cryptographic hash of the buffer.

if(CryptHashData(
   hHash, 
   pbBuffer, 
   dwBufferLen, 
   0)) 
{
     printf("The new hash has been created.\n");
}
else
{
     MyHandleError("Error during CryptHashData.");
}
//-------------------------------------------------------------------
// Validate the digital signature.

if(CryptVerifySignature(
   hHash, 
   pbSignature, 
   dwSigLen, 
   hPubKey,
   NULL, 
   0)) 
{
     printf("The signature has been verified.\n");
}
else
{
     printf("Signature not validated!\n");
}
//-------------------------------------------------------------------
// Free memory to be used to store signature.

if(pbSignature)
  free(pbSignature);

//-------------------------------------------------------------------
// Destroy the hash object.

if(hHash) 
  CryptDestroyHash(hHash);

//-------------------------------------------------------------------
// Release the provider handle.

if(hProv) 
   CryptReleaseContext(hProv, 0);
} //  End of main

//-------------------------------------------------------------------
//  This example uses the function MyHandleError, a simple error
//  handling function, to print an error message to the  
//  standard error (stderr) file and exit the program. 
//  For most applications, replace this function with one 
//  that does more extensive error reporting.

void MyHandleError(char *s)
{
    fprintf(stderr,"An error occurred in running the program. \n");
    fprintf(stderr,"%s\n",s);
    fprintf(stderr, "Error number %x.\n", GetLastError());
    fprintf(stderr, "Program terminating. \n");
    exit(1);
} // End of MyHandleError