CryptVerifySignature

The CryptVerifySignature function is used to verify a signature against a hash object.

Before calling this function, the CryptCreateHash function must be called to get a handle to a hash object. The CryptHashData and/or CryptHashSessionKey functions are then used to add the data and/or session keys to the hash object.

After this function has been completed, the only hash function that can be called by using the hHash handle is the CryptDestroyHash function.

#include <wincrypt.h>
BOOL WINAPI CryptVerifySignature(
  HCRYPTHASH hHash,      // in
  BYTE *pbSignature,     // in
  DWORD dwSigLen,        // in
  HCRYPTKEY hPubKey,     // in
  LPCTSTR sDescription,  // in
  DWORD dwFlags          // in
);

Parameters

hHash
A handle to the hash object to verify against.
pbSignature
The address of the signature data to be verified.
dwSigLen
The number of bytes in the pbSignature signature data.
hPubKey
A handle to the public key to use to authenticate the signature. This public key must belong to the key pair that was originally used to create the digital signature.
sDescription
This parameter should no longer be used, and should be set to NULL to prevent security vulnerabilities. However, it is still supported for backward compatibility in the Microsoft Base Cryptographic Provider.
dwFlags
The flag values. This parameter is reserved for future use and should always be zero.

Return Values

If the function succeeds, the return value is TRUE. If it fails, the return value is FALSE. To retrieve extended error information, use the GetLastError function.

The following table lists the error codes most commonly returned by the GetLastError function. The error codes prefaced by "NTE" are generated by the particular CSP you are using.

Error code Description
ERROR_INVALID_HANDLE One of the parameters specifies an invalid handle.
ERROR_INVALID_PARAMETER One of the parameters contains an invalid value. This is most often an illegal pointer.
NTE_BAD_FLAGS The dwFlags parameter is nonzero.
NTE_BAD_HASH The hash object specified by the hHash parameter is invalid.
NTE_BAD_KEY The hPubKey parameter does not contain a handle to a valid public key.
NTE_BAD_SIGNATURE The signature was not verified. This could be because the data itself has changed, the description string did not match, or the wrong public key was specified by hPubKey.

This error can also be returned if the hashing or signature algorithms do not match the ones used to create the signature.
NTE_BAD_UID The CSP context that was specified when the hash object was created cannot be found.
NTE_NO_MEMORY The CSP ran out of memory during the operation.

Example

#include <wincrypt.h>

HCRYPTPROV hProv = 0;
#define BUFFER_SIZE 256
BYTE pbBuffer[BUFFER_SIZE];
HCRYPTHASH hHash = 0;
HCRYPTKEY hPubKey = 0;
BYTE *pbSignature = NULL;
DWORD dwSigLen;
LPTSTR szDescription = NULL;

// 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;
}

// Load 'pbBuffer' with 'BUFFER_SIZE' bytes of test data. This must
// be the same data that was originally signed.
...
// Point 'pbSignature' at the signature created by a previous call
// to CryptSignHash. Set 'dwSigLen' to the number of bytes in the
// signature.
...

// Point 'szDescription' at the text describing the data being 
// signed. This must be the same description text that was originally
// passed to CryptSignHash.
...

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

// Create a hash object.
if(!CryptCreateHash(hProv, CALG_MD5, 0, 0, &hHash)) {
    printf("Error %x during CryptCreateHash!\n", GetLastError());
    goto done;
}

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

// Validate the digital signature.
if(!CryptVerifySignature(hHash, pbSignature, dwSigLen, hPubKey, 
                            szDescription, 0)) {
    if(GetLastError() == NTE_BAD_SIGNATURE) {
        printf("Signature not validated!\n");
    } else {
        printf("Error %x during CryptVerifySignature!\n", 
                GetLastError());
    }
} else {
    printf("Signature validated\n");
}

done:
...

// Release the public key.
if(hPubKey != 0) CryptDestroyKey(hPubKey);

// Destroy the hash object.
if(hHash != 0) CryptDestroyHash(hHash);

// Release the provider handle.
if(hProv != 0) CryptReleaseContext(hProv, 0);

QuickInfo

  Windows NT: Requires version 4.0 or later.
  Windows: Requires Windows 95 OSR2 or later (or Windows 95 with IE 3.02 or later).
  Windows CE: Unsupported.
  Header: Declared in wincrypt.h.
  Import Library: Use crypt32.lib.
  Unicode: Defined as Unicode and ANSI prototypes.

See Also

CryptCreateHash, CryptDestroyHash, CryptHashData, CryptHashSessionKey, CryptSignHash