The following example code demonstrates the process of encoding, decoding, and verifying the signature of a signed message.
//=============================================================
// EXAMPLE CODE FOR CREATING A SIGNED MESSAGE.
//=============================================================
// Use this constant definition (or one similar to it) to define
// a single encoding type that can be used in all parameters and
// data members that require one or the other or both.
#define MY_ENCODING_TYPE (PKCS_7_ASN_ENCODING | CRYPT_ASN_ENCODING)
//-------------------------------------------------------------
// Get a pointer to the message content- this code creates
// the message content and gets a pointer to it. In reality,
// the content will usually exist somewhere and a pointer to it
// will get passed to the application.
//-------------------------------------------------------------
char szContent[] = "A Razzle-Dazzle \n"
"Signed Message"; // The message
char* pszContent = szContent; // A string pointer
BYTE* pbContent = (BYTE*) pszContent; // A byte pointer
DWORD cbContent = sizeof(szContent); // Size of message
AfxMessageBox(pszContent); // Display original message
//-------------------------------------------------------------
// Get a handle to a cryptographic provider.
//-------------------------------------------------------------
HCRYPTPROV hCryptProv = NULL; // Handle returned here
BOOL fReturn = FALSE;
fReturn = CryptAcquireContext(
&hCryptProv, // Address for handle to be returned
NULL, // Use the current user's logon name
NULL, // Use the default provider
PROV_RSA_FULL, // Specifies the provider type
0); // Zero allows access to private keys
if(!fReturn)
{
AfxMessageBox("CryptAcquireContext failed");
return;
}
// If the function succeeded, the handle to the cryptographic
// provider resides at hCryptProv.
//-------------------------------------------------------------
// Open the system certificate store.
//-------------------------------------------------------------
HCERTSTORE hStoreHandle = NULL;
// Call CertOpenSystemStore to open the store.
hStoreHandle = CertOpenSystemStore(hCryptProv, "MY");
if (!hStoreHandle)
{
AfxMessageBox( "Error Getting Store Handle");
return;
}
// If the call was successful, the cert store handle now resides
// at the location pointed to by hStoreHandle.
//-------------------------------------------------------------
// Get a pointer to your signature certificate.
//-------------------------------------------------------------
PCCERT_CONTEXT pSignerCert = NULL;
// For the implementation of GetMySignerCert(), see the function
// at the end of this example code.
pSignerCert = GetMySignerCert(hStoreHandle);
if(!pSignerCert)
{
AfxMessageBox( "Error Getting Signer Cert");
return;
}
//-------------------------------------------------------------
// Initialize the Algorithm Identifier structure.
//-------------------------------------------------------------
DWORD HashAlgSize;
CRYPT_ALGORITHM_IDENTIFIER HashAlgorithm;
HashAlgSize = sizeof(HashAlgorithm);
memset(&HashAlgorithm, 0, HashAlgSize); // Init. to zero.
HashAlgorithm.pszObjId = szOID_RSA_MD5; // Then set the
// necessary field.
//-------------------------------------------------------------
// Initialize the CMSG_SIGNER_ENCODE_INFO structure.
//-------------------------------------------------------------
CMSG_SIGNER_ENCODE_INFO SignerEncodeInfo;
memset(&SignerEncodeInfo, 0, sizeof(CMSG_SIGNER_ENCODE_INFO));
SignerEncodeInfo.cbSize = sizeof(CMSG_SIGNER_ENCODE_INFO);
SignerEncodeInfo.pCertInfo = pSignerCert->pCertInfo;
SignerEncodeInfo.hCryptProv = hCryptProv;
SignerEncodeInfo.dwKeySpec = AT_SIGNATURE;
SignerEncodeInfo.HashAlgorithm = HashAlgorithm;
SignerEncodeInfo.pvHashAuxInfo = NULL;
// Create an array of one. NOTE: Currently, there can be only one
// signer.
CMSG_SIGNER_ENCODE_INFO SignerEncodeInfoArray[1];
SignerEncodeInfoArray[0] = SignerEncodeInfo;
//-------------------------------------------------------------
// Initialize the CMSG_SIGNED_ENCODE_INFO structure.
//-------------------------------------------------------------
CERT_BLOB SignerCertBlob;
SignerCertBlob.cbData = pSignerCert->cbCertEncoded;
SignerCertBlob.pbData = pSignerCert->pbCertEncoded;
// Create an array of one. Only one certificate is included.
CERT_BLOB SignerCertBlobArray[1];
SignerCertBlobArray[0] = SignerCertBlob;
CMSG_SIGNED_ENCODE_INFO SignedMsgEncodeInfo;
memset(&SignedMsgEncodeInfo, 0, sizeof(CMSG_SIGNED_ENCODE_INFO));
SignedMsgEncodeInfo.cbSize = sizeof(CMSG_SIGNED_ENCODE_INFO);
SignedMsgEncodeInfo.cSigners = 1;
SignedMsgEncodeInfo.rgSigners = SignerEncodeInfoArray;
SignedMsgEncodeInfo.cCertEncoded = 1;
SignedMsgEncodeInfo.rgCertEncoded = SignerCertBlobArray;
SignedMsgEncodeInfo.rgCrlEncoded = NULL;
//-------------------------------------------------------------
// Get the size of the encoded message blob.
//-------------------------------------------------------------
DWORD cbEncodedBlob;
BYTE *pbEncodedBlob = NULL;
cbEncodedBlob = CryptMsgCalculateEncodedLength(
MY_ENCODING_TYPE, // Msg encoding type
0, // Flags
CMSG_SIGNED, // Msg type
&SignedMsgEncodeInfo, // Ptr. to struct.
NULL, // Inner content ObjID
cbContent); // size of content
if(0 == cbEncodedBlob)
{
AfxMessageBox("Getting cbEncodedBlob length failed");
return;
}
//-------------------------------------------------------------
// Allocate memory for the encoded blob.
//-------------------------------------------------------------
pbEncodedBlob = (BYTE *) malloc(cbEncodedBlob);
if(NULL == pbEncodedBlob)
{
AfxMessageBox("Malloc failed");
return;
}
//-------------------------------------------------------------
// Open a message to encode.
//-------------------------------------------------------------
HCRYPTMSG hMsg = NULL;
hMsg = CryptMsgOpenToEncode(
MY_ENCODING_TYPE, // Encoding type
0, // Flags
CMSG_SIGNED, // Msg type
&SignedMsgEncodeInfo, // Pointer to structure
NULL, // Inner content ObjID
NULL); // Stream Info (not used)
if(NULL == hMsg)
{
AfxMessageBox("OpenToEncode failed");
free(pbEncodedBlob);
return;
}
//-------------------------------------------------------------
// Update the message with the data.
//-------------------------------------------------------------
BOOL fResult;
fResult = CryptMsgUpdate(
hMsg, // Handle to the message
pbContent, // Pointer to the content
cbContent, // Size of the content
TRUE); // Last call
if(!fResult)
{
AfxMessageBox("MsgUpdate failed");
free(pbEncodedBlob);
return;
}
//-------------------------------------------------------------
// Get the resultant message.
//-------------------------------------------------------------
fResult = CryptMsgGetParam(
hMsg, // Handle to the message
CMSG_CONTENT_PARAM, // Parameter type
0, // Index
pbEncodedBlob, // Pointer to the blob
&cbEncodedBlob); // Size of the blob
if(!fResult)
{
AfxMessageBox("MsgGetParam failed");
free(pbEncodedBlob);
CryptMsgClose(hMsg);
return;
}else
AfxMessageBox("Message encoded sucessfully");
// If the call was successful, pbEncodedBlob now points to the
// encoded, signed content.
CertFreeCertificateContext(pSignerCert);
CertCloseStore(hStoreHandle, CERT_CLOSE_STORE_FORCE_FLAG);
CryptMsgClose(hMsg);
//=============================================================
// EXAMPLE CODE FOR DECODING A SIGNED MESSGE
//=============================================================
//-------------------------------------------------------------
// Open a message for decoding.
//-------------------------------------------------------------
hMsg = CryptMsgOpenToDecode(
MY_ENCODING_TYPE, // Encoding type
0, // Flags
0, // Message type (get from msg.)
hCryptProv, // Cryptographic provider
NULL, // Recipient info
NULL); // Stream info
if(NULL == hMsg)
{
AfxMessageBox("OpenToDecode failed");
free(pbEncodedBlob);
return;
}
//-------------------------------------------------------------
// Update the message with the encoded blob, using the data
// that was encoded in the previous section of code.
//-------------------------------------------------------------
fResult = CryptMsgUpdate(
hMsg, // Handle to the message
pbEncodedBlob, // Pointer to the encoded blob
cbEncodedBlob, // Size of the encoded blob
TRUE); // Last call
if(!fResult)
{
AfxMessageBox("Decode MsgUpdate failed");
free(pbEncodedBlob);
return;
}
//-------------------------------------------------------------
// Get the message type.
//-------------------------------------------------------------
DWORD cbData = sizeof(DWORD);
DWORD dwMsgType = 0;
fResult = CryptMsgGetParam(
hMsg, // Handle to the message
CMSG_TYPE_PARAM, // Parameter type
0, // Index
&dwMsgType, // Address for returned info
&cbData); // Size of the returned info
if(!fResult)
{
AfxMessageBox("Decode CMSG_TYPE_PARAM failed");
free(pbEncodedBlob);
return;
}
// Some applications may need to use a "switch" statement here
// and process the message differently depending on the
// message type.
if(dwMsgType != CMSG_SIGNED)
{
AfxMessageBox("Wrong message type");
free(pbEncodedBlob);
return;
}
//-------------------------------------------------------------
// Get the size of the content.
//-------------------------------------------------------------
DWORD cbDecoded = 0;
fResult = CryptMsgGetParam(
hMsg, // Handle to the message
CMSG_CONTENT_PARAM, // Parameter type
0, // Index
NULL, // Address for returned info
&cbDecoded); // Size of the returned info
if(!fResult)
{
AfxMessageBox("Decode CMSG_CONTENT_PARAM failed");
free(pbEncodedBlob);
return;
}
//-------------------------------------------------------------
// Allocate Memory.
//-------------------------------------------------------------
BYTE *pbDecoded = NULL;
pbDecoded = (BYTE *) malloc(cbDecoded);
if(NULL == pbDecoded)
{
AfxMessageBox("Decode memory allocation failed");
free(pbEncodedBlob);
return;
}
//-------------------------------------------------------------
// Get a pointer to the content.
//-------------------------------------------------------------
LPSTR pDecodedString;
fResult = CryptMsgGetParam(
hMsg, // Handle to the message
CMSG_CONTENT_PARAM, // Parameter type
0, // Index
pbDecoded, // Address for returned info
&cbDecoded); // Size of the returned info
if(!fResult)
{
AfxMessageBox("Decode CMSG_CONTENT_PARAM #2 failed");
free(pbEncodedBlob);
free(pbDecoded);
return;
}else
AfxMessageBox("Message decoded successfully");
pDecodedString = (LPSTR) pbDecoded;
AfxMessageBox(pDecodedString);
//=============================================================
// Verify the signature.
//=============================================================
//-------------------------------------------------------------
// Get the signer CERT_INFO from the message.
//-------------------------------------------------------------
// Get the size of memory required.
DWORD cbSignerCertInfo = 0;
fResult = CryptMsgGetParam(
hMsg, // Handle to the message
CMSG_SIGNER_CERT_INFO_PARAM, // Parameter type
0, // Index
NULL, // Address for returned info
&cbSignerCertInfo); // Size of the returned info
if(!fResult)
{
AfxMessageBox("Verify SIGNER_CERT_INFO #1 failed");
free(pbEncodedBlob);
free(pbDecoded);
return;
}
// Allocate memory.
PCERT_INFO pSignerCertInfo = NULL;
pSignerCertInfo = (PCERT_INFO) malloc(cbSignerCertInfo);
if(NULL == pSignerCertInfo)
{
AfxMessageBox("Verify memory allocation failed");
free(pbEncodedBlob);
free(pbDecoded);
return;
}
// Get the message certificate information (CERT_INFO
// structure).
fResult = CryptMsgGetParam(
hMsg, // Handle to the message
CMSG_SIGNER_CERT_INFO_PARAM, // Parameter type
0, // Index
pSignerCertInfo, // Address for returned info
&cbSignerCertInfo); // Size of the returned info
if(!fResult)
{
AfxMessageBox("Verify SIGNER_CERT_INFO #2 failed");
free(pbEncodedBlob);
free(pbDecoded);
free(pSignerCertInfo);
return;
}
//-------------------------------------------------------------
// Open a certificate store in memory using CERT_STORE_PROV_MSG,
// which initializes it with the certificates from the message.
//-------------------------------------------------------------
HCERTSTORE hTempStore;
hTempStore = CertOpenStore(
CERT_STORE_PROV_MSG, // Store Provider type
MY_ENCODING_TYPE, // Encoding type
hCryptProv, // Cryptographic provider
0, // Flags
hMsg); // Handle to the message
if(NULL == hTempStore)
{
AfxMessageBox("Verify OpenStore failed");
free(pbEncodedBlob);
free(pbDecoded);
free(pSignerCertInfo);
return;
}
//-------------------------------------------------------------
// Find the signer's certificate in the store.
//-------------------------------------------------------------
PCCERT_CONTEXT pSignerCertContext = NULL;
pSignerCertContext = CertGetSubjectCertificateFromStore(
hTempStore, // Handle to store
MY_ENCODING_TYPE, // Encoding type
pSignerCertInfo); // Pointer to
// retrieved
// CERT_CONTEXT
if(NULL == pSignerCertContext)
{
AfxMessageBox("Verify GetSubjectCert failed");
free(pbEncodedBlob);
free(pbDecoded);
free(pSignerCertInfo);
CertCloseStore(hTempStore, CERT_CLOSE_STORE_FORCE_FLAG);
return;
}
//-------------------------------------------------------------
// Use the CERT_INFO from the signer's certificate to verify
// the signature.
//-------------------------------------------------------------
PCERT_INFO pSignerCertificateInfo = pSignerCertContext->pCertInfo;
fResult = CryptMsgControl(
hMsg, // Handle to the message
0, // Flags
CMSG_CTRL_VERIFY_SIGNATURE, // Control type
pSignerCertificateInfo); // Pointer to the
// retrieved CERT_INFO
if(!fResult)
{
AfxMessageBox("Verify Signature failed");
free(pbEncodedBlob);
free(pbDecoded);
free(pSignerCertInfo);
CertCloseStore(hTempStore, CERT_CLOSE_STORE_FORCE_FLAG);
return;
}
else
AfxMessageBox("Verify Signature Succeeded");
//-------------------------------------------------------------
// Clean up.
//-------------------------------------------------------------
free(pbEncodedBlob);
free(pbDecoded);
free(pSignerCertInfo);
CertCloseStore(hTempStore, CERT_CLOSE_STORE_FORCE_FLAG);
CryptMsgClose(hMsg);
}
//************************************************************
// GetMySignerCert() - A function to list all the certs in the
// store and get a handle to one of them.
//************************************************************
PCCERT_CONTEXT GetMySignerCert(HCERTSTORE hCertStore)
{
PCCERT_CONTEXT pPrevCertContext = NULL;
PCCERT_CONTEXT pCertContext = NULL;
BOOL fReturnFlag = FALSE;
BOOL fTestFlag;
DWORD dwSize = NULL;
KEY_PROV_INFO* pKeyInfo = NULL;
while(TRUE)
{
pCertContext = CertEnumCertificatesInStore(
hCertStore,
pPrevCertContext);
if(!pCertContext)
{
fTestFlag = FALSE;
break;
}
else
{
// For clarity, this code searches for the first
// occurrence of a certificate containing an exchange key.
// This works here, as there is only one pair of
// certificates in the store. Normally you would search
// for a name as well as the key type.
// Call CertGetCertificateContextProperty to get the
// structure size.
fReturnFlag = CertGetCertificateContextProperty(
pCertContext,
CERT_KEY_PROV_INFO_PROP_ID,
NULL,
&dwSize);
if(FALSE == fReturnFlag)
AfxMessageBox("Error Getting Key Property");
// Allocate memory for the returned structure.
if(pKeyInfo)
free(pKeyInfo);
pKeyInfo = (KEY_PROV_INFO*)malloc(dwSize);
if(!pKeyInfo)
AfxMessageBox("Error Allocating Memory");
// Get the key info structure.
fReturnFlag = CertGetCertificateContextProperty(
pCertContext,
CERT_KEY_PROV_INFO_PROP_ID,
pKeyInfo,
&dwSize);
// Does it have a signature key?
if(pKeyInfo->dwKeySpec == AT_SIGNATURE)
{
AfxMessageBox("Signature Cert Found");
fTestFlag = TRUE;
break;
}
}
pPrevCertContext = pCertContext;
}
CertFreeCertificateContext(pPrevCertContext);
if(pKeyInfo)
free(pKeyInfo);
if(FALSE == fTestFlag)
{
AfxMessageBox("Signature Cert Not Found");
return NULL;
}else
return (pCertContext);
}