Example C Program: Encoding and Decoding a Message Using a Stream
The following example demonstrates how to use the CryptMsgOpenToEncode, CryptMsgOpenToDecode, and CryptMsgUpdate functions with the CMSG_STREAM_INFO structure to encode and decode a message using the streaming features of these functions.
Signing and encoding a message does not ensure privacy for that message. Rather it ensures the authenticity of the message. Because the message is signed with the sender's private key, when the receiver of the message decrypts the signature with the sender's public key (available from the certificate that is sent along with the message), the receiver can be sure that the message was sent by the person or entity associated with the certificate and that the message was not changed after it was signed.
This encoding signing portion of this example illustrates the following tasks and CryptoAPI functions:
- Opening a certificate store by using CertOpenStore.
- Retrieving a certificate with a specific subject name by using CertFindCertificateInStore.
- Getting and printing a certificate's subject name by using CertGetNameString.
- Getting the handle to a cryptographic provider that can provide a private key with the CryptAcquireCertificatePrivateKey function.
- Initializing the CMSG_SIGNED_ENCODE_INFO and CMSG_STREAM_INFO structures to be used in a call to CryptMsgOpenToEncode.
- Signing and encoding a message by using CryptMsgOpenToEncode and CryptMsgUpdate.
- Implementing a stream callback function that can save an encoded and signed message in any persistent format, such as writing it to a file.
The decoding portion of this example illustrates the following tasks and CryptoAPI functions:
- Initializing a CMSG_STREAM_INFO structure to be used in a call to CryptMsgOpenToDecode.
- Implementing a stream callback function that can save a decoded message in any persistent format, such as printing it to the screen.
- Reading an encoded message from a file and decoding the message by using CryptMsgUpdate.
For an example of how to perform these same operations without using a stream callback, see Example C Program: Signing, Encoding, Decoding, and Verifying a Message.
This example uses the function MyHandleError. Code for this function is included with the sample. Code for this and other auxiliary functions is also listed under General_Purpose_Functions.
#include <stdio.h>
#include <tchar.h>
#include <windows.h>
#include <wincrypt.h>
#pragma comment(lib, "crypt32.lib")
// Link with the Crypt32.lib file.
#pragma comment (lib, "Crypt32")
#define MY_ENCODING_TYPE (PKCS_7_ASN_ENCODING | X509_ASN_ENCODING)
#define MAX_NAME 256
#define ENCODED_FILE_NAME L"testStream.p7s"
//-------------------------------------------------------------------
// Define function MyHandleError.
void MyHandleError(LPTSTR psz)
{
_ftprintf(stderr, TEXT("An error occurred in the program. \n"));
_ftprintf(stderr, TEXT("%s\n"), psz);
_ftprintf(stderr, TEXT("Error number %x.\n"), GetLastError());
_ftprintf(stderr, TEXT("Program terminating. \n"));
exit(1);
} // End of MyHandleError.
//+----------------------------------------------
// Callback function used for streamed Signing.
//-----------------------------------------------
BOOL
WINAPI
EncodeCallback(
const void *pvArg,
BYTE *pbData,
DWORD cbData,
BOOL fFinal)
{
DWORD dwWrittenBytes = 0;
HANDLE hFileToWrite = INVALID_HANDLE_VALUE;
hFileToWrite = *((HANDLE *)pvArg);
if ( !WriteFile(
hFileToWrite,
pbData,
cbData,
&dwWrittenBytes,
NULL) ||
(dwWrittenBytes != cbData))
{
return FALSE;
}
return TRUE;
}
//+----------------------------------------------
// Callback function used for decoding streamed Signing.
//-----------------------------------------------
BOOL
WINAPI
DecodeCallback(
const void *pvArg,
BYTE *pbData,
DWORD cbData,
BOOL fFinal)
{
if (pbData != NULL && cbData > 0)
{
*(pbData+cbData) = 0;
printf("%s", (char*)pbData);
}
return TRUE;
}
void EncodeMessageWithStream(LPWSTR pwszSignerName)
{
//---------------------------------------------------------------
// Declare and initialize variables. This includes declaring and
// initializing a pointer to message content to be countersigned
// and encoded. Usually, the message content will exist somewhere
// and a pointer to it is passed to the application.
BYTE* pbContent1 = (BYTE*)"First sentence. ";
DWORD cbContent1 = lstrlenA((char *)pbContent1);
BYTE* pbContent2 = (BYTE*)"Second sentence. ";
DWORD cbContent2 = lstrlenA((char *)pbContent2);
HCRYPTPROV hCryptProv; // CSP handle
HCERTSTORE hStoreHandle; // store handle
PCCERT_CONTEXT pSignerCert; // signer certificate
CMSG_SIGNER_ENCODE_INFO SignerEncodeInfo;
CMSG_SIGNER_ENCODE_INFO SignerEncodeInfoArray[1];
CERT_BLOB SignerCertBlob;
CERT_BLOB SignerCertBlobArray[1];
CMSG_SIGNED_ENCODE_INFO SignedMsgEncodeInfo;
HCRYPTMSG hMsg;
LPWSTR pszNameString;
DWORD dwKeySpec;
//---------------------------------------------------------------
// Open the My system certificate store.
if(!(hStoreHandle = CertOpenStore(
// The system store will be a virtual store.
CERT_STORE_PROV_SYSTEM,
// Encoding type not needed with this PROV.
0,
// Accept the default HCRYPTPROV.
NULL,
CERT_SYSTEM_STORE_CURRENT_USER,
// Set the system store location in the registry. Other
// predefined system stores could have been used, including
// trust, Ca, or root.
L"MY")))
{
MyHandleError(L"Could not open the MY system store.");
}
//---------------------------------------------------------------
// Get a pointer to a signer's signature certificate.
if(pSignerCert = CertFindCertificateInStore(
hStoreHandle,
MY_ENCODING_TYPE,
0,
CERT_FIND_SUBJECT_STR,
pwszSignerName,
NULL))
{
//-----------------------------------------------------------
// A certificate was found. Get and print the name of the
// subject of the certificate.
if(CertGetNameString(
pSignerCert,
CERT_NAME_SIMPLE_DISPLAY_TYPE,
0,
NULL,
pszNameString,
MAX_NAME) > 1)
{
printf("The message signer is %s \n",pszNameString);
}
else
{
MyHandleError(L"CertGetNameString failed.\n");
}
}
else
{
MyHandleError(L"Cert not found.\n");
}
//---------------------------------------------------------------
// Initialize the CMSG_SIGNER_ENCODE_INFO structure.
//---------------------------------------------------------------
// Get a handle to a cryptographic provider.
if(!(CryptAcquireCertificatePrivateKey(
pSignerCert,
0,
NULL,
&hCryptProv,
&dwKeySpec,
NULL)))
{
DWORD dwError = GetLastError();
if(NTE_BAD_PUBLIC_KEY == dwError)
{
printf("NTE_BAD_PUBLIC_KEY\n");
}
MyHandleError(L"CryptAcquireContext failed");
}
memset(&SignerEncodeInfo, 0, sizeof(CMSG_SIGNER_ENCODE_INFO));
SignerEncodeInfo.cbSize = sizeof(CMSG_SIGNER_ENCODE_INFO);
SignerEncodeInfo.pCertInfo = pSignerCert->pCertInfo;
SignerEncodeInfo.hCryptProv = hCryptProv;
SignerEncodeInfo.dwKeySpec = dwKeySpec;
SignerEncodeInfo.HashAlgorithm.pszObjId = szOID_RSA_MD5;
SignerEncodeInfo.pvHashAuxInfo = NULL;
//---------------------------------------------------------------
// Initialize the first element of an array of signers.
// Note: Currently, there is only one signer.
SignerEncodeInfoArray[0] = SignerEncodeInfo;
//---------------------------------------------------------------
// Initialize the CMSG_SIGNED_ENCODE_INFO structure.
SignerCertBlob.cbData = pSignerCert->cbCertEncoded;
SignerCertBlob.pbData = pSignerCert->pbCertEncoded;
//---------------------------------------------------------------
// Initialize the first element of an array of signer BLOBs.
// Note: In this program, there is only one signer BLOB used.
SignerCertBlobArray[0] = SignerCertBlob;
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;
// Fill the CMSG_STREAM_INFO structure.
CMSG_STREAM_INFO stStreamInfo;
// BER_ENCODING
stStreamInfo.cbContent = 0xffffffff;
// DER_ENCODING
// stStreamInfo.cbContent = cbContent;
stStreamInfo.pfnStreamOutput = EncodeCallback;
HANDLE hOutMsgFile = INVALID_HANDLE_VALUE;
hOutMsgFile = CreateFile(
ENCODED_FILE_NAME,
GENERIC_WRITE,
FILE_SHARE_WRITE,
NULL,
CREATE_ALWAYS,
FILE_ATTRIBUTE_NORMAL,
NULL);
if (INVALID_HANDLE_VALUE == hOutMsgFile)
{
MyHandleError(L"CreateFile (OUT MSG)");
}
stStreamInfo.pvArg = &hOutMsgFile;
//---------------------------------------------------------------
// Open a message to encode.
if(!(hMsg = CryptMsgOpenToEncode(
MY_ENCODING_TYPE, // encoding type
0, // flags
CMSG_SIGNED, // message type
&SignedMsgEncodeInfo, // pointer to structure
NULL, // inner content OID
&stStreamInfo))) // stream information
{
MyHandleError(L"OpenToEncode failed");
}
//---------------------------------------------------------------
// Update the message with the data.
if(!(CryptMsgUpdate(
hMsg, // handle to the message
pbContent1, // pointer to the content
cbContent1, // size of the content
FALSE))) // first call
{
MyHandleError(L"MsgUpdate failed");
}
if(!(CryptMsgUpdate(
hMsg, // handle to the message
pbContent2, // pointer to the content
cbContent2, // size of the content
TRUE))) // last call
{
MyHandleError(L"MsgUpdate failed");
}
//---------------------------------------------------------------
// The message is signed and encoded.
// Close the message handle and the certificate store.
CryptMsgClose(hMsg);
CertCloseStore(hStoreHandle, CERT_CLOSE_STORE_FORCE_FLAG);
CryptReleaseContext(hCryptProv,0);
CloseHandle(hOutMsgFile);
}
void DecodeMessageWithStream()
{
//---------------------------------------------------------------
// Open the message for decoding.
HCRYPTMSG hMsg;
// Fill the CMSG_STREAM_INFO structure.
CMSG_STREAM_INFO stStreamInfo2;
// BER_ENCODING
stStreamInfo2.cbContent = 0xffffffff;
stStreamInfo2.pfnStreamOutput = DecodeCallback;
if(!(hMsg = CryptMsgOpenToDecode(
MY_ENCODING_TYPE, // encoding type
0, // flags
0, // message type (get from message)
NULL, // cryptographic provider
// use NULL for the default provider
NULL, // recipient information
&stStreamInfo2))) // stream information
{
MyHandleError(L"OpenToDecode failed.");
}
HANDLE hInMsgFile = INVALID_HANDLE_VALUE;
hInMsgFile = CreateFile(
ENCODED_FILE_NAME,
GENERIC_READ,
FILE_SHARE_READ,
NULL,
OPEN_EXISTING,
FILE_ATTRIBUTE_NORMAL,
NULL);
if (INVALID_HANDLE_VALUE == hInMsgFile)
{
MyHandleError(L"CreateFile (IN MSG)");
}
const DWORD cbBytesToRead = 256;
byte pbEncodedBlob[cbBytesToRead];
DWORD cbBytesRead;
BOOL lastCall = FALSE;
while (ReadFile(
hInMsgFile,
pbEncodedBlob,
cbBytesToRead,
&cbBytesRead,
NULL))
{
if (cbBytesRead < cbBytesToRead)
{
lastCall = TRUE;
}
if(!(CryptMsgUpdate(
hMsg, // handle to the message
pbEncodedBlob, // pointer to the encoded BLOB
cbBytesRead, // size of the encoded BLOB
lastCall))) // last call
{
MyHandleError(L"Decode MsgUpdate failed.");
}
if (lastCall)
{
break;
}
}
CryptMsgClose(hMsg);
CloseHandle(hInMsgFile);
}