Example C Program: Encoding an Enveloped, Signed Message

The following example creates, signs, and envelopes a message, and it illustrates the following tasks and CryptoAPI functions:

This example will fail if a usable private key does not exist in the default key container. If the needed private key is not available, code using CryptAcquireCertificatePrivateKey, as demonstrated in the code sample Example C Program: Sending and Receiving a Signed and Encrypted Message, can be used.

This example uses the function MyHandleError. The code for this function is included with the sample. Code for this and other auxiliary functions is also listed under General Purpose Functions.


#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)
{
//-------------------------------------------------------------------
// Copyright (C) Microsoft.  All rights reserved.
// Declare and initialize variables.

BYTE* pbContent = (BYTE*) "Security is our only business.";
                                    // a byte pointer
DWORD cbContent = strlen((char *) pbContent)+1;
                                    // the size of the message
HCERTSTORE hStoreHandle;
HCRYPTPROV hCryptProv;
PCCERT_CONTEXT pSignerCert;         // signer's certificate
PCCERT_CONTEXT pRecipCert;          // receiver's certificate
LPWSTR pswzRecipientName = L"Hortense";
LPWSTR pswzCertSubject = L"Full Test Cert";
PCERT_INFO RecipCertArray[1];
DWORD ContentEncryptAlgSize;
CRYPT_ALGORITHM_IDENTIFIER ContentEncryptAlgorithm;
CMSG_ENVELOPED_ENCODE_INFO EnvelopedEncodeInfo;
DWORD cbEncodedBlob;
BYTE *pbEncodedBlob;
DWORD cbSignedBlob;
BYTE *pbSignedBlob;
HCRYPTMSG hMsg;
DWORD HashAlgSize;
CRYPT_ALGORITHM_IDENTIFIER HashAlgorithm;
CMSG_SIGNER_ENCODE_INFO SignerEncodeInfo;
CERT_BLOB SignerCertBlob;
CERT_BLOB SignerCertBlobArray[1];
CMSG_SIGNER_ENCODE_INFO SignerEncodeInfoArray[1];
CMSG_SIGNED_ENCODE_INFO SignedMsgEncodeInfo;

//-------------------------------------------------------------------
// Begin processing. Display the original message.

printf("The original message => %s\n",pbContent);  

//-------------------------------------------------------------------
// Acquire a cryptographic provider. 

if(CryptAcquireContext(
        &hCryptProv,      // address for handle to be returned
        NULL,             // use the current user's logon name
        NULL,             // use the default provider
        PROV_RSA_FULL,    // provider type
        0))               // zero allows access to private keys
{
    printf("Context CSP acquired. \n");
}
else
{
   if ( GetLastError() == NTE_BAD_KEYSET)
   {
        printf("A Usable private key was not found \n");
        printf("in the default key container. Either a \n");
        printf("private key must be generated in that container \n");
        printf("or CryptAquireCertificatePrivateKey can be used \n");
        printf("to gain access to the needed private key.");
   }
    MyHandleError("CryptAcquireContext failed.");
}
//-------------------------------------------------------------------
// Open the My system certificate store.

if(hStoreHandle = CertOpenStore(
       CERT_STORE_PROV_SYSTEM,
       0,
       NULL,
       CERT_SYSTEM_STORE_CURRENT_USER,
       L"MY"))
{
    printf("The MY system store is open. \n");
}
else
{
    MyHandleError( "Error getting store handle.");
}
//-------------------------------------------------------------------
// Get the signer's certificate. This certificate must be in the
// My store, and its private key must be available.

if(pSignerCert = CertFindCertificateInStore(
   hStoreHandle,
   MY_ENCODING_TYPE,
   0,
   CERT_FIND_SUBJECT_STR,
   pswzCertSubject,
   NULL))
{
    printf("Found certificate for %S.\n",pswzCertSubject);
}
else
{
    MyHandleError("Signer certificate not found.");
}
//-------------------------------------------------------------------
// Initialize the algorithm identifier structure.

HashAlgSize = sizeof(HashAlgorithm);
memset(&HashAlgorithm, 0, HashAlgSize);    // initialize to zero
HashAlgorithm.pszObjId = szOID_RSA_MD5;    // initialize the 
                                           // necessary member
//-------------------------------------------------------------------
// Initialize the CMSG_SIGNER_ENCODE_INFO structure.

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_KEYEXCHANGE;
SignerEncodeInfo.HashAlgorithm = HashAlgorithm;
SignerEncodeInfo.pvHashAuxInfo = NULL;

//-------------------------------------------------------------------
// Create an array of one. 
// Note: The current program is set up for only a single signer.

SignerEncodeInfoArray[0] = SignerEncodeInfo;

//-------------------------------------------------------------------
// Initialize the CMSG_SIGNED_ENCODE_INFO structure.

SignerCertBlob.cbData = pSignerCert->cbCertEncoded;
SignerCertBlob.pbData = pSignerCert->pbCertEncoded;

//-------------------------------------------------------------------
// Initialize the array of one CertBlob.

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;
SignedMsgEncodeInfo.rgCrlEncoded = NULL;

//-------------------------------------------------------------------
// Get the size of the encoded, signed message BLOB.

if(cbSignedBlob = CryptMsgCalculateEncodedLength(
    MY_ENCODING_TYPE,       // message encoding type
    0,                      // flags
    CMSG_SIGNED,            // message type
    &SignedMsgEncodeInfo,   // pointer to structure
    NULL,                   // inner content OID
    cbContent))             // size of content
{
    printf("%d, the length of data calculated. \n",cbSignedBlob);
}
else
{
   if ( GetLastError() == NTE_BAD_KEYSET)
   {
        printf("A Usable private key was not found \n");
        printf("in the default key container. Either a \n");
        printf("private key must be generated in that container \n");
        printf("or CryptAquireCertificatePRivateKey can be used \n");
        printf("to gain access to the needed private key.");
    }
    MyHandleError("Getting cbSignedBlob length failed.");
}
//-------------------------------------------------------------------
// Allocate memory for the encoded BLOB.

if(pbSignedBlob = (BYTE *) malloc(cbSignedBlob))
{
   printf("Memory has been allocated for the signed message. \n");
}
else
{
   MyHandleError("Memory allocation failed.");
}
//-------------------------------------------------------------------
// 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
    NULL))                   // stream information (not used)
{
    printf("The message to be encoded has been opened. \n");
}
else
{
    MyHandleError("OpenToEncode failed.");
}
//-------------------------------------------------------------------
// Update the message with the data.

if(CryptMsgUpdate(
     hMsg,         // handle to the message
     pbContent,    // pointer to the content
     cbContent,    // size of the content
     TRUE))        // last call
{
    printf("Content has been added to the encoded message. \n");
}
else
{
    MyHandleError("MsgUpdate failed.");
}
//-------------------------------------------------------------------
// Get the resulting message.

if(CryptMsgGetParam(
    hMsg,                      // handle to the message
    CMSG_CONTENT_PARAM,        // parameter type
    0,                         // index
    pbSignedBlob,              // pointer to the BLOB
    &cbSignedBlob))            // size of the BLOB
{
    printf("Message encoded successfully. \n");
}
else
{
    MyHandleError("MsgGetParam failed.");
}
//-------------------------------------------------------------------
// pbSignedBlob now points to the encoded, signed content.

//-------------------------------------------------------------------
// Get a pointer to the recipient certificate.
// For this program, the recipient's certificate must also be in the
// My store. At this point, only the recipient's public key is needed.
// To open the enveloped message, however, the recipient's 
// private key must also be available.

if(pRecipCert=CertFindCertificateInStore(      
      hStoreHandle,
      MY_ENCODING_TYPE,            // use X509_ASN_ENCODING
      0,                           // no dwFlags needed
      CERT_FIND_SUBJECT_STR,       // find a certificate with a
                                   // subject that matches the 
                                   // string in the next parameter
      pswzRecipientName,           // the Unicode string to be found
                                   // in a certificate's subject
      NULL))                       // NULL for the first call to the
                                   // function
                                   // in all subsequent
                                   // calls, it is the last pointer
                                   // returned by the function
{
     printf("Certificate for %S found. \n", pswzRecipientName);
}
else
{
     MyHandleError("Could not find the countersigner's "
         "certificate.");
}
//-------------------------------------------------------------------
// Initialize the first element of the array of CERT_INFOs. 
// In this example, there is only a single recipient.

RecipCertArray[0] = pRecipCert->pCertInfo;

//-------------------------------------------------------------------
// Initialize the symmetric-encryption algorithm identifier
// structure.

ContentEncryptAlgSize = sizeof(ContentEncryptAlgorithm);
memset(&ContentEncryptAlgorithm, 
       0,
       ContentEncryptAlgSize);               // initialize to zero

//-------------------------------------------------------------------
// Initialize the necessary members. This particular OID does not
// need any parameters. Some OIDs, however, will require that
// the other members be initialized.

ContentEncryptAlgorithm.pszObjId = szOID_RSA_RC4;

//-------------------------------------------------------------------
// Initialize the CMSG_ENVELOPED_ENCODE_INFO structure.

memset(&EnvelopedEncodeInfo, 
       0,
       sizeof(CMSG_ENVELOPED_ENCODE_INFO));
EnvelopedEncodeInfo.cbSize = sizeof(CMSG_ENVELOPED_ENCODE_INFO);
EnvelopedEncodeInfo.hCryptProv = hCryptProv;
EnvelopedEncodeInfo.ContentEncryptionAlgorithm = 
                                        ContentEncryptAlgorithm;
EnvelopedEncodeInfo.pvEncryptionAuxInfo = NULL;
EnvelopedEncodeInfo.cRecipients = 1;
EnvelopedEncodeInfo.rgpRecipients = RecipCertArray;

//-------------------------------------------------------------------
// Get the size of the encoded message BLOB.

if(cbEncodedBlob = CryptMsgCalculateEncodedLength(
     MY_ENCODING_TYPE,        // message encoding type
     0,                       // flags
     CMSG_ENVELOPED,          // message type
     &EnvelopedEncodeInfo,    // pointer to structure
     szOID_RSA_signedData,    // inner content OID
     cbSignedBlob))           // size of content
{
    printf("Length of the encoded BLOB will be %d.\n",cbEncodedBlob);
}
else
{
    MyHandleError("Getting enveloped cbEncodedBlob length failed.");
}
//--------------------------------------------------------------------
// Allocate memory for the encoded BLOB.

if(pbEncodedBlob = (BYTE *) malloc(cbEncodedBlob))
{
    printf("Memory has been allocated for the BLOB. \n");
}
else
{
    MyHandleError("Enveloped malloc operation failed.");
}
//-------------------------------------------------------------------
// Open a message to encode.

if(hMsg = CryptMsgOpenToEncode(
     MY_ENCODING_TYPE,        // encoding type
     0,                       // flags
     CMSG_ENVELOPED,          // message type
     &EnvelopedEncodeInfo,    // pointer to structure
     szOID_RSA_signedData,    // inner content OID
     NULL))                   // stream information (not used)
{
    printf("The message to encode is open. \n");
}
else
{
    MyHandleError("Enveloped OpenToEncode failed.");
}
//-------------------------------------------------------------------
// Update the message with the data.

if(CryptMsgUpdate(
     hMsg,              // handle to the message
     pbSignedBlob,      // pointer to the signed data BLOB
     cbSignedBlob,      // size of the data BLOB
     TRUE))             // last call
{
    printf("The signed BLOB has been added to the message. \n");
}
else
{
    MyHandleError("Enveloped MsgUpdate failed.");
}
//-------------------------------------------------------------------
// Get the resulting message.

if(CryptMsgGetParam(
     hMsg,                  // handle to the message
     CMSG_CONTENT_PARAM,    // parameter type
     0,                     // index
     pbEncodedBlob,         // pointer to the enveloped,
                            // signed data BLOB
     &cbEncodedBlob))       // size of the BLOB
{
    printf("Enveloped message encoded successfully. \n");
}
else
{
    MyHandleError("Enveloped MsgGetParam failed.");
}
//-------------------------------------------------------------------
// Clean up.

CertFreeCertificateContext(pRecipCert);
if(CertCloseStore(
     hStoreHandle,
     CERT_CLOSE_STORE_CHECK_FLAG))
{
  printf("The certificate store closed without a certificate "
      "left open. \n");
}
else
{
  printf("The store closed but a certificate was still open. \n");
}
if(hMsg)
   CryptMsgClose(hMsg);
if(hCryptProv) 
   CryptReleaseContext(hCryptProv,0);
} // end main

//-------------------------------------------------------------------
// This example uses the function MyHandleError, a simple error
// handling function to print an error message 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 MyHandleError


 

 

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