Example C Program: Encoding and Decoding Data
The following example encodes and decodes simple, general data, and illustrates the following tasks and CryptoAPI functions.
- Determining the length needed for the buffer to hold the encoded data using CryptMsgCalculateEncodedLength.
- Opening a message for encoding using CryptMsgOpenToEncode.
- Adding content to the encoded message using CryptMsgUpdate.
- Copying the encoded message into a buffer using CryptMsgGetParam.
- Closing the encoded message using CryptMsgClose.
- Opening a message to decode using CryptMsgOpenToDecode.
- Using CryptMsgUpdate and CryptMsgGetParam to get the decoded data.
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.
//-------------------------------------------------------------------
// Copyright (C) Microsoft. All rights reserved.
// Example of encoding and decoding a message.
#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. This includes getting a pointer
// to the message content. This sample program creates the message
// content and gets a pointer to it. In most situations,
// the content will exist somewhere and a pointer to it
// will get passed to the application.
HCRYPTMSG hMsg;
BYTE* pbContent; // a byte pointer to the message
DWORD cbContent; // the size of message
DWORD cbEncodedBlob;
BYTE *pbEncodedBlob;
//-------------------------------------------------------------------
// The following variables are used only in the decoding phase.
DWORD cbDecoded;
BYTE *pbDecoded;
//-------------------------------------------------------------------
// Begin processing. Display the original message.
pbContent = (BYTE*) "Security is our only business";
cbContent = strlen((char *) pbContent)+1;
printf("The original message => %s\n",pbContent);
//-------------------------------------------------------------------
// Get the size of the encoded message BLOB.
if(cbEncodedBlob = CryptMsgCalculateEncodedLength(
MY_ENCODING_TYPE, // message encoding type
0, // flags
CMSG_DATA, // message type
NULL, // pointer to structure
NULL, // inner content object ID
cbContent)) // size of content
{
printf("The length of the data has been calculated. \n");
}
else
{
MyHandleError("Getting cbEncodedBlob length failed");
}
//-------------------------------------------------------------------
// Allocate memory for the encoded BLOB.
if(pbEncodedBlob = (BYTE *) malloc(cbEncodedBlob))
{
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_DATA, // message type
NULL, // pointer to structure
NULL, // inner content object ID
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_BARE_CONTENT_PARAM, // parameter type
0, // index
pbEncodedBlob, // pointer to the BLOB
&cbEncodedBlob)) // size of the BLOB
{
printf("Message encoded successfully. \n");
}
else
{
MyHandleError("MsgGetParam failed");
}
//-------------------------------------------------------------------
// pbEncodedBlob now points to the encoded, signed content.
//-------------------------------------------------------------------
// Close the message.
if(hMsg)
CryptMsgClose(hMsg);
//-------------------------------------------------------------------
// The following code decodes a message.
// This code may be included here or could be used
// in a stand-alone program if the message
// to be decoded and its size were input.
// The encoded message BLOB and its length could be read
// from a disk file or could be extracted from an email message
// or other input source.
//-------------------------------------------------------------------
// Open a message for decoding.
if(hMsg = CryptMsgOpenToDecode(
MY_ENCODING_TYPE, // encoding type.
0, // flags.
CMSG_DATA, // look for a data message.
NULL, // cryptographic provider.
NULL, // recipient information.
NULL)) // stream information.
{
printf("The message to decode is open. \n");
}
else
{
MyHandleError("OpenToDecode failed");
}
//-------------------------------------------------------------------
// Update the message with an encoded BLOB.
// Both pbEncodedBlob, the encoded data,
// and cbEncodedBlob, the length of the encoded data,
// must be available.
printf("\nThe length of the encoded message is %d.\n\n",
cbEncodedBlob);
if(CryptMsgUpdate(
hMsg, // handle to the message
pbEncodedBlob, // pointer to the encoded BLOB
cbEncodedBlob, // size of the encoded BLOB
TRUE)) // last call
{
printf("The encoded BLOB has been added to the message. \n");
}
else
{
MyHandleError("Decode MsgUpdate failed");
}
//-------------------------------------------------------------------
// Get the size of the content.
if(CryptMsgGetParam(
hMsg, // handle to the message
CMSG_CONTENT_PARAM, // parameter type
0, // index
NULL, // address for returned
// information
&cbDecoded)) // size of the returned
// information
{
printf("The decoded message size is %d. \n", cbDecoded);
}
else
{
MyHandleError("Decode CMSG_CONTENT_PARAM failed");
}
//-------------------------------------------------------------------
// Allocate memory.
if(pbDecoded = (BYTE *) malloc(cbDecoded))
{
printf("Memory has been allocated for the decoded message.\n");
}
else
{
MyHandleError("Decoding memory allocation failed.");
}
//-------------------------------------------------------------------
// Get a pointer to the content.
if(CryptMsgGetParam(
hMsg, // handle to the message
CMSG_CONTENT_PARAM, // parameter type
0, // index
pbDecoded, // address for returned
// information
&cbDecoded)) // size of the returned
// information
{
printf("The message is %s.\n",(LPSTR)pbDecoded);
}
else
{
MyHandleError("Decode CMSG_CONTENT_PARAM #2 failed");
}
//-------------------------------------------------------------------
// Clean up.
if(pbEncodedBlob)
free(pbEncodedBlob);
if(pbDecoded)
free(pbDecoded);
if(hMsg)
CryptMsgClose(hMsg);
printf("This program ran to completion without error. \n");
} // End of 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)
{
printf("An error occurred in running the program.\n");
printf("%s\n",s);
printf("Error number %x\n.",GetLastError());
printf("Program terminating.\n");
exit(1);
}
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