Beispiel-C-Programm: Signieren, Codieren, Decodieren und Überprüfen einer Nachricht
Im folgenden Beispiel wird das Signieren und Codieren einer Nachricht sowie das Decodieren einer signierten Nachricht und das Überprüfen der Signatur kombiniert. Die beiden Vorgänge befinden sich in der Regel in separaten Programmen. Im Codierungsbeispiel wird die codierte Nachricht erstellt, in einer Datenträgerdatei gespeichert oder auf andere Weise an einen anderen Benutzer gesendet. Das Decodierungsbeispiel würde die codierte Nachricht empfangen, decodieren und die Signatur überprüfen. Die beiden Prozesse wurden hier kombiniert, um zu zeigen, dass beide Verfahren funktionieren.
Das Signieren und Codieren einer Nachricht gewährleistet nicht den Datenschutz für diese Nachricht. Sie stellt vielmehr die Authentizität der Nachricht sicher. Da die Nachricht mit dem privaten Schlüssel des Absenders signiert ist und der Empfänger der Nachricht die Signatur mit dem öffentlichen Schlüssel des Absenders entschlüsselt (verfügbar über das Zertifikat, das zusammen mit der Nachricht gesendet wird), kann der Empfänger sicher sein, dass die Nachricht von der Person oder Entität gesendet wurde, die dem Zertifikat zugeordnet ist, und dass die Nachricht nach dem Signieren nicht geändert wurde.
In diesem Beispiel werden die folgenden Aufgaben und CryptoAPI-Funktionen zum Codieren einer Nachricht veranschaulicht:
- Öffnen eines Zertifikatspeichers mithilfe von CertOpenStore.
- Abrufen eines Zertifikats mit einem bestimmten Antragstellernamen mithilfe von CertFindCertificateInStore.
- Abrufen und Drucken des Antragstellernamens eines Zertifikats mithilfe von CertGetNameString.
- Initialisieren einer CRYPT_SIGN_MESSAGE_PARA Struktur, die in einem Aufruf von CryptSignMessage verwendet werden soll.
- Signieren und Codieren einer Nachricht mit CryptSignMessage.
In diesem Beispiel werden die folgenden Aufgaben und CryptoAPI-Funktionen zum Decodieren einer Nachricht und zum Überprüfen der Signatur veranschaulicht:
- Öffnen einer Nachricht zum Decodieren mit CryptMsgOpenToDecode.
- Hinzufügen des codierten BLOB zur Nachricht, die mithilfe von CryptMsgUpdate decodiert werden soll.
- Decodieren sie die Nachricht mithilfe von CryptMsgGetParam.
- Öffnen eines Zertifikatspeichers im Arbeitsspeicher mit CertOpenStore mithilfe der empfangenen und decodierten Nachricht.
- Verwenden von CertGetSubjectCertificateFromStore zum Abrufen des Zertifikats des Signaturgebers der Nachricht.
- Überprüfen der Signatur einer Nachricht mithilfe von CryptMsgControl.
- Freigeben von Arbeitsspeicher, Schließen von Zertifikatspeichern und Freigeben des Zertifikatkontexts.
Ein Beispiel für das Ausführen ähnlicher Vorgänge mithilfe eines Streamrückrufs finden Sie unter Beispiel C-Programm: Codieren und Decodieren einer Nachricht mithilfe eines Datenstroms.
In diesem Beispiel wird die Funktion MyHandleError verwendet. Code für diese Funktion ist im Beispiel enthalten. Code für diese und andere Hilfsfunktionen ist ebenfalls unter General_Purpose_Functions aufgeführt.
//-------------------------------------------------------------------
// Copyright (C) Microsoft. All rights reserved.
// Example of encoding and decoding a signed message.
#include <stdio.h>
#include <tchar.h>
#include <windows.h>
#include <Wincrypt.h>
// 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 CERTIFICATE_STORE_NAME L"MY"
//-------------------------------------------------------------------
// Declare local functions.
//-------------------------------------------------------------------
BOOL EncodeMessage(PCRYPT_DATA_BLOB pEncodedData,
LPWSTR pwszSignerName);
void DecodeMessage(PCRYPT_DATA_BLOB pEncodedData,
LPWSTR pwszSignerName);
// 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.
void ReportFailure()
{
switch (GetLastError())
{
case CRYPT_E_AUTH_ATTR_MISSING:
printf("Message does not contain an expected "
"attribute.\n");
break;
case CRYPT_E_BAD_ENCODE:
printf("An error encountered encoding or decoding.\n");
break;
case CRYPT_E_HASH_VALUE:
printf("The hash value is not correct.\n");
break;
case CRYPT_E_INVALID_MSG_TYPE:
printf("The message type is not valid.\n");
break;
case CRYPT_E_OSS_ERROR:
printf("OSS error.\n");
break;
case CRYPT_E_SIGNER_NOT_FOUND:
printf("Signer not found.\n");
break;
case CRYPT_E_UNEXPECTED_ENCODING:
printf("Unexpected encoding. \n");
break;
case CRYPT_E_UNKNOWN_ALGO:
printf("Unknown algorithm.\n");
break;
case E_OUTOFMEMORY:
printf("Out of memory.\n");
break;
case ERROR_INVALID_HANDLE:
printf("The handle from verify signature is not valid." \
"function.\n");
break;
case ERROR_INVALID_PARAMETER:
printf("The parameter from verify signature "
"is not valid.\n");
break;
case NTE_BAD_FLAGS:
printf("Bad Flags from verify signature function.\n");
break;
case NTE_BAD_HASH:
printf("Bad Hash from verify signature function.\n");
break;
case NTE_BAD_KEY:
printf("Bad Key from verify signature function.\n");
break;
case NTE_BAD_SIGNATURE:
printf("Bad signature from verify signature " \
"function.\n");
break;
case NTE_BAD_UID:
printf("Bad UID from verify signature function.\n");
break;
} // End switch.
} // End ReportFailure.
void EncodeAndDecodeMessage(LPWSTR pwszSignerName)
{
CRYPT_DATA_BLOB EncodedBlob;
if(EncodeMessage(&EncodedBlob, pwszSignerName))
{
DecodeMessage(&EncodedBlob, pwszSignerName);
}
}
BOOL EncodeMessage(PCRYPT_DATA_BLOB pEncodedBlob,
LPWSTR pwszSignerName)
{
/*---------------------------------------------------------------
Declare and initialize variables. This includes getting a
pointer to the message content. This sample 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.
---------------------------------------------------------------*/
HCERTSTORE hSystemStoreHandle;
CRYPT_SIGN_MESSAGE_PARA SignMessagePara;
//---------------------------------------------------------------
// The message to be signed and encoded.
BYTE* pbContent = (BYTE*) "The quick brown fox jumped over " \
"the lazy dog.";
/*---------------------------------------------------------------
The length of the message. This must be one more than the
value returned by strlen() to include the terminal NULL
character.
---------------------------------------------------------------*/
DWORD cbContent = lstrlenA((char *) pbContent) + 1;
//---------------------------------------------------------------
// Arrays to hold the message to be signed and its length.
const BYTE *rgpbToBeSigned[1] ;
DWORD rgcbToBeSigned[1];
//---------------------------------------------------------------
// The signer's certificate.
PCCERT_CONTEXT pSignerCert;
//---------------------------------------------------------------
// Buffer to hold the name of the subject of a certificate.
char pszNameString[MAX_NAME];
//---------------------------------------------------------------
// The following variables are used only in the decoding phase.
DWORD cbData = sizeof(DWORD);
//---------------------------------------------------------------
// Begin processing. Display the original message.
rgpbToBeSigned[0] = pbContent;
rgcbToBeSigned[0] = cbContent;
printf("The original message = \n%s\n\n",
rgpbToBeSigned[0]);
//---------------------------------------------------------------
// Open a certificate store.
if(hSystemStoreHandle = CertOpenStore(
CERT_STORE_PROV_SYSTEM,
0,
NULL,
CERT_SYSTEM_STORE_CURRENT_USER,
CERTIFICATE_STORE_NAME))
{
printf("The certificate store is open. \n");
}
else
{
MyHandleError( "Error Getting Store Handle");
}
/*---------------------------------------------------------------
Find a certificate in the store. This certificate will be
used to sign the message. To sign the message, the
certificate must have a private key accessible.
---------------------------------------------------------------*/
if(pSignerCert = CertFindCertificateInStore(
hSystemStoreHandle,
MY_ENCODING_TYPE,
0,
CERT_FIND_SUBJECT_STR,
pwszSignerName,
NULL))
{
//-----------------------------------------------------------
// 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("Getting the name of the signer " \
"failed.\n");
}
}
else
{
MyHandleError("Signer certificate not found.");
}
/*---------------------------------------------------------------
Initialize the CRYPT_SIGN_MESSAGE_PARA structure. First, use
memset to set all members to zero or NULL. Then set the values of
all members that must be nonzero.
---------------------------------------------------------------*/
memset(&SignMessagePara, 0, sizeof(CRYPT_SIGN_MESSAGE_PARA));
SignMessagePara.cbSize = sizeof(CRYPT_SIGN_MESSAGE_PARA);
SignMessagePara.HashAlgorithm.pszObjId = szOID_RSA_MD2;
SignMessagePara.pSigningCert = pSignerCert;
SignMessagePara.dwMsgEncodingType = MY_ENCODING_TYPE;
SignMessagePara.cMsgCert = 1;
SignMessagePara.rgpMsgCert = &pSignerCert;
/*---------------------------------------------------------------
In two steps, sign and encode the message. First, get the
number of bytes required for the buffer to hold the signed
and encoded message.
---------------------------------------------------------------*/
if( CryptSignMessage(
&SignMessagePara,
FALSE,
1,
rgpbToBeSigned,
rgcbToBeSigned,
NULL,
&pEncodedBlob->cbData))
{
printf("The needed length is %d \n", pEncodedBlob->cbData);
}
else
{
MyHandleError("Getting the length failed.\n");
}
//---------------------------------------------------------------
// Allocate memory for the required buffer.
pEncodedBlob->pbData = (BYTE *)malloc(pEncodedBlob->cbData);
if(!pEncodedBlob->pbData)
{
MyHandleError("Memory allocation failed.");
}
//---------------------------------------------------------------
// Call CryptSignMessage a second time to
// copy the signed and encoded message to the buffer.
if( CryptSignMessage(
&SignMessagePara,
FALSE,
1,
rgpbToBeSigned,
rgcbToBeSigned,
pEncodedBlob->pbData,
&pEncodedBlob->cbData))
{
printf("Signing worked \n");
}
else
{
MyHandleError("Signing failed.\n");
}
//---------------------------------------------------------------
// Clean up after signing and encoding.
if(pSignerCert)
{
CertFreeCertificateContext(pSignerCert);
}
if(hSystemStoreHandle)
{
CertCloseStore(hSystemStoreHandle,
CERT_CLOSE_STORE_FORCE_FLAG);
}
return TRUE;
}
void DecodeMessage(PCRYPT_DATA_BLOB pEncodedBlob,
LPWSTR pwszSignerName)
{
//---------------------------------------------------------------
// Buffer to hold the name of the subject of a certificate.
char pszNameString[MAX_NAME];
//---------------------------------------------------------------
// The following variables are used only in the decoding phase.
HCRYPTMSG hMsg;
HCERTSTORE hStoreHandle; // certificate store handle
DWORD cbData = sizeof(DWORD);
DWORD cbDecoded;
BYTE *pbDecoded;
DWORD cbSignerCertInfo;
PCERT_INFO pSignerCertInfo;
PCCERT_CONTEXT pSignerCertContext;
/*---------------------------------------------------------------
The following code decodes the message and verifies the
message signature. This code would normally be in a
stand-alone program that would read the signed and encoded
message and its length from a file from an email message,
or from some other source.
---------------------------------------------------------------*/
//---------------------------------------------------------------
// Open a message for decoding.
if(hMsg = CryptMsgOpenToDecode(
MY_ENCODING_TYPE, // encoding type
0, // flags
0, // use the default message type
// the message type is
// listed in the message header
NULL, // cryptographic provider
// use NULL for the default 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.
if(CryptMsgUpdate(
hMsg, // handle to the message
pEncodedBlob->pbData, // pointer to the encoded BLOB
pEncodedBlob->cbData, // 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 number of bytes needed for a buffer
// to hold the decoded message.
if(CryptMsgGetParam(
hMsg, // handle to the message
CMSG_CONTENT_PARAM, // parameter type
0, // index
NULL,
&cbDecoded)) // size of the returned information
{
printf("The message parameter has been acquired. \n");
}
else
{
MyHandleError("Decode CMSG_CONTENT_PARAM failed.");
}
//---------------------------------------------------------------
// Allocate memory.
if(!(pbDecoded = (BYTE *) malloc(cbDecoded)))
{
MyHandleError("Decode memory allocation failed.");
}
//---------------------------------------------------------------
// Copy the content to the buffer.
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 decoded message is =>\n%s\n\n",
(LPSTR)pbDecoded);
}
else
{
MyHandleError("Decode CMSG_CONTENT_PARAM #2 failed");
}
//---------------------------------------------------------------
// Verify the signature.
// First, get the signer CERT_INFO from the message.
//---------------------------------------------------------------
// Get the size of memory required for the certificate.
if(CryptMsgGetParam(
hMsg, // handle to the message
CMSG_SIGNER_CERT_INFO_PARAM, // parameter type
0, // index
NULL,
&cbSignerCertInfo)) // size of the returned
// information
{
printf("%d bytes needed for the buffer.\n",
cbSignerCertInfo);
}
else
{
MyHandleError("Verify SIGNER_CERT_INFO #1 failed.");
}
//---------------------------------------------------------------
// Allocate memory.
if(!(pSignerCertInfo = (PCERT_INFO) malloc(cbSignerCertInfo)))
{
MyHandleError("Verify memory allocation failed.");
}
//---------------------------------------------------------------
// Get the message certificate information (CERT_INFO
// structure).
if(!(CryptMsgGetParam(
hMsg, // handle to the message
CMSG_SIGNER_CERT_INFO_PARAM, // parameter type
0, // index
pSignerCertInfo, // address for returned
// information
&cbSignerCertInfo))) // size of the returned
// information
{
MyHandleError("Verify SIGNER_CERT_INFO #2 failed");
}
//---------------------------------------------------------------
// Open a certificate store in memory using CERT_STORE_PROV_MSG,
// which initializes it with the certificates from the message.
if(hStoreHandle = CertOpenStore(
CERT_STORE_PROV_MSG, // store provider type
MY_ENCODING_TYPE, // encoding type
NULL, // cryptographic provider
// use NULL for the default
0, // flags
hMsg)) // handle to the message
{
printf("The certificate store to be used for message " \
"verification has been opened.\n");
}
else
{
MyHandleError("Verify open store failed");
}
//---------------------------------------------------------------
// Find the signer's certificate in the store.
if(pSignerCertContext = CertGetSubjectCertificateFromStore(
hStoreHandle, // handle to the store
MY_ENCODING_TYPE, // encoding type
pSignerCertInfo)) // pointer to retrieved CERT_CONTEXT
{
if(CertGetNameString(
pSignerCertContext,
CERT_NAME_SIMPLE_DISPLAY_TYPE,
0,
NULL,
pszNameString,
MAX_NAME) > 1)
{
printf("The message signer is %s \n",pszNameString);
}
else
{
MyHandleError("Getting the signer's name failed.\n");
}
}
else
{
MyHandleError("Verify GetSubjectCert failed");
}
//---------------------------------------------------------------
// Use the CERT_INFO from the signer certificate to verify
// the signature.
if(CryptMsgControl(
hMsg,
0,
CMSG_CTRL_VERIFY_SIGNATURE,
pSignerCertContext->pCertInfo))
{
printf("Verify signature succeeded. \n");
}
else
{
printf("The signature was not verified. \n");
ReportFailure();
}
//---------------------------------------------------------------
// Clean up.
if(pEncodedBlob->pbData)
{
free(pEncodedBlob->pbData);
pEncodedBlob->pbData = NULL;
}
if(pbDecoded)
{
free(pbDecoded);
}
if(pSignerCertInfo)
{
free(pSignerCertInfo);
}
if(pSignerCertContext)
{
CertFreeCertificateContext(pSignerCertContext);
}
if(hStoreHandle)
{
CertCloseStore(hStoreHandle, CERT_CLOSE_STORE_FORCE_FLAG);
}
if(hMsg)
{
CryptMsgClose(hMsg);
}
}