Encrypting a Message

The following example code shows a message being encrypted before it is sent to a remote computer over the secure connection.

The example assumes that a SecHandle variable named phContext and a SOCKET named s are initialized. For the declarations and initiations of these variables, see Using SSPI with a Windows Sockets Client and Using SSPI with a Windows Sockets Server. This code also uses the function SendMsg and HandleError. Code for these functions can be seen in Utility Functions for Windows Sockets Client and Server.

SSPI functions are shown as available through the global SecurityFunctionTable structure g_SecurityFunc.

//--------------------------------------------------------------------
//   Declare and initialize local variables.

SecPkgContext_StreamSizes  Sizes;
SECURITY_STATUS            scRet;
SecBufferDesc              Message;
SecBuffer                  Buffers[4];
SecBuffer                  *pDataBuffer;

PBYTE                       pbIoBuffer;
DWORD                       cbIoBuffer;
DWORD                       cbIoBufferLength;
PBYTE                       pbMessage;
DWORD                       cbMessage;

//--------------------------------------------------------------------
// Get the stream encryption sizes. This needs to 
// be done once per connection. 
// phContext must have been initialized during the handshake process.

scRet = g_SecurityFunc.QueryContextAttributes(
            phContext,
            SECPKG_ATTR_STREAM_SIZES,
            &Sizes);

if(FAILED(scRet))
{
    HandleError("Error reading SECPKG_ATTR_STREAM_SIZES");
}

//--------------------------------------------------------------------
// Allocate a working buffer. The plaintext sent to EncryptMessage
// can never be more than 'Sizes.cbMaximumMessage', so a buffer 
// size of Sizes.cbMaximunMessage plus the header and trailer sizes 
// is sufficient for the longest message.

cbIoBufferLength = Sizes.cbHeader + 
                   Sizes.cbMaximumMessage +
                   Sizes.cbTrailer;

if(!(pbIoBuffer = malloc((BYTE *), cbIoBufferLength)))
{
    HandleError("Out of memory");
}

//--------------------------------------------------------------------
// Create a plaintext message to be encrypted offset into the data 
// buffer by "header size" bytes. This allows encryption in place.

pbMessage = pbIoBuffer + Sizes.cbHeader;

sprintf(pbMessage, "This is the plaintext message.");
cbMessage = strlen(pbMessage);

//--------------------------------------------------------------------
// Encrypt the plaintext message.

Buffers[0].pvBuffer     = pbIoBuffer;
Buffers[0].cbBuffer     = Sizes.cbHeader;
Buffers[0].BufferType   = SECBUFFER_STREAM_HEADER;

Buffers[1].pvBuffer     = pbMessage;
Buffers[1].cbBuffer     = cbMessage;
Buffers[1].BufferType   = SECBUFFER_DATA;

Buffers[2].pvBuffer     = pbMessage + cbMessage;
Buffers[2].cbBuffer     = Sizes.cbTrailer;
Buffers[2].BufferType   = SECBUFFER_STREAM_TRAILER;

Buffers[3].BufferType   = SECBUFFER_EMPTY;

Message.ulVersion       = SECBUFFER_VERSION;
Message.cBuffers        = 4;
Message.pBuffers        = Buffers;

scRet = g_SecurityFunc.EncryptMessage(phContext, 0, &Message, 0);

if(FAILED(scRet))
{
    HandleError("Error returned by EncryptMessage.");
}

//--------------------------------------------------------------------
// Send the encrypted data.

if(!(SendMsg(
     s,
     pbIoBuffer,
     Buffers[0].cbBuffer + Buffers[1].cbBuffer + 
           Buffers[2].cbBuffer)))
{
     HandleError("SendMsg failed.");
}