Schannel messages larger than cbmaximummessage - c++

Question

I have problem sending SChannel TLS message larger than the negotiated maximum length.
When "EncryptSend" is called with a buffer larger than SecPkgContext_StreamSizes.cbMaximumMessage, the part greater than SecPkgContext_StreamSizes.cbMaximumMessage is not understood by the server (nor by Wireshark).

Answer 1

You should be able to break your data into chunks that are less than or equal to the cbMaximumMessage size. For example, if you are sending VOID* pvData of ULONG cbData bytes, then...
while(0 < cbData)
{
ULONG cbChunk = (cbData > m_Sizes.cbMaximumMessage) ? m_Sizes.cbMaximumMessage : cbData;
Message.ulVersion = SECBUFFER_VERSION;
Message.cBuffers = ARRAYSIZE(Buffers);
Message.pBuffers = Buffers;
Buffers[0].pvBuffer = m_pSendBuffer;
Buffers[0].cbBuffer = m_Sizes.cbHeader;
Buffers[0].BufferType = SECBUFFER_STREAM_HEADER;
Buffers[1].pvBuffer = m_pSendBuffer + m_Sizes.cbHeader;
Buffers[1].cbBuffer = cbChunk;
Buffers[1].BufferType = SECBUFFER_DATA;
CopyMemory(Buffers[1].pvBuffer, pvData, cbChunk);
Buffers[2].pvBuffer = m_pSendBuffer + m_Sizes.cbHeader + cbChunk;
Buffers[2].cbBuffer = m_Sizes.cbTrailer;
Buffers[2].BufferType = SECBUFFER_STREAM_TRAILER;
Buffers[3].BufferType = SECBUFFER_EMPTY;
hr = EncryptMessage(&m_hContext, &Message, 0, 0);
if(FAILED(hr))
break;
hr = pSocket->Send(m_pSendBuffer, Buffers[0].cbBuffer + cbChunk + Buffers[2].cbBuffer);
if(FAILED(hr))
break;
pvData = reinterpret_cast<PBYTE>(pvData) + cbChunk;
cbData -= cbChunk;
}
In each iteration of the loop, a chunk that is less than or equal to the maximum size is encrypted and sent. For this to work, the mechanism used to send data to the socket will likely need to employ a buffering strategy in case the socket's internal buffer is filled to capacity.