How to send a close_notify at the end of an SSL connection
One of the more confusing parts of writing code to correctly work an SSL connection is the final act - the closure.
Here's how to do it in Windows' SChannel:
// phCtx is the pointer to the context handle you've already been using for SSL.
static DWORD dwshut=SCHANNEL_SHUTDOWN;
SecBuffer sbshut={sizeof(dwshut), SECBUFFER_TOKEN, &dwshut};
SecBufferDesc sdshut={SECBUFFER_VERSION,1,&sbshut};
DWORD sec_ret;
sec_ret=s_pSecFuns->ApplyControlToken(phCtx,&sdshut);
ASSERT(secret==SEC_E_OK); // You'll want to do better handling than just "assert".
DWORD dwSSPIFlags=ASC_REQ_SEQUENCE_DETECT |
ASC_REQ_REPLAY_DETECT |
ASC_REQ_CONFIDENTIALITY |
ASC_REQ_EXTENDED_ERROR |
ASC_REQ_ALLOCATE_MEMORY |
ASC_REQ_STREAM;
DWORD dwOutFlags=0;
sbshut.BufferType=SECBUFFER_TOKEN;
sbshut.cbBuffer=0;
sbshut.pvBuffer=0;
sdshut.cBuffers=1;
sdshut.pBuffers=&sbshut;
sec_ret=s_pSecFuns->AcceptSecurityContext(&tls_credhandle,phCtx,
0,dwSSPIFlags,SECURITY_NATIVE_DREP,0,&sdshut,&dwOutFlags,NULL);
At this point, you'll need to send the contents of sbshut.pvBuffer (length is in sbshut.cbBuffer) in the stream (after anything else encrypted you've queued up), because it contains the close_notify message. You'll likely have to read - and decrypt - more response back from your peer, checking for it to either close the stream, or send a matching close_notify.
[The documentation for DecryptMessage online at Microsoft's MSDN now correctly describes how to recognise and react to a peer's close_notify alert.]
After verifying that you're receiving a close_notify from the other end, you'll be in a loop with AcceptSecurityContext, responding to the peer, and sending what AcceptSecurityContext tells you to, until ASC (as we insiders call it) returns SEC_I_CONTEXT_EXPIRED or SEC_E_OK.