Microsoft will sometimes split GSS tokens when they exceed a certain
size in some protocols. This is specified in
[MS-SPNG]: Simple and Protected GSS-API Negotiation
Mechanism (SPNEGO) Extension
https://winprotocoldoc.blob.core.windows.net/productionwindowsarchives/MS-SPNG/%5bMS-SPNG%5d.pdf
sections 3.1.5.4 to 3.1.5.9.
We extend gss_accept_sec_context() to recognise partial tokens and
to accumulate the fragments until an entire token is available to
be processed. If the entire token is not yet available,
GSS_S_CONTINUE_NEEDED is returned with a zero length output token.
This is specified in RFC2744 page 25-26 to indicate that no reply
need be sent.
We include updates to the test framework to test split tokens when
using SPNEGO.
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