[gnutls-help] about ecnrypting pkts with lenght 0

[Andrea Dal Bo]

Hi,
recently we moved from gnults 2.13.23 to gnutls 3.6.5.
By doing this I encountered an issue with applications using the library. In fact it happens that the our TR-069 agent is sending pkts over https by means of gnutls_record_send. It might happens that gnutls_record_send is called with data_size=0.  This was accepted in the older gnutls 2.13.23.
With gnutls 3.6.5 it is not. In fact gnutls_record_send calls gnutls_record_send2 with pad=0, and subsequently _gnutls_send_tlen_int  that verifies that if both data_size and pad are 0 returns GNUTLS_E_INVALID_REQUEST (record.c line 492).
I tried to modify the application to add a padding>=1 in case of data_size=0 by calling  gnutls_record_can_use_length_hiding. In fact even if this function was returning 1, the subsequent call of gnutls_record_send2 was still returning GNUTLS_E_INVALID_REQUEST. In fact the discrimination was that tls1.3 handshake semantics was set to false. That is a bit misleading, IMHO.
Can you explain the reason for that?
Moreover why this restriction on the data_size length has been added to?
Thanks

Andrea

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