Re: [quicwg/base-drafts] 5.6 Packet Number Protection is hard to follow (#1387)

[elenagurevich <[e-mail]>]

No i mean the following:
a single QUIC short packet may be contained within a single UDP datagram --> hence UDP header length may be used to determine protected packet length. 
packet_length = sizesof(ShortHeader) + sizeof(Protected_Payload)+sizeof(Authentication Tag)
but if  
_sample_offset + sample_length > packet_length 
then 
sample_offset = packet_length - sample_length_ 
So it may happen  that sample will take bytes from authentication tag (ICV) depending on actual payload size. 
Once the sample is known we can remove protection from the packet_number.
Once packet_number is decrypted,  its size is known, its value can be decoded, so we can form nonce, locate start of protected payload and decrypt& verify authentication tag of a packet.





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