[cifs-protocol] [EXTERNAL] RE: [REG:120060621000848] [MS-XCA] L77+Huffman negative huffman symbol update

[Jeff McCashland]

Are there any specific parts that are particularly confusing? How far do you get with it? 

Best regards,
Jeff McCashland | Senior Escalation Engineer | Microsoft Protocol Open Specifications Team 
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-----Original Message-----
From: Aurélien Aptel <aaptel at suse.com> 
Sent: Monday, June 22, 2020 9:59 AM
To: Jeff McCashland <jeffm at microsoft.com>; cifs-protocol at lists.samba.org
Cc: support <support at mail.support.microsoft.com>
Subject: RE: [EXTERNAL] RE: [REG:120060621000848] [MS-XCA] L77+Huffman negative huffman symbol update

Hi Jeff,

I've read and reread this and MS-XCA multiple times over but I still cannot understand :(

Jeff McCashland <jeffm at microsoft.com> writes:
> We have some explanation of the pseudocode in question:
> A negative HuffmanSymbol indicates that the size of the value exceeds the value of the table (is > 64k). The location for the actual value is based of the decoded value:
>
>         Each 16-bit entry in the table/tree follows this protocol:
>           If the high bit is zero, the table/tree value is
>           (decoded symbol) * 16 + (symbol bit length)
>           If the high bit is one, the table/tree value is:
>           -(index of tree node to decode remaining bits)
>
> The high bit being 1 is what makes the number negative.

The table has 2^15 entries. We know this from:

"The decoding table will have 2^15 entries because 15 is the maximum bit length permitted by the Xpress Compression Algorithm for a Huffman code."

The decoding table construction code indicates the same thing:

  If CurrentTableEntry does not equal 2^15
	 The compressed data is not valid. Return with error.

Each entry index range from 0 to 2^15-1 (15 bits). *All* these indexes have the highest bit to zero (2^15-1 = 0111 1111 1111 1111). There cannot be any negative indexes.

Each entry value is in the range 0-511 ("For Symbol = 0 to 511"). There cannot be any negative values.

Could you double-check if the table construction pseudocode (not the final decoding) is up to date? It really seems like it is off.

Thanks,
--
Aurélien Aptel / SUSE Labs Samba Team
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