[cifs-protocol] [MS-XCA] L77+Huffman negative huffman symbol update

[Aurélien Aptel]

Hi,

The latest version 6.0 of [MS-XCA] the pseudo code of the final
LZ77+huffman decoding added a branch to the pseudocode in 2.2.4 for the
decompression loop which I don't understand.

Huffman decoding table
======================

     CurrentTableEntry = 0
     For BitLength = 1 to 15
         For Symbol = 0 to 511
             If the encoded bit length of Symbol equals BitLength
                 EntryCount = (1 << (15 – BitLength))
                 Repeat EntryCount times
                     If CurrentTableEntry >= 2^15
                         The compressed data is not valid. Return with error.
                     DecodingTable[CurrentTableEntry] = Symbol
                     CurrentTableEntry = CurrentTableEntry + 1
     If CurrentTableEntry does not equal 2^15
         The compressed data is not valid. Return with error.


In the decoding table construction we set all 2^15 possible bits to an
existing symbol that occured at least once. So this property must be
true:

(1)    For any X in [0 - (2^15)-1)],  0 <= DecodeTable[X] < 512.


Final decoding
==============

     [...omited beginning...]
     Loop until a literal processing terminating condition
         Next15Bits = NextBits >> (32 – 15)
         HuffmanSymbol = DecodingTable[Next15Bits]
         HuffmanSymbolBitLength = the bit length of HuffmanSymbol, from the table in
                                  the input buffer
(a)      If HuffmanSymbol <= 0
             NextBits <<= HuffmanSymbolBitLength
             ExtraBits -= HuffmanSymbolBitLength


            Do
                HuffmanSymbol = - HuffmanSymbol
                HuffmanSymbol += (NextBits >> 31)
                NextBits *= 2
                ExtraBits = ExtraBits - 1
                HuffmanSymbol = DecodingTable[HuffmanSymbol]
            While HuffmanSymbol <= 0
        Else
            DecodedBitCount = HuffmanSymbol & 15
            NextBits <<= DecodedBitCount
(b)         ExtraBits -= DedcodedBitCount
        [...omited end...]

In line (a), from propery (1) we know HuffmanSymbol cannot be negative.
So the test is really checking for HuffmanSymbol == 0, which is the
valid literal byte 0 but cannot appear?

Basically I don't understand the purpose of this branch started in (a).

In line (b), there is a typo: Dedcoded => Decoded.

Cheers,
-- 
Aurélien Aptel / SUSE Labs Samba Team
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