[Speex-dev] Steganographic extension to Speex

[Björn Thalheim]

Hello,

Jean-Marc Valin schrieb:
>>I am planning on creating a steganographic extension for speex/CELP.
>>Since speex/CELP is lossy, there should be quite many bits that one
>>can use for hiding data.
> 
> Quite the opposite. Speex uses very few bits, so every bit you corrupt
> is likely to be heard. You would need to use "fractions of bits" at a
> time. Even then, it may be hard to hide data efficiently (so it's hard
> to detect whether steganography was applied).

Of course, I cannot blindly change bits. But everywhere, where
truncation is done, one can change things to embed data.
Besides, even if I find no good possibility to embed data in the
CELP encoding process, that would still be an ok result for this work.
At this time, I see the possibility to hide data in the process of
mapping the residues onto codebook entries, in the LPC to LSP
conversion and in the comfort noise generation (I guess a more
commonly used term for this is "noise shaping").

>>- which bits of a transmitted frame are the two filters, which are
>>codebook keys?
> 
> what do you mean?

I mean, that we send the speech frame by frame. So a frame needs to
contain the two filters (pitch and formants), the first couple of
values (raw, now we can apply the filter to predict the rest of the
frame), and then the codebook keys of which the corresponding
entries are added to get raw speech data back.

Ciao,

Björn


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