Re: Real-world steganography
On 2002-10-01, Ben Laurie uttered to Peter Gutmann: Yeah, right - and green felt-tip around the edges of your CD improves the sound, too. I'm not sure about HDCD as a technology, but the principle is sound. If we can compress sound transparently, we can also transparently embed quite a lot of data into the part which is perceptually irrelevant. We might also depart with perceptual equivalence and go with perceptual similarity instead -- e.g. multiband compress the audio, and embed data which allows us to expand to a higher perceptual resolution. Whatever the implementation, putting data in the gap between statistical (i.e. computed against a Markov model) and perceptual (against a perceptual similarity model) entropy which compensates for some of the perceptual shortcomings (like total dynamic range) of a particular recording technology seems like an excellent idea. However, applications like these have very little to do with steganography proper. In this case, we can (and want) to fill up the entire gap between statistical and perceptual entropy estimates with useful data, leaving us with signals which have statistical entropies consistently higher than we'd expect of a typical recording with similar perceptual characteristics. That is, the encoded signal will appear manifestly random compared to typical unencoded material from a similar source, and we can easily see there is hidden communication going on. Such encodings will be of little value in the context of industrial strength steganography used for hidden communication. Steganography used in the latter sense will also have to be imperceptible, true, but but here the entropic gap we're filling is the one between the entropy estimates of our best model of the source material vs. that of the adversary's. Be the models Markov ones, perceptual, something else, or composites of the above. Consequently the margin is much thinner (bandwidths are probably at least a decade or two lower), and the aims remain completely separate. Consequently, I don't believe encodings developed for the first purpose could ever be the best ones for the latter, or that HDCD-like endeavors really have that much to do with the subject matter of this list. -- Sampo Syreeni, aka decoy - mailto:[EMAIL PROTECTED], tel:+358-50-5756111 student/math+cs/helsinki university, http://www.iki.fi/~decoy/front openpgp: 050985C2/025E D175 ABE5 027C 9494 EEB0 E090 8BA9 0509 85C2 - The Cryptography Mailing List Unsubscribe by sending unsubscribe cryptography to [EMAIL PROTECTED]
Re: Real-world steganography
Peter Gutmann wrote: I recently came across a real-world use of steganography which hides extra data in the LSB of CD audio tracks to allow (according to the vendor) the equivalent of 20-bit samples instead of 16-bit and assorted other features. According to the vendors, HDCD has been used in the recording of more than 5,000 CD titles, which include more than 250 Billboard Top 200 recordings and more than 175 GRAMMY nominations, so it's already fairly widely deployed. Yeah, right - and green felt-tip around the edges of your CD improves the sound, too. Cheers, Ben. -- http://www.apache-ssl.org/ben.html http://www.thebunker.net/ There is no limit to what a man can do or how far he can go if he doesn't mind who gets the credit. - Robert Woodruff - The Cryptography Mailing List Unsubscribe by sending unsubscribe cryptography to [EMAIL PROTECTED]
Re: Real-world steganography
At 09:38 PM 09/30/2002 -0700, Bram Cohen wrote: Peter Gutmann wrote: I recently came across a real-world use of steganography which hides extra data in the LSB of CD audio tracks to allow (according to the vendor) the equivalent of 20-bit samples instead of 16-bit and assorted other features. I don't think that's really 'steganography' per se, since no attempt is made to hide the fact that the information is in there. The quasi-stego used is just to prevent bad audio artifacts from happening. Traditional digital telephone signalling uses a robbed-bit method that steals the low-order bit from every sixth voice sample to carry information like whether the line is busy or idle or wants to set up a connection. (That's why you only get 56kbps and not 64kbps in some US formats, since it doesn't want to keep track of which low bits got robbed.) In a sense both of these are steganography, because they're trying to hide the data channel from the audio listener by being low level noise in ways that equipment that isn't looking for it won't notice. That's not really much different from encoding Secret Data in the LSB of uncompressed graphics or audio - it's about the second-crudest form of the stuff, and if you think there are Attackers trying to decide if you're using stego, you need more sophisticated stego - at minimum, encoding the stegotext so it looks like random noise, or encoding the stegotext with statistics resembling the real noise patterns, or whatever. The definition of hidden writing doesn't specify how hard you tried to hide it or how hard the Attacker is looking - you need to Bring Your Own Threat Model. Since I don't speak Audiophile Engineering / Human perceptual modelspeak, which the paper was written in, I wasn't able to figure out where the HDCD stuff hides the extra bits. Are they really there (in the CDROM's error-correction bits or something)? It sounded like they were either saying that they make part-time use of the one LSB bit to somehow encode the LSB and 4 more bits, which sounded really unlikely given that there weren't any equations there about the compression models, or else that they had some perceptual model and were using that to make a better choice of LSB than a simple 50% cut-off of the A-to-D converter (more absolute distortion, but better-sounding distortion.) Or did I miss the implications of the reference to oversampling and the real difference is that HDCD disks really have more pixels on the disk with only the LSB different, so a conventional reader reads it fine but needs the ECC to get the LSB? A separate question is - so is there some internet-accessible list of disks using HDCD, or do I just have to look at the labels for a logo? - The Cryptography Mailing List Unsubscribe by sending unsubscribe cryptography to [EMAIL PROTECTED]
Re: Real-world steganography
--On Tuesday, 01 October, 2002 13:54 +1200 Peter Gutmann [EMAIL PROTECTED] wrote: I recently came across a real-world use of steganography which hides extra data in the LSB of CD audio tracks to allow (according to the vendor) the equivalent of 20-bit samples instead of 16-bit and assorted other features. According to the vendors, HDCD has been used in the recording of more than 5,000 CD titles, which include more than 250 Billboard Top 200 recordings and more than 175 GRAMMY nominations, so it's already fairly widely deployed. maybe. i'm not sure how many players support it (my spectral D/A convertor does, but then some of the people at spectral seem to have invented HDCD). while the CDs i have that use it sound pretty good, i don't have any good way to compare them when played back over a non-HDCD capable convertor (i could hook up one of my computer CD drives, but that doesn't seem fair compared to the spectral transport-D/A combination). but when i do play such CDs on other gear, i don't notice any audible degradation, so it isn't obviously harmful. i've seen comments in reviews of professional CD mastering gear that there are other, seemingly preferred, technologies, although i've never found details of them. -paul - The Cryptography Mailing List Unsubscribe by sending unsubscribe cryptography to [EMAIL PROTECTED]
