details of steganography_lecture-7
DESCRIPTION
SteganographyTRANSCRIPT
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Steganography and Watermarks
Trust and Reputation
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Steganography
• Steganography is the science of embedding a secret message within another message.
• Secret is carried innocuously within a harmless-looking wrapper.– Useful when an encrypted message might draw
suspicion.
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Watermarks
• Traditionally, a watermark has been used to verify the authenticity of a document.– Difficult to reproduce.– Tampering will destroy watermark.
– Driver’s Licenses, diplomas, official letterhead.
• More recently, used to track or prevent redistribution– TV logos
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Digital Watermarks
• Three purposes:– Ensure authenticity of digital goods
• Should be difficult to copy watermark.
– Prevent unauthorized use/ensure copyright– Prevent copying
• Should be difficult to remove watermark.
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Digital Watermarks
• Adding the watermark to the image itself prevents removal by changing the format. (e.g. GIF->JPEG)
• Research challenge: How to construct a watermark that is resistant to manipulation of the image– Cropping, editing, rotation, scaling, D/A/D
conversion, noise addition, etc.
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Authentication
• Proof of authenticity can be embedded into a digital good.– Author generates a watermark, signs it, and
embeds it.• Commercial services might assign an ID
– Presence of watermark is advertised.– User can verify, creator, date created, etc.
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Copy Protection• Watermarking can be used to prevent illicit copies
from being made.• Requires hardware support.
• CD -> DAT: Audio watermark included a flag; allowed one copy (for personal use).– Difficulty: manufacturer compliance.
• DVD: Proposed schemes allow manufacturer to specify copy protection– No copies, one copy, many copies.– Reliance on hardware a problem
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Broadcast Encryption
• A closely related problem is that of broadcast encryption– Sender sends an encrypted signal– A subset of the population can decode this
signal.– Typically implemented using smartcards or
tamper-resistant hardware– Difficult to completely avoid piracy
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Usage tracking
• Content Providers can also use a watermark to track usage.– Help find and track unauthorized usage, ensure
copyright.
• Each copy of an image has a unique identifier– Referred to as a fingerprint
• Buyer, timestamp, etc.
• Images also have a watermark embedded– Provides notification of copyright
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Usage Tracking
• Finding the user who originally posted/gave away the image is called the traitor tracing problem. • Similar: who allowed their smartcard to be used to
build a pirate decoder?
• Web spiders can be used to crawl sites, download images, check for watermarks and extract the corresponding fingerprints.
• Database of images (or their hashes) maintained.• Legal issues are unresolved
• Am I responsible for all loss that results from giving away copyrighted material?
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Example: Replacing bits
• Image, sound, and video are resistant to changes in the low-order bits.– This is what makes compression possible.– In a 24-bit AIFF, the lowest bits can be treated
as noise.
• We can replace those low-order bits with bits that encode a message.– This could be a string, another image, or
anything else that can be represented digitally.
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Wrinkles
• Simply changing all the lower-order bits is very brittle.– Attackers need only flip a few bits to remove a
watermark.– Depends on keeping the hiding mechanism
secret.
• A key can be used to specify which blocks contain the watermark.
• The watermark may be redundantly embedded.
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Wrinkles
• Manipulating low-order bits is easy to understand, but not very secure.– Easy to detect and defeat.– e.g. uncompress and recompress, crop, shear.– This is called a bit-plane or least-significant-bit
watermark.
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Wrinkles
• More secure watermarks can be generated by transforming the image and changing bits in the transformed space.– Luminance, quantization in images
• Choose random pairs and vary contrast
– Frequency, harmonics in sounds • (Fourier transform)
– This falls into the realm of signal processing – beyond our scope!
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Existing Approaches
• Usage– Combination of watermarks and hardware
control.
• Tracking– Central database where images are registered.
• Digital rights management– Watermark contains usage conditions (copying,
modification, number of viewings), billing info• Digimarc, Philips, Verance
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Existing Applications
• Linking– Possesion of a watermarked good allows access
to other goods.
• Inclusion of metadata– Content, creator/creation date, category
• Broadcast verification– Automated tracking of ads, promos, music
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Reading watermarks
• LSB methods allow a user to extract the watermark directly.– Watermark easily damaged or corrupted.
• Many transform methods require the use of an original, non-watermarked image.– Watermark is extracted through the equivalent
of a diff.
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Attacks
• Add jitter– Moves the location of blocks containing a message.
• Mosaic– Single image is chopped into several subimages.
– Defeats spiders.
• Addition of watermarks– It is possible in some schemes for an attacker to embed
his own watermark and mark it appear to be the original.
– Timestamping by a trusted third party can solve this.
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Case Study: Giovanni
• Ctyptographic/watermarking technique developed by BlueSpike.
• Goals: embed a watermark in an audio signal so that:– It cannot be removed without severely
degrading the signal. – Ownership can be verified– Inaudible to users.
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1. Generate keys for encryption and placement.- Watermark will be randomly placed.
2. Generate a watermark and hash it.- The hash will be embedded.
3. Construct a perceptual model of the audio signal- Watermark is placed in most perceptually significant parts of the signal
4. Embed watermark at random locations governed by the perceptual model.- Humans are insensitive to small changes in amplitude- Low-amplitude echoes can also be inserted.
Case Study: Giovanni
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Summary
• Watermark uses include:– Provide authenticity– Copyright protection/usage
tracking/verification– Copy protection– Embedded metadata– Linking to other data
• In general, cooperation of hardware vendors makes the problem much easier.
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Larger Issues in Watermarking
• The assumption underlying watermarking is that information providers can prevent copying and earn profits by selling their work directly.– Prevent copying or enforce copyright law
• It’s not clear that this assumption is reasonable.– History is full of examples of these schemes being
circumvented.
• What are alternative ways for information producers to get paid?– ???