Re: [WM]: Difference between watermarking and digital signatures

["Jeffrey Bloom" <jbloom@sarnoff.com>]

Salman,

Digital Signature: a cryptographic technique for generating a string indicating that a particular digital file was sent by a
particular person.  Relies on one-way cryptographic hashing and public key encryption.  Receiver can decrypt the digital signature
using my public key.  This insures that it was created using my private key (and thus was created by me).  The decrypted signature
is typically a one-way cryptographic hash of the digital file.  Receiver calculates the one-way cryptographic hash of the file and
compares it to the decrypted signature.  If they match, then the file has not been altered.  As an alternative, the sender could
simply encrypt the entire digital file with his private key, but this is too computationally costly.  So, the digital file is
usually sent in the clear and a digital signature is sent as metadata.

Watermarking: A technique for embedding metadata into a digital file.  In this discussion group, we often talk about digital
multimedia files and robust watermarking.  The fact that it is a multimedia file (image, audio,
video) means that the embedded data should not affect the perceptual quality.  Watermarked audio should sound the same as the
original audio, watermarked images and video should look the same as the unwatermarked versions.  The fact that the embedding should
be robust, implies that the data cannot be stored in a file header or in a user data field because those can easily be stripped.
The data must be embedded by changing the actual data values: pixel values.

Clearly, one could use watermarking to embed a digital signature as long as the watermarking technique did not affect the hash.
There may be a number of approaches here, but here are three popular ones.
1. Break the multimedia file into two parts, calculate hash of one part and embed in the other part.  The two parts need to make
sense.  Perhaps one is a low-pass version and the other is a high-pass version.  Note that this approach can only authenticate the
part on which the hash was calculated.
2. Use a removable watermark.  The use the watermark detection algorithm to recover the digital signature and then would remove the
watermark to restore the digital file to it's unwatermarked state.  Then the digital signature could be verified.
3.  There is work on "robust hashing" techniques and "perceptual hashing" 
techniques.  Here, two files that are perceptually similar (e.g. the original multimedia file and the watermarked multimedia file)
would hash to the same value.  Using such a robust hash in the digital signature, the watermark would not need to be removed prior
to verifying the signature.

Hope this helps.

- Jeffrey

At 10:57 AM 4/7/2005, Salman Bhatti wrote:
>Hi all,
>Can anybody tell me the difference between digital signatures and 
>watermarking?
>Cheers
>
>Salman

=============================
Dr. Jeffrey A Bloom, Sarnoff Corporation, 201 Washington Road, CN 5300, Princeton, NJ 08543-5300 jbloom@sarnoff.com,
http://www.geocities.com/Jeffrey_Bloom, (609) 734-3287,
(609) 734-2662 fax


______________________________________________________________________________

Watermarking Mailing List - http://www.watermarkingworld.org/ml.html
To unsubscribe send email to "majordomo@watermarkingworld.org" with
"unsubscribe watermarking YOURMAIL" in the body.
______________________________________________________________________________