Authenticity is always an important
issue as long as the multimedia contents are also available to other parties
besides the owner. In the context of cryptography, the authenticity of the
data is provided by cryptographic hash functions (MD5, SHA-1) in which the
data is mapped to short bit strings that make up the hash value, which is
then attached to the data. Whenever one would like to test the authenticity
of the data, one simply recalculates the hash value from the data itself and
compares it to the attached hash value.
Perceptual hash functions are
designated hash functions for multimedia contents. Similar to cryptographic
hash functions, they are required to generate different hash values for
different inputs. However, here the definition of difference is changed from
bitwise difference to perceptual difference. In other words, cryptographic
hash functions generate a totally different hash value even if the input is
changed by a single bit, while robust hash functions are expected to change
the hash value only if the input is perceptually changed. For instance, the
hash value of an image and its JPEG compressed version should be the same
since they have no perceptual difference although their bit-string
representation is completely different.
A good perceptual hash function
1) robust: Manipulations that don't change the perceptual
information should not change the hash value.
2) unique: Perceptually
different inputs should have completely different hash values.
It should be very hard to find (forge) perceptually different inputs having
similar hash values.
Apart from authentication, perceptual hash functions
can be used in applications where the multimedia content must be identified
efficiently, such as in database indexing, broadcast monitoring, etc.In
ISIS we are designing perceptual hash functions for multimedia contents with
maximized robustness, uniqueness and security.
- B. Coskun, N. Memon, "On the
Confusion/Diffusion Properties of Perceptual Hash Functions", CISS
2006: Conference on Information Sciences and Systems, March 22-24,
2006, Princeton, NJ
- B. Coskun, B. Sankur, N.
Transform-Based Video Hashing", IEEE Trans.
on Multimedia, Vol. 8, No. 6, pp. 1190-1208, December 2006
- B. Coskun, B. Sankur, "Robust Video Hash
Extraction", EUSIPCO 2004: European Conf. on Signal Processing,
2295-2298, September 2004, Vienna
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