Cryptography Reference
In-Depth Information
ability. Although the Internet is a public access network, certain data require
secrecy, such as commercial or military images, to protect them from illegiti-
mate users during transmission. Steganography is a technique for transmitting
secret data without being noticed. This technique hides secret messages into
cover media to avoid malicious attacks. The cover media can be digital images,
digital videos, source codes, or even HyperText Markup Language (HTML)
codes. The digital image used to embed the secret data is called a cover im-
age, which becomes a stego-image once the secret data are embedded. With
steganography, malicious attackers do not know that a stego-image carries se-
cret data. Therefore, they will not try to extract the data or otherwise trespass
on it. However, there is a weakness common to all steganographic techniques.
If one of the stego-media is lost or corrupted, the secret data cannot be re-
vealed exactly and completely. Therefore, several secret sharing techniques
have been proposed to overcome this weakness. With regard to the concept of
secret sharing, the well-known (k, n)-threshold schemes pioneered by Shamir
[20] and Blakley [2], respectively, have four characteristics in common:
(1) k n.
(2)
The secret is shared among n participants.
(3)
Only k
or more participants can reconstruct the original secret.
(4)
When the number of participants is (k 1) or less, they cannot
recover the original secret.
Based on the (k, n )-threshold schemes, in 1995 Naor and Shamir [17] devel-
oped the first secret image sharing technique to safeguard and share image-
based secrets. Their approach, called visual secret sharing (VSS), makes it
possible to publicly transmit secret images eciently and safely. The detailed
principles of such a technique follow:
(1)
The secret image is divided into n noise-like images, called shadows
or shares, with each noise-like shadow containing no original image
information.
(2)
n noise-like shares can be transmitted to n participants via the In-
ternet instead of the secret image itself, so that each participant
keeps only one single share. Thereby, VSS can prevent attackers
from gaining access to the secret image directly.
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