background image
DigiCULT
.
Info
40
or invisible. Invisible watermarks cannot be
seen but can be detected by special algo-
rithms, facilitating tracking of illegal distri-
bution of copyrighted materials. There are
two types of invisible watermarks: `destroy-
able' watermarks and watermarks that are
highly resistant to destruction and can
therefore be used to prove ownership.
HOW DOES IT WORK?
M
essages in electronic form can be
hidden within any digital media
type (images, audio files, or even other text
messages) by exploiting the storage capacity
within inessential pixels or lines of code in
a way that does not alter the look or sound
of the original file.
I
n steganography, two types of file are
usually used when embedding data into
an image. A `container' file (for exam-
ple, a digitised work of art) is the packag-
ing for a secret `message' file (the content
of which could range from a digital water-
mark to a military transmission). Data
describing the message are then embedded
into the least significant bits (LSBs) of the
container file. This technique works par-
ticularly well on images which have sub-
tle colour gradations, as the bits which are
altered to store the message change the col-
our of the pixel so little as to be imper-
ceptible to the human eye. This method is
sometimes also known as `wrapping'. For an
example of digital wrappers, see InterTrust's
DigiBoxes (http://www.intertrust.com). It
is important to note that many images are
compressed using algorithms in order to
reduce the file size for transmission over
networks. Image compression techniques
that are `lossy' such as JPEG are not suitable
for `wrapped' data, as much of the original
information cannot be recovered and this
risks damaging the hidden message. Lossless
compression such as GIF should be used
instead. 50
I
n watermarking, a visible watermark
(which is not strictly steganography as it
does not hide itself) can simply be added to
the image itself, with no hidden message.
A visible watermark can act as a deterrent
to misuse; however, it does interfere with
the study of the image and can be relative-
ly easily removed using most image manip-
ulation software. Invisible watermarks are
less obvious and therefore more difficult to
remove.
T
o find out more about steganography,
look at the NINCH Guide to Good
Practice: Rights Management (http://
www.nyu.edu/its/humanities/ninchguide/
IV/) or, to find a suitable tool, try the
Digital Image Steganography and Digital
Watermarking Tool Table (http://www.jjtc.
com/Steganography/toolmatrix.htm).
M
E M O RY
O F
T
H E
N
E T H E R L A N D S
(H
E T
G
E H E U G E N
VA N
N
E D E R L A N D
):
A
P R O
-
G R A M M AT I C
A P P R OAC H
TO
T H E
D I G I T I S AT I
-
O N
O F
C U LT U R A L
-
H I S TO R I C A L
R E S O U R C E S
I N
T
H E
N
E T H E R L A N D S
P
AUL
D
OORENBOSCH
, MA
K
ONINKLIJKE
B
IBLIOTHEEK
,
N
ATIONAL
L
IBRARY
OF
T
HE
N
ETHERLANDS
INTRODUCTION
M
emory of The Netherlands (http://
www.memoryofthenetherlands.nl) is
the Dutch national digitisation programme.
The programme aims at making available
a large amount of cultural-historical col-
lections to a wide audience, in an efficient
way and in a relatively short period of time
(three to four years). Another aim of the
programme is to develop educational appli-
cations for a number of the collections.
The collections are contributed by vari-
ous heritage organisations, including muse-
ums, archives and libraries. Currently more
than 45 organisations are involved. Memory
of The Netherlands has been available on
the Internet since May 2003. By the end of
2004 (the start of the second phase), it will
contain 750,000 images and text pages, 250
hours of moving images, and 100 hours of
audio. By then, at least twenty educational
applications will have become available, too.
A BRIEF HISTORY
I
n 1999 the Koninklijke Bibliotheek
(http://www.kb.nl/) submitted a project
proposal to the national government. This
proposal formed an important step towards
the creation of a digital collection of
resources on Dutch history, art and culture:
Screenshot Memory of the Netherlands website

Memor
y
of
the
Netherlands,

2004
50 For more information, see Steganography: Seeing the
Unseen http://www.jjtc.com/pub/r2026a.htm or the
detailed descriptions available from http://www.infosyssec.
org/infosyssec/cry2.htm.