Cryptography Reference
In-Depth Information
We will classify the types of certificate-document forgeries into two cate-
gories.
Type-1 Forgery: Altering a part of the authentic certificate
Example: Tampering a drivers permit, credit card, passport, student tran-
script, etc.
Type 2 Forgery: Creating a new fake certificate
Example: PhD diploma, pedigree, expert evidence, medical certificate, etc.
BPCS steganography can protect both types of forgeries in a digital manner.
Digital documents include several types of data such as plain texts, graph-
ics, images, and motion pictures in some case. When people see them on a
computer monitor, they look just image information. People do not care if the
original data consist of several different data types or not. So, it does not cause
any problem to convert the document data into all image data. However, the
converted image still have a single layered information structure.
A multi-layered information structure of media data, such as an image,
is as follows. It has an external layer, which is visible, and an internal layer,
which is invisible for human eyes. The internal layer can be further structured
as sub-external and sub-internal layers. It is very obvious that this kind of
multi-layered information structure is implemented by steganographic data
embedding method.
In order to make a digital document to serve as a certification document,
we will do the following.
Step-1: Convert a given digital document consisting of several data categories
into one single image file. We call this file conversion as vessel image
creation. The data size will significantly increases in this step.
Step-2: Embed the original document data in the created vessel image ac-
cording to a steganographic scheme. This step needs a large data-
embedding capacity. So, BPCS method is the best. The external view
of the embedded vessel-image, i.e., stego image, is the external infor-
mation, and the embedded original document is the internal infor-
mation. The internal information can be extracted only if it is not
altered at all.
As it is clear from Step-2, the external and the internal layer have the
same visual information. In order for someone to alter this multi-layered digi-
tal document successfully, he/she must alter the two layers at the same time.
However, this is impossible from the nature of steganography. On top of that,
once he/she tries to alter a part of the external layer, they will damage the
internal information immediately because the embedded data is so fragile, es-
pecially in BPCS steganography. Once the internal layer is damaged, it will
never be extracted.
Type-1 forgery detection
Determining whether a given digital certificate having a multi-layered infor-
mation structure is authentic or not in a Type-1 sense is very easy. If the
