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To address this problem, the ANSI/NIST-ITL 1-2011 standard states: “Images shall be
compressed only from an original uncompressed image. If an image has been received in
compressed format, it shall not be uncompressed and re-compressed in the same or different
format.”
H) Comparison of the probe to the target set
The actual comparison of biometric information may be automated, partially automated, or
manual. Automated fingerprint matching systems typically rely on a specified set of features
within the fingerprint. The need to standardize the encoding of these ‘fingerprint minutiae’
for use by multiple matchers was recognised very early. In 1986, the first version of what
eventually became the ANSI/NIST-ITL standard addressed fingerprint minutiae with the goal
of ensuring that law enforcement organizations would be able to send information to one
another without extensive re-coding of the data.
However, forensic examiners must rely upon more types of information than where ridges
end and divide (bifurcations), which form the basis for minutiae. They must also be able to
state their findings in a way that can be understood years later by other examiners. This led
to the development of the Extended Feature Set, which is now incorporated into the ANSI/
NIST-ITL 1-2011 standard. Forensic examiners can now specify in a fixed manner features such
as the location of pores, the number of ridges in an area and other important characteristics.
Fingerprint examiners in other locations, and perhaps separated by time, can refer to these
features in a way that could have very important results in criminal prosecutions.
I) Biometric sample and metadata storage
In many applications, there is a requirement to use a minimum amount of space. An example
is biometric data stored on an identification card used for building access. The data used by
iris matchers can be stored in a very efficient manner (in some cases in as little as 3 kilobytes).
This has been demonstrated through research conducted at NIST 18 . This analysis also found
that one form of compact storage (polar format) resulted in degraded performance. The
ISO and ANSI/NIST-ITL standards now both allow the ‘crop and mask’ format that has
been shown to retain fidelity to the original biometric sample yet simultaneously reduce
storage requirements. In order to maintain system accuracy, both the ISO and ANSI/NIST-ITL
standard do not allow iris data to be stored in the ‘polar’ format.
J) Reporting and use of comparison results
The output of a biometric system is not necessarily a ‘yes’ or a ‘no.’ A probe of a biometric
will always have slightly different characteristics than data in the target set, so a ‘match’ is
never exact 19 . In fact, if it is exact, then that means that the probe and the target set data are
from the exact same sample, which should raise suspicions about attempts to compromise
the system. In many cases, there is only one set of data in the target set that is ‘close’ in
comparison to the probe. In other cases, there may be several sets of data in the target set
18 See http://www.nist.gov/itl/iad/ig/irex.cfm
19 It is possible under certain circumstances to have an exact match with DNA.
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