hundred years,signatures have been recognized
using thesimplest possible method: visual matching.
If the signature in question closely matched the
appearance of a known sample, it would be judged to
be from thesame person as the known sample.
As we became more sophisticated about signatures
and dealing with forgeries, we started looking at
nuances of the shaping, strokes, and pressure when
possible. For example, an original signature
(one that has not been reproduced by photocopying,
faxing, or other means) retains details such as the varied pressure applied to the
paper whilethe signer was creating it. Using sensitive instruments, we can actually
measure the depth of the slight groove created as the pen moved across the paper.
Depending on the paper, you can also measure the speed of the pen in various
areas of the signature, depending on how the ink bleeds into the surrounding
paper because slower strokes deposit more ink, which bleeds more. Strokes
that leave the paper also tend to trail into a thin point rather than ending
abruptly. The way this happens varies from person to person.
Many of these items are fodder for biometric signature recognition, since
they can also be measured directly while the signature is collected. Wet ink
signatures can also supply other factual aspects that signatures collected
purely electronically cannot — for instance, ink composition and color, paper
absorption and bleed, and the aging characteristics of some inks. Although
these chemical and physical characteristics are interesting and useful for
authenticating signatures from historical documents or paintings, they’re not
all that helpful in authenticating a credit-card transaction.
The most basic form of signature recognition for biometrics is quite similar
to what happens in fingerprint biometrics. We call this basic form an imageonly
signing — only the image of the signature itself — to authenticate the signer.biometric signature since we are not using anything about the act ofThe method collects various characteristics of the signature from a known
signature sample (or multiple samples) and compares their characteristics to
the sample presented for verification or authentication.
Full comparison of any two signatures will almost always fail, since it’s nearly
impossible for anyone to exactly duplicate a signature. Minor variances in
paper, ink flow, pen weight, and muscle control will nearly always introduce
variations between any two signatures, even from the same person.
To deal with these variations, signature comparison algorithms use other
characteristics to help correctly identify authentic signatures. In later sections
of this chapter, we explain how movement and pressure are used to
help with this process; right now we are primarily concerned with the physical
image of the signature.
Although any two signatures from the same person may not be identical, the
relationships between the letters, the relative sizes of loops and character
spacing should match. For example, if your lowercase letter t is exactly threequarters
the height of your letter l, that ratio will generally remain constant
across all your signatures. Are the two letter ts in Littleton crossed with a single
stroke or with two? From the left or the right? Do you pause after the second t
for the cross-stroke, or wait until the end? Do you bother to dot the i or leave it
undotted? Is your i dot a speck, a line, a circle, or more comma-shaped? Each
of these characteristics, though it may differ somewhat from the sample, will
be repeated in ways that can be compared to the original and authenticated.
Letter shape is also an obvious place to look for uniformity and biometric
Signature uniqueness, but in many cases it’s behaviorally less distinct
than other gross characteristics of the physical form of the signature.
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