978-1-5386-1585-0/17/$31.00 ©2017 European Union
Stability of a dynamic biometric signature
created on various devices
Vladimír Smejkal
Department of Informatics
Faculty of Business and Management
Brno University of Technology
Brno, Czech Republic
smejkal@znalci.cz
Jindřich Kodl
Authorized expert of cryptology and
information systems security
Prague, Czech Republic
jm.kodl@gmail.com
Ladislav Sieger
Department of Physics
Faculty of Electrical Engineering
Czech Technical University in Prague
Prague, Czech Republic
sieger@fel.cvut.cz
František Hortai
Department of Informatics
Faculty of Business and Management
Brno University of Technology
Brno, Czech Republic
hortai.frantisek@gmail.com
Petr Tesař
Department of Informatics and Mathematics
Faculty of Economic Studies
University of Finance and Administration
Prague, Czech Republic
petr.tesar@post.cz
Abstract—The paper directly follows on from the prior
research on the dynamic biometric signature (DBS), its
properties, security, its resistance to forgery, and its stability. In
our experiments, we used all the available pads produced by
Signotec, which differ from each other in terms of their design,
the size of the signature field, resolution, sampling rate, and even
the scanning method used – a regular pen or a special pen using
the ERT (Electromagnetic Resonance Technology). A less
heterogenous sample was used than in the previous cases, as the
objective of the experiments was to demonstrate a potential
change in the DBS connected with the use of a different device,
nevertheless the size of the sample means it is sufficiently
statistically representative.
The results showed that irrespective of the device used, the
stability of scanning of the dynamic biometric signature was high
for each person. The signature variability did not significantly
differ between the devices for individual people.
Once again it was confirmed that the use of the first signature
as a “trial”, not included in the results, reduces the signature
variability for each participant.
Keywords—dynamic biometric signature; biometric data of the
signature; stability of the dynamic biometric signature; dynamic
biometric signature capture device
I. INTRODUCTION
The dynamic biometric signature is increasingly being
implemented primarily in such application areas as financial
services, healthcare and public administration. Its importance
grows due to the increasing demands on the trust in
authenticity of the signature and authentication of the person
signing in a digital environment. Last but not least, it is the
dynamic character of the method that significantly reduces the
possibility of identity theft, or forgery of the authentication
factor by another person.
The presented results of our initial research continually
follow those of our earlier papers published within the ICCST
conference. Issues of the indisputable connection of the created
DBS with the text of the electronic document that is being
signed were investigated in our previous ICCST papers [1], [2].
We also performed basic experiments demonstrating the
uniqueness of the DBS and its resistance to forgery [3].
In addition to confirming the uniqueness of the DBS, its
stability is another crucial aspect. That is why, in 2015, we
dealt with the impact of alcohol on the stability of the dynamic
biometric signature in a more homogeneous group as well as
the uniqueness and resistance of the signature to forgery in a
highly heterogeneous group of people aged 12 to 92 [4]. In
2016, we presented a report on the impact of stress on the
stability of the dynamic biometric signature [5].
So far, we have dealt with the changes relating to a signer
or the influences of his surrounding on the signing situation, in
which case the used equipment was always the same, i.e.
invariant. Therefore, we were now interested whether there
was an impact of using different scanning devices by the same
person on the quality of data and stability of the DBS and,
alternatively, what this impact was. Our paper describes the
experiments that were related to the impact of using various
devices (pads and tablets) on the dynamic biometric signatures
of a particular person. The obtained results continually follow
those of our earlier published papers.
In our experiments, we used all the available pads produced
by Signotec, which differ from each other in terms of their
design, the size of the signature field, resolution, sampling rate,
and even the scanning method used – a regular pen or a special
pen using the ERT (Electromagnetic Resonance Technology).
The purpose of the experiments was to show the possible
change of the stability of the DBS of a signer depending on the
scanning device. As the sample represented people of both
sexes aged 20 to 65, the size of the heterogeneous sample used
was statistically representative enough.
II. HYPOTHESES
The following hypotheses were formulated:
I. The participants will cope with the difficulties connected
with changing circumstances of the signing depending on the
technical design of the pad in a different way:
H0 – the stability of signatures of a particular person on
each device does not significantly change (mean and variance
of the degree of compliance of signatures for each device
belong to the same basic set),
H1 – there is a statistically significant difference in the
means and variances of the degree of compliance of signatures
of a particular person on individual devices.
II. The stability of signatures achieved on individual
devices will statistically significantly differ.
H0 – the mean degree and variance of compliance of
signatures for each device do not significantly change (mean
and variance of the degree of compliance of signatures for each
device belong to the same basic set),
H1 – there is a statistically significant difference in the
means and variances of the degree of compliance of signatures
on individual devices.
III. METHODS AND DATA
Testing was carried out on the following dynamic biometric
signature devices with the various technical parameters
produced by the company Signotec GmbH in the last five
years:
TABLE I. OVERVIEW OF THE TESTED DEVICES
Method of the
signature
capture
Model of the dynamic biometric signature
device
The active pen,
display, and pen
are mutually
synchronized
Signotec Alpha Pad (hereinafter referred to
as Alpha – ERT)
ST-A4E-2-UFTE100: Colour LCD Signature
Pad Alpha ERT (Electromagnetic Resonance
Technology)
https://en.signotec.com/portal/seiten/a4-
colour-lcd-signature-pad-signotec-alpha-
900000330-10002.html?rubrik=900000001
The display is
electromagnetic,
the pressure is
captured on the
basis of the
outward pressure
of the passive
pen on the
display
Signotec Delta Pad (hereinafter referred to
as Delta – ERT)
Touch display ST-DERT-3-U100 ERT
(Electromagnetic Resonance Technology)
https://en.signotec.com/portal/seiten/colour-
lcd-signature-pad-signotec-delta-900000406-
10002.html?rubrik=900000001
The display is
electromagnetic,
the pressure is
captured on the
basis of the
outward pressure
of the passive
pen on the
display
Signotec Gamma Pad (hereinafter referred
to as Gamma – ERT)
Touch display ST-GERT-3-U100: 5" Colour
LCD Signature Pad Gamma ERT
(Electromagnetic Resonance Technology)
https://en.signotec.com/portal/seiten/colour-
lcd-signature-pad-signotec-gamma-
900000375-10002.html?rubrik=900000001
The display is a
touch-screen, the
pressure is
captured on the
basis of the
outward pressure
of the passive
pen
Signotec Omega Pad revision B (hereinafter
referred to as OmegaOld – TD)
Touch display ST-CE1075-2-U100
(old version, reference is no longer available)
Signotec Omega Pad revision E (hereinafter
referred to as OmegaNew – TD)
Touch display ST-CE1075-2-U100
(current version)
https://en.signotec.com/portal/seiten/colour-
lcd-signature-pad-signotec-omega-
900000237-10002.html?rubrik=900000001
Signotec Sigma Pad revision B (hereinafter
referred to as SigmaOld – TD)
Touch display ST-ME105-2-U100-B (old
version)
Signotec Sigma Pad revision E (hereinafter
referred to as SigmaNew – TD)
Touch display ST-ME105-2-U100-B (current
version)
https://en.signotec.com/portal/seiten/lcd-
signature-pad-signotec-sigma-900000301-
10002.html?rubrik=900000001
There is no
display, only the
touch area
Signotec Sigma Lite (hereinafter referred to
as SigmaLite – WD)
Touch area without a display function ST-
LT105-2-U100
https://en.signotec.com/portal/seiten/signature-
pad-signotec-sigma-lite-without-lcd-
900000411-10002.html?rubrik=900000001
A total of 8 scanning devices were used. The sampling
frequency of the used devices can be set up to 150 Hz, 250 Hz
or 500 Hz. The scan rate (sampling) was set up to
recommended 250 points/sec. The x, y, time and pressure
coordinates are scanned. The experiment was attended by 40
people in one session.
IV. EXPERIMENTS AND RESULTS
DBS were recorded on the devices using the program
signoSign2 version 10.4.5. produced by Signotec. Each
participant made 10 signatures on each device, so the matrix of
signatures of each participant and all devices was formed
as follows:
i, j = 1, … , 10 , 1
where i is a serial number of the device, j is a serial number
of the participant, xk is a particular signature.
In accordance with the findings from the previous works
([3), [4], [5]), the first signatures made by each person on a
particular device were not included in the evaluation. The
signature match rate is automatically evaluated by the
analytical software of device manufacturer.
i, j
A. Testing of the hypothesis I.
In the first part of the evaluation of the experiments, the
degree of compliance among the signatures of each
person within each device was investigated (in %) – the output
is a triangular matrix of compliances {sm,n} m=2,…8, n=m+1,
where for each device the selective mean of compliance of
signatures of a particular person on this device, the selective
variance of compliance M2 and the selective standard deviation
σ were calculated. That is how the vector of the selective
means of compliances to the vector of the
selective variances M2,1 to M2,8, and the vector of the selective
standard deviations σ1 to σ8 were obtained for each person and
all the equipment.
A three-dimensional chart in Fig. 1 illustrates the obtained
structure of data used to ascertain the selective standard
deviation of the degree of compliance among the signatures (2
numbers of people are not assigned – No. 6 and No. 23):
Fig. 1. Selective standard deviation of the degree of compliance
The result characterizing the technology as a whole, i.e.
without differentiation of types of devices and signers (i.e. for
all people on all devices) is as follows:
TABLE II. SUMMARY RESULTS ON THE DEGREE OF COMPLIANCE
OF SIGNATURES
x [%] M2 σ [%]
79.330 173.290 13.164
The selective mean of the degree of compliance of
signatures came under an accepted level of compliance
of biometric signatures 60% only in case of two people
(No. 16 and No. 34) – see Fig. 2:
Fig. 2. Selective mean of the degree of compliance for individual people
It is assumed that independent random selections originate
from normal distributions with mean values µ1, µ2 …. µr with
the same variance σ2
.
In order to test the homogeneity of variances of the degree
of compliance of signatures of each participant on all devices,
the Bartlett's test was used [6]. The values in the B test ranged
from 20.341 to 609.934, i.e. the P-value was between 0.000
and 0.005. For all participants, the null hypothesis was
therefore rejected at the significance level 0.01 and thus at the
significance level 0.05 (as the P-value was <0.01 for all
participants).
Using the Cochran-Cox test [7], a pair of devices, where
the hypothesis on compliance of means of the degree of
compliance was rejected at the significance level 0.01, was
found for each participant. The simple sorting test (analysis of
variance, ANOVA) that would keep the probability of error of
the first kind at the level 0.05 or 0.01 could not be used with
regard to the results of the Bartlett's test.
Due to the facts that selective variances of the degree of
compliance are significantly different for all participants on
individual devices and there are differences in means of
compliances, we can conclude that the participants coped badly
with various designs of devices.
B. Testing of the hypothesis II.
The following values of selective means and unbiased
estimates for variances of the degree of compliance of
signatures were detected on the stated devices:
̅
̅1 ̅8,
≥
TABLE III. SELECTIVE MEANS AND UNBIASED ESTIMATES FOR
VARIANCES OF THE DEGREE OF COMPLIANCE OF SIGNATURES ON THE TESTED
DEVICES
Device and scanning
method
x [%] S2
Alpha - ERT 80.342 113.019
Delta - ERT 76.749 238.268
Gamma - ERT 78.971 232.027
OmegaNew - TD 76.022 228.052
OmegaOld - TD 83.002 125.844
SigmaLite - WD 77.097 148.574
SigmaNew - TD 85.233 139.194
SigmaOld - TD 77.195 120.338
Fig. 3. Selective means and unbiased estimates for variances of the degree
of compliance of signatures on individual devices
Compliance of variances was verified by the Bartlett's test
(B = 13.597, k-1 = 7, α = 0.01 and 0.05, P-value = 0.059), so
the hypothesis on compliance of all variances was accepted at
the significance level 0.01 and at the significance level 0.05.
The simple sorting test (ANOVA) [7] gave the following
results F = 2.565, k = 7, n-k = 306, P-value = 0.014, α = 0.01
and 0.05, F0.01= 2.700 and F0.05= 2.039, where F1-α (k-1, n-k) is
(1-α) quantile of the Fisher-Snedecor distribution for the
significance level α, so compliance of all means was accepted
at the significance level 0.01 and rejected at the significance
level 0.05.
The results of the Scheffe's test of multiple comparisons [8]
would enable to determine between which two abovementioned
means the statistically significant differences exist.
The Scheffe's test accepts the equality of all 28 pairs of means
of the degree of compliance of signatures at the significance
level 0.01 and 0.05 (28 is the number of possible options for all
pairs of devices).
When using different devices, there were found no
differences between the mean values of the degree of
compliance (x) and values of variance of the degree of
compliance (σ2
), that is at the significance level 0.05 for
variances and at the significance level 0.01 for means. It can
therefore be noted that, despite the technological differences
among individual devices, the stability of signatures (indicated
by variance) does not change when changing the device. Also,
the degree of compliance of signatures on all devices does not
statistically significantly differ at the significance level 0.01.
V. DISCUSSION
The results showed that the mean degree of compliance is
high for individual people (it fell below 60% only in case of 2
people), but the selective variances of the degree of compliance
are significantly different for all participants on each od all
used devices, which means that the participants coped badly
with various designs of devices. This follows from the fact that
the DBS is a highly automated activity given by an acquired
stereotype. The different design of the pad (thick pen, nondisplayable
signature, small signature area, hand resting
beyond the pad, etc.) can lead to the distortion of the stereotype
and higher variance between the signatures.
By contrast, regardless of the individual properties of the
signers, the mean values of compliance of signatures of all
people on all devices were high (from 76 to 85) and there were
no differences between the values of means and variances of
the degree of compliance when using different devices, that is
at the significance level 0.05 for variances and at the
significance level 0.01 for means.
The different scanning technology does not affect the
degree of compliance and variability of signatures – see Fig. 3
above. In the opinion of the authors, the "user-friendliness" is a
key factor in creating the signature. Another factor is then the
individual characteristics of the signer. The variability of the
signature, and hence the low degree of compliance among
individual signatures, which is exceptionally manifested among
the signers, is closely related to the stability of the signature.
The greater the intra-personal variability is, the less stable the
signer is. [9] There are two types of the variability of
signatures: the short-term (depends on the psychological state
of the person and on the conditions of writing) and the longterm
variability (depends on the change of the system of
physical writing, or modification of the motor program in the
brain) [10] – e.g. due to the influence of the disease or aging.
According to our experiments, where instead of the optical
evaluation of the signature images the biometric characteristics
were analyzed, it was however proved that the influence of
external factors on the short-term variability is negligible [2],
[3], [4], [5].
In our paper, we describe experiments that related to the
influence of different devices (pads) on the DBS performed by
the signing person. An interesting contribution to this issue is a
paper [11] dealing with the creation of DBS database in
relation to variable devices (pads)
Our experiments used pads when DBP was verified using
time functions (X, Y, Z coordinates etc.), that we consider to
be most reliable today. Here, it will be suitable to focus on
other evaluation algorithms (see [12]), as more and more types
of signing devices are currently being used.
In our experiments, we have confirmed the stability of the
DBS in the changing environment surrounding the signing
persons. The results of the experiments are consistent with the
papers [13], [14] where DBS stability models are proposed
with regards to the significant aspects of the signature,
respectively, to the behaviour of the signers.
We have dealt with the changes relating to a signer or the
influences of his surrounding on the signing situation.
However, the impact of the aging of the signer has not been
included in these experiments, as long-term results have to be
taken into account. Modelling aging issues [15] deals with the
design of time-based systems such as the Hidden Markov
Model (HMM), in which case the results of the studies will be
confirmed experimentally, which is also the subject of our
future work.
VI. CONCLUSION
Hypothesis I. – The participants will cope with the
difficulties connected with changing circumstances of the
signing depending on the technical design of the pad in a
different way: The null hypothesis H0 claiming that the
stability of signatures of a particular person on each device
does not significantly change, that is at the significance level
0.01 and thus at the significance level 0.05, was disproved. A
pair of devices, where the hypothesis on compliance of means
of the degree of compliance was rejected at the significance
level 0.01, was found for each participant. Therefore, the
hypothesis H1 claiming that there is a statistically significant
difference in the means and variances of the degree of
compliance of signatures of a particular person on individual
devices was confirmed.
Hypothesis II. – The stability of signatures achieved on
individual devices will statistically significantly differ: The
null hypothesis H0 claiming that the mean degree and variance
of compliance of signatures for each device do not significantly
change, because there were found no differences between the
values of means and variances of the degree of compliance
when using different devices, that is at the significance level
0.05 for variances and at the significance level 0.01 for means,
was confirmed.
ACKNOWLEDGMENT
The authors wish to thank the Moravian University College
Olomouc, its academic staff, and students for active
participation in carrying out the tests. Also, they thank to the
company Contrisys spol. s r. o. for the lease of equipment and
technical support.
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