REMOTE HANDWRITTEN SIGNATURE AUTHENTICATION
Jarrod Trevathan
James Cook University
Townsville, Australia
Alan McCabe
James Cook University
Townsville, Australia
Keywords:
Biometrics, authentication, client/server, handwritten signature, network security.
Abstract:
This paper presents a secure real-time remote user authentication system based on dynamic handwritten signa-
ture verification. The system allows users to establish their identities to other parties in real-time via a trusted
verification server. The system can be used to gain remote access to restricted content on a server or to verify
a signature on a legal document. State of the art dynamic verification techniques are combined with proven
cryptographic methods to develop a secure model for remote handwritten signature verification.
1 INTRODUCTION
Dynamic handwritten signature verification (HSV)
is a computerised method of establishing a person’s
identity This is achieved by examining the character-
istic way in which the person signs his/her name. The
handwriting of the person is captured and digitised in
real-time using a graphics tablet. The result is a series
of values representing the person’s handwritten signa-
ture. This is referred to as an “electronic signature”.
An electronic signature can be verified by comparing
it with a reference set of the user’s handwritten signa-
ture attributes.
Biometric security systems (including HSV) are
typically used in simple, stand alone applications.
However, this is now changing as the emphasis of
computing shifts towards distributed systems where
the qualities offered by a handwritten signature are
highly desirable. This paper describes a system for
performing HSV over a network.
This paper is organised as follows: Section 2 pro-
vides some background on HSV and the motivation
for constructing a networked based HSV system. Sec-
tion 3 presents a client/server model whereby a hand-
written signature can be remotely verified. Section
4 examines the HSV algorithm. Section 5 discusses
network security issues and its implications for HSV.
Section 6 describes several protocols for remote HSV.
Section 7 provides a security analysis of the proposed
system. Section 8 gives some concluding remarks.
2 HSV
Handwritten signatures have long been used as proof
of authorship of, or at least agreement with, the con-
tents of a document (Schneier, 1994). Humans find
signatures compelling as they offer authenticity, and
prevent forgery, alteration, repudiation and reuse of
the signed item (Schneier, 1996).
Despite the increasing popularity of other biomet-
ric authentication schemes such as retinal scanning
and automated fingerprint identification, HSV is still
regarded as one of the most reliable and natural meth-
ods of authenticating one’s identity (Gupta and Mc-
Cabe, 1997). There are a number of factors contribut-
ing to this including:
• high level of acceptability to the general public
• low cost of hardware compared to most other com-
puterised schemes
• high level of reliability compared to most other
schemes
• low error rate with little real deviation due to exter-
nal factors
The growth of web based commerce and banking
has been accompanied by a rise in new and innova-
tive attacks of defrauding or impersonating legitimate
users. Each year the costs associated with Internet
and credit card fraud continue to accumulate. Much
of this loss is attributed to ineffective user authentica-
tion protocols.
335
Trevathan J. and McCabe A. (2005).
REMOTE HANDWRITTEN SIGNATURE AUTHENTICATION.
In Proceedings of the Second International Conference on e-Business and Telecommunication Networks, pages 335-339
DOI: 10.5220/0001419203350339
Copyright
c
 SciTePress

A remote HSV system alleviates this problem by
allowing a user to verify his/her identity on demand.
For example, the user can verify his/her identity at the
point of sale when using a credit card or can sign bids
in an electronic auction. Similar systems involving
fingerprinting and hand geometry have been proposed
in (Techweb, 1998; Jain et al., 1998). However, hand-
written signatures are much more versatile.
Handwritten signatures are familiar to humans and
have many network-based applications, which con-
ventional biometric recognition systems are unable to
effectively emulate. For example, remote HSV can
be used to grant approval by “signing” a legal or con-
senting document such as a will or business contract
without having to be physically present. Furthermore,
remote HSV can allow a doctor’s prescription to be
authenticated at a pharmacy during sale to ensure the
purchaser is the correct recipient of the medication.
Another application for remote HSV is where a
client must authenticate themself to gain access to a
database or file system under a server’s control. In
addition, the recipient of a business or personal email
can use HSV to authenticate the sender.
While other biometric and cryptographic schemes
can be used to accomplish the aforementioned tasks,
HSV is clearly the most natural and least intrusive for
humans. For example, people would prefer to sign
their name rather than place their heads in a machine
to photograph their retina or face. Furthermore, cryp-
tographic methods often rely on people remembering
large numbers. While these values can be placed in a
smart card, cards can be lost or stolen and are incon-
venient to carry. Furthermore, HSV cannot be sub-
verted like other biometric systems. For example, it is
almost impossible to get someone to sign their name
under duress, whereas fingers, eyeballs and heads can
be severed from a person’s body and can be used to
identify the dead (or maimed) person (as seen in ac-
tion movies such as Demolition Man and The Sixth
Day).
3 REMOTE HSV
3.1 The Model
Figure 1 presents a client/server model for perform-
ing remote HSV. The main components of the system
include:
• A set of clients C = {c
1
, c
2
, ..., c
n
} who each pos-
sess a unique handwritten signature h; h
i
denotes
the signature corresponding to c
i
, 1 ≤ i ≤ n;
• A trusted server S to which clients can make re-
quests for verification. The server contains a verifi-
cation algorithm V, and secure reference database;
Figure 1: Remote Handwritten Signature Verification
• A network providing a bi-directional communica-
tion link between each client and the server.
A client, c
i
, provides a signature for verification (in
which case he/she is trying to establish or “prove”
his/her identity). Alternately, c
i
can be the recipient of
the verification results (in which case the server sends
them the verified identity of the signer).
The server, S, verifies signatures presented on the
client’s behalf. The verification algorithm can be up-
dated on the server and hence technology may be im-
proved without requiring any changes to the client
software. Furthermore, having the verification soft-
ware on the server removes the bias that would exist
if a client also contained the same software. For ex-
ample, there is a possibility that malicious users may
tamper with or simulate the verification algorithm.
The database is used to store handwritten signature
reference files, client information and security logs.
The verification algorithm, V , makes a binary de-
cision to ascertain whether the client is genuine. The
algorithm must be able to successfully authenticate le-
gitimate signatures and identify forgeries. An outline
of the verification algorithm is presented in Section 4.
The network links clients to the server. This net-
work is assumed to be insecure and any information
passing through it can be tampered with. Section 5
discusses network security and its implications for
HSV.
3.2 Stages in Verification
There are two main stages in a remote HSV system
(Figure 2):
• Registration - Add users to the system. Update sig-
nature profiles;
• Verification - Verify an electronic signature.
Registration is required in order to add new users to
the scheme. This must generally occur in some form
of supervised environment where the potential user
must prove his/her identity through traditional means
ICETE 2005 - SECURITY AND RELIABILITY IN INFORMATION SYSTEMS AND NETWORKS
336

Figure 2: Registration and Verification Phases
such as a birth certificate, driver license, etc. If suc-
cessful, a reference file is created for the registered
user. This involves storing samples of the user’s sig-
nature in a secure database. The user is provided with
a unique identifier (number or name) and any addi-
tional information pertinent to the verification proce-
dure.
Unlike other biometric systems, an individual’s sig-
nature tends to change over time due to various factors
such as age and health (Gupta and McCabe, 1997).
Therefore, the registration stage requires a method
to update a user’s reference file when needed after
his/her initial registration.
During verification an algorithm is run on a client’s
electronic signature. The electronic signature is com-
pared with the client’s reference file. The server logs
the outcome and notifies the client.
4 VERIFICATION ALGORITHM
This section contains a brief description of the verifi-
cation algorithm used in the scheme. Further details
can be found in (McCabe, 1997; McCabe, 2004).
The dynamic HSV approach consists of two sepa-
rate phases: conversion and comparison. The conver-
sion phase involves tracing the path of the signature.
It then examines several static and dynamic details of
the handwriting such as shape, spacing, velocity, ac-
celeration and timing details. These details are con-
verted into a string of characters (different characters
are used to represent different levels of velocity, ac-
celeration etc. and different aspects of shape, timing
etc.). The end result of this process is a string of sev-
eral hundred characters in length which attempts to
characterise the handwritten signature.
In order to compare two signatures it is possible
to use any one of a number of existing string dis-
tance algorithms to determine the level to which the
signatures differ. In (McCabe, 1997) several string
distance algorithms were implemented and experi-
mented with. The Wagner-Fischer algorithm (de-
scribed in (Stephen, 1994)) proved to be the most suc-
cessful.
In order to create a reference, a user of the system
is asked to provide five reference signatures on reg-
istration. The strings are extracted from each of the
reference signatures, each signature is compared to
one another and details of the common types and av-
erage levels of variation are recorded. When a test
signature is processed, it is compared to the reference
and a figure for the overall difference is calculated. If
this figure is below the threshold the test signature is
accepted as genuine, otherwise rejected as a forgery.
5 NETWORK SECURITY
In a network environment the signature data is at risk
from eavesdropping, interception, modification, fab-
rication and disruption. Measures such as encryption,
digital signatures and time stamping, are needed to
provide protection against these threats. This section
examines network security requirements for the HSV
scheme presented in this paper.
A client is provided with an initial session key, k
i
,
during registration. This allows encrypted communi-
cation with the server using any off-the-shelf symmet-
ric cryptosystem. Key updates are performed using
Diffie-Hellman key exchange (Diffie and Hellman,
1976).
The results of verification are signed using a digi-
tal signature (not to be confused with handwritten or
electronic signatures). S publishes a public key S
ku
.
This enables S to sign the results of verification using
the private key S
kr
. Anybody can verify the signature
using S
ku
. This can be achieved using RSA (Rivest
et al., 1978) or ElGamal (ElGamal, 1985) signatures.
Time stamping is required to prevent an intercep-
tor from resending captured messages. A time stamp
is appended to the handwritten signature data before
encryption. When a message is received, its time
stamp is examined to determine how old the mes-
sage is and if it has been previously received. This
presents a unique problem to HSV as any captured
electronic signature could be reused indefinitely. A
remedy stated in (McCabe, 2000), is to use a signed
password (rather than a signature) to verify a user.
This enables compromised handwritten passwords to
be changed, rather than handwritten signatures.
S logs all information regarding verification. Users
can access these logs in accordance with S’s policies.
REMOTE HANDWRITTEN SIGNATURE AUTHENTICATION
337

Table 1: Registration Protocol
User S
ID,r,c
ku
−→
r,s
←−
s,ref
h
i
−→
t,k
i
←−
Table 2: Client/Server Verification Protocol
c
i
S
t,r,h
i
−→
t,r,V (h
i
),S
kr
(t,r,V (h
i
))
←−
6 REMOTE HSV PROTOCOLS
An electronic signature (and the results of veri-
fication) is more binding and versatile than other
biometric authentication schemes. This gives rise to
many possible authentication scenarios. This section
describes the setup and registration of the scheme and
authentication protocols for performing verification.
Setup - S sets up the system, publishes a public key,
S
ku
, and keeps private key, S
kr
, secret.
Registration - A user must register with S. This
protocol is shown in Table 1. A user first obtains S
ku
in order to encrypt information sent to S. A user
submits a request to S with proof of identity, ID, a
random number, r, and the user’s public key, c
ku
.
S acknowledges this request, returns r and another
random number, s (encrypted using c
ku
). The user
generates a signature reference file, ref
h
i
. S saves
ref
h
i
and issues the new client with a certificate, t,
and an initial session key, k
i
.
Client/Server Verification - To begin the verification
process the client c
i
must first establish a connection
with the server S (see Table 2). All information is
encrypted using k
i
. c
i
transmits his/her handwritten
signature, h
i
, to S. Given h
i
, the verification algo-
rithm either returns a true or f alse value indicating
the success of the verification, that is, V (h
i
) = true
means that the signature has been accepted as authen-
tic or V (h
i
) = false meaning that the signature was
rejected as a forgery. S signs this information using
S
kr
.
One to One Verification - One to one verification
involves two clients c
i
and c
j
, i 6= j. Client c
j
wants to establish the identity of c
i
. c
i
provides h
i
Table 3: One to One Verification Protocol
c
i
S
t,r,h
i
,t
c
j
−→
c
j
S
t
c
i
,r,V (h
i
),S
kr
(t
c
i
,r,V (h
i
))
←−
Table 4: Multiple Verification Protocol
t
c
i
list
S
t,r,h
i
,t
c
j
list
−→
t
c
j
list
S
t
c
i
list
,r,V (h
i
),S
kr
(t
c
i
,r,V (h
i
))
←−
and the id of the intended recipient, t
c
j
, to S. S
performs V (h
i
) and forwards the result and the id of
the signer, t
c
i
, to c
j
. This process is shown in Table 3.
Multiple Client Verification - Multiple client verifi-
cation allows a set of clients to verify themselves to
another set of clients. For example, two signatures
may be required to perform a transaction on a jointly
held bank account. Another situation is where a group
of t out of n clients must authenticate themselves.
For example, 2 out of 3 CEO’s must be authenticated
to provide consent on a business contract or 6 out of
10 Senator’s votes are needed to pass a bill. This is
reminiscent of cryptographic secret sharing schemes.
The general protocol is shown in Table 4, where
t
c
i
list
, t
c
j
list
, i 6= j denote sets of clients.
7 SECURITY ANALYSIS
This section provides a security analysis of the pro-
posed system. As stated in Section 2, HSV is a supe-
rior form of biometric authentication system as it can-
not be subverted in the same manner as fingerprinting,
retinal scanning and facial recognition setups. The
security of the verification algorithm is based on the
HSV methods described in (McCabe, 1997) (see Sec-
tion 4).
The network security of the scheme is dependent
on the underlying cryptographic protocols. All in-
formation sent between the clients and the Server is
encrypted and digitally signed, therefore it cannot be
modified. Key updates can be performed at any time.
Timestamping prevents signature data, or the results
of verification from being replayed. The Server signs
the results of verification, therefore anyone can verify
that the handsignature was legitimately accepted or
ICETE 2005 - SECURITY AND RELIABILITY IN INFORMATION SYSTEMS AND NETWORKS
338

rejected. The Server retains logs of all transactions.
In the case of a dispute (repudiation of a signature,
etc.) the logs can be used to resolve the problem.
8 CONCLUSION
This paper describes a system for conducting dynamic
handwritten signature verification across a network.
A user is able to establish his/her identity in real-
time for the purposes of commerce, indication of in-
tent or simply to ensure the identity of communicat-
ing parties. The scheme allows complex authentica-
tion scenarios to occur between users of the system.
The system is more natural/less intrusive to humans,
and has a larger application area than other biometric
techniques. Security of the scheme is based on the
strength of the HSV methods of (McCabe, 1997; Mc-
Cabe, 2004) and the underlying cryptographic mea-
sures.
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