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A Secure Biometric Authentication Scheme Based
on Robust Hashing
Yagiz Sutcu
Polytechnic University
Six Metrotech Center
Brooklyn, NY 11201
ysutcu01@utopia.poly.edu
Husrev Taha Sencar
Polytechnic University
Six Metrotech Center
Brooklyn, NY 11201
taha@isis.poly.edu
Nasir Memon
Polytechnic University
Six Metrotech Center
Brooklyn, NY 11201
memon@poly.edu
ABSTRACT
In this paper, we propose a secure biometric based authentication
scheme which fundamentally relies on the use of a robust hash
function. The robust hash function is a one-way transformation
tailored specifically for each user based on their biometrics. The
function is designed as a sum of properly weighted and shifted
Gaussian functions to ensure the security and privacy of biometric
data. We discuss various design issues such as scalability,
collision-freeness and security. We also provide test results
obtained by applying the proposed scheme to ORL face database
by designating the biometrics as singular values of face images.
Categories and Subject Descriptors
E.m [Data]: Miscellaneous – biometrics, security, robust hashing.
General Terms
Security, Design, Human Factors.
Keywords
Authentication, Biometrics, Robust Hashing, Security, Privacy.
1. INTRODUCTION
Today, as a member of technology driven society, we are faced
with many security and privacy related issues and one of them is
reliable user authentication. Although for most of the cases,
traditional password based authentication systems may be
considered secure enough, the level of security is limited to
relatively weak human memory and therefore, it is not a preferred
method for systems which require high level of security. An
alternative approach is to use biometrics (fingerprints, iris data,
face and voice characteristics) instead of passwords for
authentication. Higher entropy and uniqueness of biometrics make
them favorable in so many applications which require high level
of security, and recent developments of biometrics technology
enable widespread use of biometrics-based authentication
systems.
Despite the qualities of biometrics, they have also some privacy
and security related shortcomings. In the privacy point of view,
most of the biometrics-based authentication systems have
common weakest link which is the need for a template database.
Typically, during the enrollment stage, every user presents some
number of samples of their biometric data and using this
information, some descriptive features of that type of biometric
(i.e., singular values, DCT coefficients, etc.) are extracted.
Analyzing these extracted features, templates for each and every
user are constructed. During authentication, a matching algorithm
tries to match the biometric data acquired by a sensor with the
templates stored in the template database. According to the result
of the matching algorithm, authentication succeeds or fails. This
enrollment and authentication process is illustrated in Figure 1.
Table 1. Properties of different authentication techniques [6]
Method Examples Properties
What you know
User ID
Password
PIN
Shared
Easy to guess
Forgotten
What you have
Cards
Badges
Keys
Shared
Duplication
Lost or stolen
What you know
+
What you have
ATM card
+
PIN
Shared
PIN is weakest link
Something
unique about
user
Fingerprint
Face, Iris,
Voice, …
Not possible to share
Forging difficult
Cannot be lost or stolen
Main weakness of the biometrics is the fact that, if biometrics
compromised, there is no way to assign a new template, and
therefore, storing biometric templates should be avoided.
However, unlike passwords, the dramatic variability of biometric
data and the imperfect data acquisition process prevents the use of
secure cryptographic hashing algorithms for securing the
biometrics data. Secure cryptographic hashing algorithms such as
MD-5 and SHA-1 give completely different outputs even if the
inputs are very close to each other. This problem made researchers
to ask the following question: Is it possible to design a robust
hashing algorithm such that, the hashes of two close inputs are
same (or close) whereas inputs which are not that close will give
completely different outputs?
In recent years, researchers have proposed many different ideas to
overcome this problem. Juels and Wattenberg [1] proposed a
fuzzy commitment scheme which simply uses quantization idea to
define closeness in the input space. Depending on the
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111
quantization level, if noisy biometric data is close enough to its
nominal value determined at the time of enrollment, user will be
successfully authenticated. Later, Juels and Sudan [4] proposed
“fuzzy vault” scheme, which combines the polynomial
reconstruction problem with error correcting codes, in order to be
able to handle unordered feature representations. Tuyls et al. [2],
[3] also used error-correction techniques with quantization to
handle the variability of biometric data. Ratha et al. [6] and
Davida et al. [5] were among the first to introduce the concept of
cancelable biometrics. In [6], the main idea is to use a
noninvertible transform to map biometric data to another space
and store that mapped template instead of the original one. This
approach will give the opportunity to cancel that template and
corresponding transformation when the biometric data is
compromised. Vielhauer et al. [?] also proposed a simple method
to calculate biometric hash values using statistical features of
online signatures. The idea behind their approach can be
summarized as follows: After the determination of the range of
feature vector components, the length of extended intervals and
corresponding offset values of each interval are calculated. At the
time of authentication, extracted feature values are first
normalized using the length and offset values determined
previously and then rounded accordingly to get the hash value.
Although this approach is simple and fast, hash values cannot be
assigned freely due to nature of the scheme and this makes the
collision resistance performance of the proposed method
questionable. Furthermore, need for storing the offset and interval
length values for each individual is another weakness from the
security point of view. More recently, Connie et al. [10], Teoh et
al. [11] and Jin et al. [12] proposed similar bio-hashing methods
for cancelable biometrics problem. A detailed survey of all these
approaches can be found in [7].
Figure 1. Enrollment and authentication process of a biometric
authentication system [13].
In this paper, we analyze the performance and feasibility of a
biometric based authentication system which relies on the
sequential use of a robust hash function and a cryptographic hash
function (i.e., MD-5, SHA-1). The robust hash function is a one-
way function designed as a sum of many Gaussian functions. In
section 2, we give the details of our approach and discuss related
design issues and challenges. In section 3, we elaborate on the
setup and present simulation results. Our conclusions and the
scope of future work are provided in Section 4.
2. PROPOSED SCHEME
In [6], Ratha et al. proposed the use of a noninvertible distortion
transform, in either the signal domain or the feature domain to
secure the biometric data of the user. This will not only eliminate
the need for storing biometric template in the database but also
provide flexibility to change the transformation from one
application to another to ensure the security and privacy of
biometric data. Figure 2 simply illustrates that noninvertible
transformation idea such that, the value of a feature x is mapped
to another space (y) meaning that, given y, it is not possible to
find the value of x since the inverse transform is one-to-many.
However, in this setup matching process needs to be performed in
transformed space, and it is not a trivial task to design such a
transform because of the characteristics of the feature vector.
Typically, depending on the type of biometric used and feature
extraction process, the components of feature vectors take
different values changing in some range, rather than taking precise
values, and therefore candidate transform has to satisfy some
smoothness criteria. While providing robustness against to
variability of same user’s biometric data, that transformation also
has to distinguish different users successfully.
Apart from the difficulty in design of such transformations, the
smoothness properties of that transformation might reveal the
range information of the feature vector components to some
extent. Furthermore, overlapping or even close ranges may pose
another problem for this design and especially it becomes more
difficult to satisfy the required robustness.
Figure 2. An one-way transformation example.
In this context, other than the one-way transform and error
tolerance requirements, there are other important design issues
that need to be addressed. One concern is the scalability of the
overall system. Since the number of users may vary over the time,
the design has to be flexible enough to accommodate new user
addition and deletion. That is, it should be possible to create new
accounts at minimum cost as well as providing collision free
operation. Another design issue is the user-dependence of these
transformations. If not impossible, it is extremely difficult to
design such a single non-invertible transformation for each user
that satisfies all design specifications. Finally, output space of the
candidate transformation needs to be quantized in order to make it
112
possible to combine this transformation with a secure hashing
algorithm.
Considering these issues, we propose an alternate form of one-
way transformation which is combined with a secure
cryptographic hash function. The one-way transformation is
designed as a combination of various Gaussian functions to
function as robust hash. The cryptographic hash is used to secure
the biometric templates stored in the database.
In this approach, we simply assume that every component of the
n-dimensional feature vector is taking some value in some
range without imposing any constraint on the values and ranges as
follows:
T
iniii
vvvV ],...,,[
21
=
is the n-dimensional feature vector of i
th
user
of the system and
njNivvv
ijijijijij
,...1;,...,1 ==+≤≤−
δδ
where 2
δ
ij
determine the range of the j
th
component of the feature
vector of the i
th
user and N is the total number of the users.
In the enrollment stage, enough number of samples of biometric
data is acquired from users. Using these data, range information of
each user’s feature vector (
δ
ij
) is obtained. Once this information
is determined, every component of the feature vectors are
considered separately and a single Gaussian (red Gaussian in
Figure 3) is fitted to corresponding range considering the output
value assigned to that component of the feature vector. Let us
explain this fitting operation with the help of an example.
Consider j
th
component, v
ij
, of the feature vector of user i. Assume
that v
ij
takes values between (v
ij
-
δ
ij
) and (v
ij
+
δ
ij
) and also
assume that o
ij
is the assigned output value for that component of
the feature vector. Set of points to be used for Gaussian fitting
will be:
{(x
1
,y
1
), (x
2
,y
2
), (x
3
,y
3
)} where
(x
1
,y
1
) = (v
ij
-
δ
ij
, o
ij
) ; (x
2
,y
2
) = (v
ij
, o
ij
+ r) and
(x
3
,y
3
) = (v
ij
+
δ
ij
, o
ij
) with r is a uniformly selected random
number between 0 and 1.
After that stage, some number of fake Gaussian functions are
generated and combined with the first one in order to cover the
whole range and hide the real range information and this process
will be repeated n times for every user. This process is illustrated
in Figure 3.
Figure 3. Design process of proposed one-way transformation.
Certainly the parameters of these transformations are determined
and given to the users by an authorized, trusted third party and
furthermore this information is stored in a smartcard or a token
which needs to be used at the time of authentication.
Authentication process will be performed in the following
manner: Firstly, user’s biometric data will be acquired with a
sensor and his/her feature vector will be extracted. Secondly, one-
way transformation, stored in the smart-card, will be generated,
and it will be evaluated at the extracted feature vector component
values. Lastly, values obtained after quantization will be
concatenated together to form a string and than hashed. The
hashed value will be compared to user’s entry for authentication,
as illustrated in Figure 3.
Assuming the fact that hashing algorithm used in this scheme is
secure, for an attacker who has access to the database,
determining the real values of the feature vector by looking at
hashed values stored in the database will not be possible.
Furthermore, even though the information on the smartcard is
compromised, it still remains difficult for an attacker to guess the
real values of the biometric data of the user by only analyzing the
shape of one-way transformation of that user.
This approach is also scalable not only because of the fact that
generating gaussians is relatively a simple task, but also it is
possible to generate and assign different output values for each
and every component of a feature vector while satisfying
collision-free operation. Considering a number of potential users,
one can generate m-by-n matrix (where m is the total number of
users and n is the dimensionality of the feature vector) ensuring
that any two rows of this matrix are not identical. By the time of a
new user account needed, one row from that matrix will be
assigned to that user and his/her one-way transformation will be
designed using these values.
113
Figure 4. Authentication process of proposed scheme.
However, since the range information is hidden by the peaks of
the gaussians, these transformations are not used in an efficient
manner. This weakness of the proposed approach may be
observed by an intelligent attacker and help him/her to reduce
brute force guessing space for biometric data. To be able to reduce
this leakage of information, number of fake gaussians should be as
high as possible but also these fake gaussians need to have
variance and magnitude parameter values close to real gaussian
fitted to the real range. But in this case, especially if the length of
user range is relatively high with compared to the length of
overall range for a specific component of his/her feature vector, it
will not be possible to generate so many number of fake
gaussians. The reason for that constraint is the consequence of the
fact that, summation of overlapping tails of gaussians will have a
relatively high value and this will make the design difficult and
resulting transformation will have a poor hiding quality.
Finally, since the proposed approach is generic, type of biometric
data may be changed regularly to assure the privacy and security
of the system. The proposed approach is tested on the ORL face
database using simple singular value based feature vectors and
performance of the scheme will be presented in the following
section.
3. EXPERIMENTAL RESULTS
In recent years, singular values have been introduced as the
feature vector for face recognition and other applications. In this
study, we also used singular values as feature vector for testing
our scheme and in the following sub-sections, we will give a brief
explanations about singular value decomposition and its
properties and then explain our experimental setup.
3.1 Singular Value Decomposition
Let us first introduce the singular value decomposition of a
matrix.
Theorem 1 (Singular Value Decomposition)
),min(0...
),,...,(
,
21
21
nmpandwith
diagwhereVUA
thatsuchRVandRU
matricesorthogonalexisttherethenRAIf
p
p
T
nxnmxm
mxn
=≥≥≥≥ =ΣΣ= ∈∈ ∈
λλλ λλλ
Following theorem provides the necessary information about the
sensitivity of singular values of a matrix.
Theorem 2 (Perturbation)
Eofnorminduced
isEwherepiforE
thenAofSVDbeVUAletand
AofonperturbatiabeREAALet
ii
T
mxn
2
,...,1
,
~
~
~
~
~
,
~
22
−=≤− Σ= ∈+=
λλ
Since SVD is one of the well-known topics of linear algebra, we
omitted to give detailed analysis of this subject and interested
reader may find more details in [9].
3.2 Experiments and Results
The ORL face database [8] is created for face recognition related
research studies and as a result, differences of facial expressions
of the subjects are more than acceptable limits for a biometric
authentication system. However, since creating a new set of face
images for our study is not trivial, we decided to make our
preliminary tests using this database.
ORL face database consists of 10 different images of 40 distinct
subjects and the size of each image is 92x112, 8-bit grey levels. In
our simulation, we randomly divide each 10 samples of subjects
into two parts namely, training and test sets while training set has
6 of the images, test set has the remaining 4 samples. In our
simulations, only first 20 singular values of the images are
considered and none of the data pre-processing techniques (such
as principal component analysis (PCA), linear discriminant
analysis (LDA), etc) are used.
The performance of the proposed scheme is determined in terms
of basic performance measures of biometric systems, namely,
False Acceptance Rate (FAR) and False Rejection Rate (FRR).
However, another type of performance measure that needs to be
considered is due to the possibility that a one-way transformation
designed for a particular user can be used in authentication of
another user. (This is the likelihood of user X authenticating
himself as user Y while using user Y’s smartcard.) This type of
error can be interpreted as a factor contributing to FAR. For the
sake of clarity, we will denote such errors by FAR-II.
In our analysis, we first extract a feature vector from the set of
training images, and then determine the range of variation for
each feature vector component. The range for each component is
calculated by measuring the maximum and minimum values
observed in the training set and expanding this interval by some
tolerance factor (e.g., 5% or 10%) in order to account for the
possible variation in a feature value that is not represented within
the available training images. Our results obtained for 5% and
114
10% tolerance factors are given in Tables 2 and 3. It should be
remembered that in our experiments, we used 6 out of 10 images
(available for each person) to estimate the range and tested the
scheme on the rest of the images
Table 2. FRR results
Correct
Authentication
Ratio
# of correctly
authenticated
subjects
(5% tolerance)
# of correctly
authenticated
subjects
(10% tolerance)
4/4 2 15
3/4 8 10
2/4 13 10
1/4 13 4
0/4 4 1
Total 40 40
Table 3. FAR-II results
Incorrect
Authentication
Ratio
#of incorrectly
authenticated
subjects
(5% tolerance)
# of incorrectly
authenticated
subjects
(10% tolerance)
0/39 12 1
1/39 12 7
2/39 9 3
3/39 6 4
≥ 4/39 1 25
Total 40 40
Table 2 summarizes the FRR performance of the proposed scheme
in the following manner: First column stands for the correct
authentication ratio, which is the ratio of correctly authenticated
number of unseen test images to the total number of unseen
images — 4 images. On the other hand, each row shows the
number of persons that were successfully authenticated at a given
authentication ratio. For example, the number 2, which stands in
the second column of first row indicates that; there are 2 subjects
(out of 40), who are authenticated successfully for all of the test
images. Similarly, the number 4 (second column and fifth row)
denotes that there are 4 subjects (out of 40) that were not
authenticated at all, indicating that the assumed tolerance factor is
not satisfactory.
In Table 3, FAR-II performance of our scheme is presented in a
similar manner. For a given user, all remaining (39) users are tried
to be authenticated using that user’s smart-card (one-way
transform function) and presenting their own biometric data and
results obtained are summarized in Table 3. First column of Table
3 represents the ratio of incorrectly authenticated users to the
number of remaining users — 39 users. For example, there are 12
(out of 40) users who were never confused by any other user,
meaning that, none of the remaining 39 users were authenticated
as one of them. On the other hand, with a tolerance factor of 10%
there are 25 users whose authentication data were collided with at
least 4 of the remaining 39 users.
In our scheme, any of the users who uses his/her own smart-card,
is authenticated as another user, which means, FAR is zero.
However, false acceptance results (FAR-II), presented in Table 3,
which actually indicate the rate of being authenticated as another
user using other user’s smart-card. One of the reasons to observe
such a relatively high false acceptance rate (especially with a
tolerance factor of 10%) is due to nature of ORL face database
which contains images captured under extensively varying
conditions. As a result, actual range information of the singular
values could not be estimated efficiently due to the high variations
depending on the differences of facial expressions of the subjects.
It should be noted that, to further improve the performance one
can employ data pre-processing techniques such as PCA or LDA.
It is reasonable to expect that, when appropriate pre-processing
techniques are employed along with higher dimensional feature
vectors (e.g., more than 20 singular values), performance of the
proposed scheme will be better. These considerations will be the
parts of our future work.
4. CONCLUSION AND FUTURE WORK
We proposed a secure biometric based authentication scheme
which employs a user-dependant one-way transformation
combined with a secure hashing algorithm. Furthermore, we
discussed its design issues such as scalability, collision-freeness
and security. We tested our scheme using ORL face database and
presented simulation results. Preliminary results show that,
proposed scheme offers a simple and practical solution to one of
the privacy and security weakness of biometrics-based
authentication systems namely, template security.
In order to improve the results, our future focus is three-fold: (1)
To find a more flexible and efficient way to design one-way
transformations with less parameters; (2) To find a metric for
measuring and comparing data hiding quality of these one-way
transformations. (3) To test our approach on larger databases also
with different types of biometric data.
5. REFERENCES
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[2] J.-P. Linnartz and P. Tuyls, “New shielding functions to
enhance privacy and prevent misuse of biometric templates,”
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[8] The ORL Database of Faces, available at
http://www.uk.research.att.com/facedatabase.html
[9] Strang, G., “Introduction to linear algebra”, 1998, Wellesley,
MA, Wellesley- Cambridge Press.
115
[10] T. Connie, A. Teoh, M. Goh, and D. Ngo, “Palmhashing: a
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[12] A. T. B. Jin, D.N.C Ling, and A. Goh, “Biohashing: two
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